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>> R.J. AQUINO: Let's just get started.

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So this is Quiz 1.

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Here is some high-level information.

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The About page for the quiz is at this
URL, no longer CS50.net, although that

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will still work.

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It's CS50.harvard.edu/quizzes/2013/1.

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It's the big About page, telling you
where and when, namely next Wednesday

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in a bunch of rooms.

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And by next Wednesday, I
mean two days from now.

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All this information is there.

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But it is cumulative.

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>> So everything from the first half of the
year is potentially on the quiz,

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because you can't really do advanced
things in C without if conditions and

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for loops and the like.

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But there will be an emphasis on the
material covered since Quiz 0,

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starting with structs and File
I/O. It's typically more

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challenging than Quiz 0.

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The average score is typically lower.

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Study hard.

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>> While you're studying, be sure to use
CS50/discuss to post your questions

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and read other people's questions.

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So if you don't have any questions,
log in and read

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your friends' questions.

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They're probably good questions.

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And take the practice quizzes.

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We've been giving quizzes for
seven or eight years now.

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They're all online.

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Future questions are similar
to old questions.

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That's how we make them.

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>> The quiz doesn't exist yet.

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None of us have seen it.

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But it will look like
previous quizzes.

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For this review session, this is not
an exhaustive list of topics.

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You can't just attend this and then
be perfectly ready for the quiz.

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Otherwise, it wouldn't be
that much of a quiz.

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And this is also not necessarily
everything you need to know about any

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given topic.

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It's meant to expose you to the things
we've covered, remind you what we

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covered, and the way in
which we covered it.

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But you will have to go further and
deeper when you study to double-check

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that you know everything about any given
topic and that you've filled in

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all the corners that were
covered in lecture.

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>> The quiz notes tell you to go to the
scribe notes, watch lecture videos.

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That's a good way to make sure you've
covered all your bases.

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So getting started, when I made these
slides, I tried to put where I found

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information.

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So for File I/O, for instance, Week 7,
Monday's lecture, and the posted

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Section 6 and Problem Set all have
information about File I/O. I've done

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this for every topic.

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So those title slides may
be helpful to you.

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>> So here we have File I/O. Remember, in
Problem Set 5, we used fopen, fclose,

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fwrite, fread, and fseek.

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Having recovered 30ish JPEGs and
having resized and messed with

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bitmaps, you should be pretty familiar
with these functions

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and how they work.

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If you are no longer familiar,
definitely review them.

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And make sure you understand what
the different arguments are,

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when they're used.

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>> But the common file-related bugs
you may be asked about--

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well, if you've forgotten to check if
fopen actually worked before you go to

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modify a file.

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That could be bad.

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If you've forgotten to fclose a file
that you've fopened, that's similar to

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a memory leak.

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That's pretty bad.

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And forgetting to check if you've
reached the end of the file before you

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start writing to it.

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>> So if you say, hey, I'm at
the end of the file.

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Give me 5 more bytes.

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Well, that's probably not going to
work out the way you expect.

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That's really it for File I/O,
because we did so much of it

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with the problem set.

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So if you understood what was going on
in Problem Set 5, remember the bitmats

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and the JPEGs, then you're probably all
set for File I/O. If that's a bit

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fuzzy, definitely review that problem
set and the associated material.

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>> Structs were the topic that were on the
line between Quiz 0 and Quiz 1.

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Didn't quite make the cut for Quiz 0.

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So they'll definitely be on
Quiz 1, Week 7, Monday.

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What is a struct?

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Here we show a struct.

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It's like a new type.

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It's like a container
for multiple fields.

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>> In this case, we've declared a struct
student that has two fields--

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a string that we're calling name and
an int that we're calling age.

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So when I pass around students or I
modify students, I'll be able to

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access their name and their age.

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Let's look at some code for that.

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Here we see that I've declared
a student s, just like

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I declare any variable--

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int x, int y, et cetera.

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>> Here's student s.

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He starts with nothing in his fields.

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So let's set them.

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You set fields of a struct with dot.

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So I've said here that s.name = RJ.

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And s.age = 21.

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You can also update fields
the same way you'd update

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the value of a variable.

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So I want to change my name from RJ with
no periods to R.J. spelled the

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correct way.

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It would be s.name = RJ, the same
as we said it originally.

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And then you can access them.

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>> So we've set them.

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We've updated them.

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You can also access them
in the very same way.

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So here, I'm printing out
R.J. Is 21 years old.

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And I'm accessing those values
with s.name and s.age.

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So that's accessing structs
with the dot notation.

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Yep, question?

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>> AUDIENCE: Is there a reason on the
previous slide that you didn't put

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student on the top line, like typedef
struct student and then

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student at the end?

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>> R.J. AQUINO: So the question was, on
this slide, we've typically seen

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typedef struct node and then the
fields of the struct and

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then the word node.

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And how come here I didn't say, typedef
struct student and then the

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fields of the struct and then student?

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The reason is that I don't need to
access it inside of the struct.

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So it's OK to leave it without a name.

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I can just leave it as
an anonymous struct.

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>> The reason we do it for linked lists and
things is because inside you need

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to reference a struct nodes star.

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So the struct has to have a name,
so you can access it later.

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It's a minor detail.

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But you'll typically see typedef struct
curly braces if you don't need

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the name and typedef struct some name
followed by curly braces if you will

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need the name.

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So that's a good question.

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>> And on that point, we tend to modify
structs and pass around structs by

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reference, not by value.

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So we'll just pass around pointers to
structs instead of passing around the

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structs themselves.

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So you're very frequently going to be
using, in this case, student* or

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struct node* or node* instead
out students or nodes.

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So here, I've said, OK, the variable ptr
is going to be the address of s.

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It's going to be the pointer
to the student R.J.

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>> So we can get at those fields
the same as we get anything.

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First, ID reference the pointer
to get the struct.

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That's *ptr and then
a dot and then age.

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So to access the field, and I've updated
it now to 22, because, let's

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say, it was my birthday.

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There's a shortcut syntax
using the arrow here.

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So ptr arrow age is just
the same as *ptr.age.

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Now, that's something you'll have
to memorize and remember.

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>> You used it a lot in pset6,
the speller pset.

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But this is actually what's going
on underneath the hood.

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It's dereferencing the pointer
and then accessing it.

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Question?

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>> AUDIENCE: [INAUDIBLE].

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>> R.J. AQUINO: So why are we using
pointers as structs instead of the

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structs themselves?

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The reason would be if you're passing
a struct to a function, you probably

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want to pass around just the 4 or so
bytes that represent the pointer, as

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opposed to the potentially 30 or
40 bytes that are the struct.

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So passing something to a function
is easier when the thing

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is smaller in short.

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Question?

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>> AUDIENCE: You might have mentioned this
in the beginning, but are there

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other slides up on [INAUDIBLE]?

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>> R.J. AQUINO: These slides will be
up after the review session.

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We'll post them on the website.

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So moving on and moving on slightly
faster, we're going to talk about data

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structures.

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There are a lot.

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We covered a bunch of them.

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Here's what you should understand
about data structure.

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>> You should really understand at a high
level what each structure is.

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Can you explain in English to your
friend that hasn't taken CS50 how

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we're organizing our data and why we'd
be using something in this way?

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That's thing one.

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Thing two, understand
the implementation.

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So understand how to use these things
in C. And we'll be going over this.

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>> And then thing three would be know the
run times and the limitations of the

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various structures you're using.

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So understand why you would use a
hash table instead of an array.

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Understand how fast, on average,
accessing a hash table is.

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Understand what operations are fast on
linked list but slow on arrays and

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vice versa.

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So to understand that, you'll have to
understand Big-O notation just to know

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how to talk about these
sorts of things.

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And we'll talk about that.

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>> So first thing, linked lists.

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Here is a high-level picture
of a linked list.

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We show this in class.

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We typically have 10 people
standing on the stage.

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But we have a series of nodes where each
node has some value and a pointer

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to its next value.

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So to get from one node to the next, you
just say, give me the next node.

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You have that node.

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Give me the next node.

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You have that node.

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Give me the next node and so on
until there's no node left.

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>> So continue to talk about
it at a high level.

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It's very easy to insert things
into a linked list.

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If you don't care about the order,
you can just drop it

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right at the beginning.

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That's constant time.

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But it's hard to find a value.

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If you're trying to ask,
is seven in my list?

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You have to go through
every single value.

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Is this seven?

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Is this seven?

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Is this seven?

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Is this seven?

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Over and over again.

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And that's O(n).

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So when studying for the quiz,
compare this with arrays.

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>> Is that OK?

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The lights went dim.

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OK.

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When is a linked list better?

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When is an array better?

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So let's look at some code.

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Here is a potential node.

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It's a struct.

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It has an int n, which
will be our value.

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And it has a struct node* next, which
is our pointer to the next node.

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So here, we can see that we happened
to have put an int in our node.

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But if this were a linked list of char
stars or a linked list of floats, we

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totally could do that too.

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>> Remember in pset6, you probably had a
linked list of char stars or just

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static char arrays.

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Let's look here at an operation.

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So we want to insert a new
n into our linked list.

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We start out with a head pointer that is
a pointer to this node that has the

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value of n and a next of a pointer that
points to this node is a value of

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n and a next of null, because
it's the last node.

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So in the interest of time, I will
put all the code on the screen.

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And we'll walk through it
a few lines at a time.

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>> So here's the code.

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I hope it's readable.

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The first thing we do is
we malloc a new node.

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So it makes a pointer to a new node that
doesn't quite have anything set

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up in it yet.

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We check to make sure that
the new node isn't null.

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Otherwise, we have to give up.

244
00:11:22,220 --> 00:11:27,680
So having checked that, we now
set the values in the node.

245
00:11:27,680 --> 00:11:31,520
So we put the new n into our n field.

246
00:11:31,520 --> 00:11:36,050
And we set the next pointer to point to
the original head, so that we can

247
00:11:36,050 --> 00:11:38,900
now have inserted this
node into our list.

248
00:11:38,900 --> 00:11:44,600
>> Finally, we have the global head point
to our new node, so that if we were to

249
00:11:44,600 --> 00:11:50,300
start at head, we would be at this
new first node instead of

250
00:11:50,300 --> 00:11:51,560
the old first node.

251
00:11:51,560 --> 00:11:55,320
And when this function exits, the
variable new node no longer exists,

252
00:11:55,320 --> 00:11:57,130
because it was local to the function.

253
00:11:57,130 --> 00:11:59,770
So this is the state of the world.

254
00:11:59,770 --> 00:12:03,570
Our global head points to our new
first node, which points to our

255
00:12:03,570 --> 00:12:06,346
original first node, which points
to the node after that.

256
00:12:06,346 --> 00:12:09,790
>> That was insertion.

257
00:12:09,790 --> 00:12:12,150
I hope that was relatively
straightforward to follow.

258
00:12:12,150 --> 00:12:14,300
When in doubt, draw a picture.

259
00:12:14,300 --> 00:12:17,820
So I find that talking about
linked lists and looking at

260
00:12:17,820 --> 00:12:19,870
code is very not helpful.

261
00:12:19,870 --> 00:12:23,790
Whereas looking at a picture of a linked
list allows me to think, oh, so

262
00:12:23,790 --> 00:12:24,970
I have this node here.

263
00:12:24,970 --> 00:12:28,980
But if I update that pointer,
it ends up disconnected.

264
00:12:28,980 --> 00:12:34,340
And I've forgotten where
the node goes.

265
00:12:34,340 --> 00:12:35,390
And the code exits.

266
00:12:35,390 --> 00:12:37,830
And you have multiple nodes
that are disconnected.

267
00:12:37,830 --> 00:12:39,970
And you don't end up with
the list you want.

268
00:12:39,970 --> 00:12:43,320
>> So if you draw the picture and do it
step by step, hopefully, you'll see

269
00:12:43,320 --> 00:12:46,840
the correct order of things in terms of
updating the pointers to make sure

270
00:12:46,840 --> 00:12:48,830
that the list comes together.

271
00:12:48,830 --> 00:12:51,240
Insert is relatively straightforward.

272
00:12:51,240 --> 00:12:55,210
A more complicated one would be
insertion into a sorted list.

273
00:12:55,210 --> 00:12:59,980
A more complicated function is delete
and find, so looking through a list to

274
00:12:59,980 --> 00:13:03,030
see if something is there.

275
00:13:03,030 --> 00:13:07,220
Perhaps you did this in pset6 when you
got into your hash table and you said,

276
00:13:07,220 --> 00:13:10,460
well, is the word apple
in my linked list?

277
00:13:10,460 --> 00:13:11,440
>> So you may have already done this.

278
00:13:11,440 --> 00:13:15,530
But definitely, refresh your memory
and try to reimplement find and

279
00:13:15,530 --> 00:13:19,150
reimplement delete for a linked list.

280
00:13:19,150 --> 00:13:22,850
Fun side note, there also doubly-linked
lists, where you have

281
00:13:22,850 --> 00:13:27,490
pointers that point both forward and
backward, so that you could go to the

282
00:13:27,490 --> 00:13:29,270
next node and to the previous node.

283
00:13:29,270 --> 00:13:33,860
And there was a question on last year's
quiz of that type, talking

284
00:13:33,860 --> 00:13:36,190
about doubly-linked lists.

285
00:13:36,190 --> 00:13:40,070
>> Now, that's a structure that you're
relatively familiar with, because most

286
00:13:40,070 --> 00:13:42,520
of you probably used them on pset6.

287
00:13:42,520 --> 00:13:45,680
Here's one that's a bit less familiar.

288
00:13:45,680 --> 00:13:51,020
As a side note, I think that Quiz 1 is
primarily harder than Quiz 0, because

289
00:13:51,020 --> 00:13:54,600
the stuff you're doing, you
have not done as much.

290
00:13:54,600 --> 00:13:58,100
To put that another way, for Quiz 0,
you had written a lot of C. And we

291
00:13:58,100 --> 00:13:58,880
asked you about C.

292
00:13:58,880 --> 00:14:02,310
>> For Quiz 1, we're going to ask you about
PHP and JavaScript, which you

293
00:14:02,310 --> 00:14:03,490
haven't written as much of.

294
00:14:03,490 --> 00:14:07,590
We're going to ask you about C-code that
you haven't written as much of,

295
00:14:07,590 --> 00:14:09,130
this advanced C stuff.

296
00:14:09,130 --> 00:14:11,520
So definitely, practice the stuff we
talked about in lecture that you

297
00:14:11,520 --> 00:14:15,260
didn't necessarily do
on the problem set.

298
00:14:15,260 --> 00:14:17,870
>> Speaking of which, you haven't written
a stack on a problem set.

299
00:14:17,870 --> 00:14:19,610
But it was in lecture.

300
00:14:19,610 --> 00:14:22,530
Here's the high-level picture of
stacks that we show every year.

301
00:14:22,530 --> 00:14:26,180
It's the stack of trays in
the Mather dining hall.

302
00:14:26,180 --> 00:14:30,570
At a high level, stacks are a last
in, first out data structure.

303
00:14:30,570 --> 00:14:32,040
That means you're going
to put things in--

304
00:14:32,040 --> 00:14:36,400
1, 3, 7, 12, 14, negative 0.

305
00:14:36,400 --> 00:14:39,180
The one thing I couldn't have
said-- negative 3, 0.

306
00:14:39,180 --> 00:14:40,250
You put all these things in.

307
00:14:40,250 --> 00:14:42,940
And the last one you put in is the first
one that's going to come out.

308
00:14:42,940 --> 00:14:44,170
>> So you have two operations--

309
00:14:44,170 --> 00:14:45,260
push and pop.

310
00:14:45,260 --> 00:14:49,180
All of the putting in that I was
gesturing like this are push.

311
00:14:49,180 --> 00:14:52,020
And then when I reach in to grab
something or reach on top to grab

312
00:14:52,020 --> 00:14:53,940
something, that's pop.

313
00:14:53,940 --> 00:14:55,540
So we're going to implement stacks.

314
00:14:55,540 --> 00:14:57,870
And we showed them in lecture
using arrays.

315
00:14:57,870 --> 00:14:59,550
But you could do them
using linked lists.

316
00:14:59,550 --> 00:15:01,770
A stack is a conceptual data
structure, not like an

317
00:15:01,770 --> 00:15:03,760
implementation-specific one.

318
00:15:03,760 --> 00:15:06,420
>> So what would that look like?

319
00:15:06,420 --> 00:15:07,970
It would look kind of like this.

320
00:15:07,970 --> 00:15:10,840
You'd have an integer size.

321
00:15:10,840 --> 00:15:16,000
And you'd have an array of values that
we're calling trays, because that's

322
00:15:16,000 --> 00:15:18,570
what the picture was for
us-- int trays--

323
00:15:18,570 --> 00:15:21,740
and then some maximum capacity.

324
00:15:21,740 --> 00:15:27,100
So what would push look like?

325
00:15:27,100 --> 00:15:33,250
Well, if we have a stack s, then to push
something onto s, we would get

326
00:15:33,250 --> 00:15:34,620
the size of s.

327
00:15:34,620 --> 00:15:42,270
And that would be the next
open spot of our array.

328
00:15:42,270 --> 00:15:50,510
So if we have three things in our stack,
then trays 3 would be the next

329
00:15:50,510 --> 00:15:54,290
open spot, because 0, 1, and
2 are already filled up.

330
00:15:54,290 --> 00:16:01,790
>> So we put the value into
s.trays[s.size], the third spot.

331
00:16:01,790 --> 00:16:06,290
And then we increment s.size to say,
hey, we had three things before.

332
00:16:06,290 --> 00:16:07,400
Now, we have four.

333
00:16:07,400 --> 00:16:10,180
So the next time you push, you're
going to put something into 4.

334
00:16:10,180 --> 00:16:15,560
Or the next time you pop, you're going
to look at 3 instead of 4 or whatever.

335
00:16:15,560 --> 00:16:18,000
And then we'll return true to
say, hey, we succeeded.

336
00:16:18,000 --> 00:16:19,160
This worked.

337
00:16:19,160 --> 00:16:22,060
As a rule of thumb, if a function that's
supposed to return true or

338
00:16:22,060 --> 00:16:26,370
false always returns true, you may
have done something wrong.

339
00:16:26,370 --> 00:16:28,350
So does this work?

340
00:16:28,350 --> 00:16:32,400
>> Well, it works fine for 1, and
2, and 3, and 4, and five.

341
00:16:32,400 --> 00:16:34,640
But let's say I reach my capacity.

342
00:16:34,640 --> 00:16:38,750
I've then run into a problem, because
if size is the same as capacity, I'm

343
00:16:38,750 --> 00:16:43,340
now trying to put something into an
array where I don't have space.

344
00:16:43,340 --> 00:16:46,980
So a short check to fix this.

345
00:16:46,980 --> 00:16:51,630
If s.size == CAPACITY, return false.

346
00:16:51,630 --> 00:16:54,130
Otherwise, go and do what we did.

347
00:16:54,130 --> 00:16:55,660
So what else could we ask
about for stacks?

348
00:16:55,660 --> 00:16:56,460
What else should you study?

349
00:16:56,460 --> 00:16:57,690
What else should you practice?

350
00:16:57,690 --> 00:17:01,030
>> Well, implementing pop.

351
00:17:01,030 --> 00:17:02,370
We already did push.

352
00:17:02,370 --> 00:17:04,280
I'll fix that.

353
00:17:04,280 --> 00:17:08,180
A non-array implementation, where
you use a linked list, perhaps.

354
00:17:08,180 --> 00:17:09,390
A non-int implementation.

355
00:17:09,390 --> 00:17:10,940
We did ints here.

356
00:17:10,940 --> 00:17:11,880
But it could have been floats.

357
00:17:11,880 --> 00:17:13,010
I could have been strings.

358
00:17:13,010 --> 00:17:14,290
It could have been char stars.

359
00:17:14,290 --> 00:17:17,960
Look at past quizzes for the kinds of
questions we've asked about stacks.

360
00:17:17,960 --> 00:17:20,000
>> I'll say that we covered stacks around
the same as we've covered them in

361
00:17:20,000 --> 00:17:20,540
years past.

362
00:17:20,540 --> 00:17:24,400
So the quiz questions should
be a good indication.

363
00:17:24,400 --> 00:17:27,010
Moving forward even faster, queues.

364
00:17:27,010 --> 00:17:28,200
They're like stacks.

365
00:17:28,200 --> 00:17:29,960
But they're first in, first out.

366
00:17:29,960 --> 00:17:33,530
If you're British, the word queue
probably made a lot of sense to you.

367
00:17:33,530 --> 00:17:36,390
Otherwise, you may have
heard of it as a line.

368
00:17:36,390 --> 00:17:38,120
They work like the line
at the Apple store.

369
00:17:38,120 --> 00:17:40,740
The first person to show up at 3:00
in the morning is the first

370
00:17:40,740 --> 00:17:42,880
person to buy his iPad.

371
00:17:42,880 --> 00:17:44,260
>> So we have two operations--

372
00:17:44,260 --> 00:17:45,720
enqueue and dequeue.

373
00:17:45,720 --> 00:17:47,560
Enqueue puts someone in the line.

374
00:17:47,560 --> 00:17:50,070
Dequeue pulls the first
person off the line.

375
00:17:50,070 --> 00:17:52,640
Again, we can implement
this with an array.

376
00:17:52,640 --> 00:17:54,880
So what is the struct we
showed in lecture?

377
00:17:54,880 --> 00:17:57,660
It was this one.

378
00:17:57,660 --> 00:17:59,300
Again, numbers.

379
00:17:59,300 --> 00:18:02,020
Again, size and this new thing front.

380
00:18:02,020 --> 00:18:04,880
Why is there something called front?

381
00:18:04,880 --> 00:18:07,050
It's the index of the next
element to dequeue.

382
00:18:07,050 --> 00:18:12,210
It's just internally keeping track of
the first guy to show up, so that we

383
00:18:12,210 --> 00:18:15,005
can pull it out when we need to.

384
00:18:15,005 --> 00:18:19,322
>> Definitely look at lecture notes and try
to implement enqueue and dequeue

385
00:18:19,322 --> 00:18:21,700
when studying for the quiz.

386
00:18:21,700 --> 00:18:23,190
Important things to think about.

387
00:18:23,190 --> 00:18:27,260
Wrapping around if the front plus the
size ends up bigger than capacity.

388
00:18:27,260 --> 00:18:32,670
Again, if your structure is full,
you're going to have a problem.

389
00:18:32,670 --> 00:18:34,780
Hash tables you've seen before.

390
00:18:34,780 --> 00:18:39,820
Most of you probably implemented
these on pset6.

391
00:18:39,820 --> 00:18:44,210
It's a structure that aims for O(1)
constant time insertion and O(1)

392
00:18:44,210 --> 00:18:46,430
constant time lookup.

393
00:18:46,430 --> 00:18:49,760
>> In CS50, we implemented this as
an array of linked lists.

394
00:18:49,760 --> 00:18:53,690
The key component to a hash table
is the hash function.

395
00:18:53,690 --> 00:18:58,350
So it converts your input, let's say,
a dictionary word, into a number,

396
00:18:58,350 --> 00:18:59,560
which is going to be our index.

397
00:18:59,560 --> 00:19:01,410
And we'll use that index
into our array.

398
00:19:01,410 --> 00:19:05,374
So here's a cute little picture
from study.50.net.

399
00:19:05,374 --> 00:19:08,060
We throw all the words into
our hash function.

400
00:19:08,060 --> 00:19:10,950
And the hash function tells us
where to put these words.

401
00:19:10,950 --> 00:19:15,650
>> This is all great in the land where
there's only one word for every slot.

402
00:19:15,650 --> 00:19:20,480
But as you remember from pset6, there
are more words than slots.

403
00:19:20,480 --> 00:19:23,080
So what happens when you
get a collision?

404
00:19:23,080 --> 00:19:26,730
Instead of storing one value in,
let's say, hash table 3, you

405
00:19:26,730 --> 00:19:27,990
store a linked list.

406
00:19:27,990 --> 00:19:32,900
And so instead of having cantaloupe
here, you would have a linked list,

407
00:19:32,900 --> 00:19:34,190
where the first node is cantaloupe.

408
00:19:34,190 --> 00:19:35,260
And the next node is cat.

409
00:19:35,260 --> 00:19:38,970
And the third node is collision, let's
say, because all these words start

410
00:19:38,970 --> 00:19:41,110
with C.

411
00:19:41,110 --> 00:19:42,700
>> So most of you did this for pset6.

412
00:19:42,700 --> 00:19:45,685
If you did not do a hash table on pset6
and you attempted something like

413
00:19:45,685 --> 00:19:47,720
a trie, definitely review hash tables.

414
00:19:47,720 --> 00:19:50,650
If you did do it on pset6, definitely
review hash tables.

415
00:19:50,650 --> 00:19:53,610
And if you did it on pset6 and it didn't
work out quite right and you

416
00:19:53,610 --> 00:19:56,150
had a lot of trouble with it, definitely
review hash tables.

417
00:19:56,150 --> 00:20:01,610
So the lesson really is definitely
review hash tables.

418
00:20:01,610 --> 00:20:07,130
>> The vast minority of you tried
out tries on pset6.

419
00:20:07,130 --> 00:20:08,570
High-level picture.

420
00:20:08,570 --> 00:20:15,150
It's something like this, where each
node has a set of children, where each

421
00:20:15,150 --> 00:20:17,100
child corresponds to a letter.

422
00:20:17,100 --> 00:20:20,520
And every node also says,
hey, I am a word.

423
00:20:20,520 --> 00:20:25,933
So in this instance, the word Maxwell,
if you follow the M to the A to the

424
00:20:25,933 --> 00:20:28,530
X-W-E-L-L and then follow it one more.

425
00:20:28,530 --> 00:20:32,800
And you get this symbol, delta, which
we signify to mean this is a word.

426
00:20:32,800 --> 00:20:34,780
So Maxwell is a word.

427
00:20:34,780 --> 00:20:38,430
These deltas are throughout signifying
which things are words and which

428
00:20:38,430 --> 00:20:40,360
things are not.

429
00:20:40,360 --> 00:20:46,400
>> So in pset6, the data we stored
alongside any of our nodes was "I am a

430
00:20:46,400 --> 00:20:52,630
word." And the cool thing about tries
is they demonstrate insertion and

431
00:20:52,630 --> 00:20:55,080
lookup in O(length of a word).

432
00:20:55,080 --> 00:20:59,450
So just to get through Maxwell, it's
M-A-X-W-E-L-L. So seven or eight--

433
00:20:59,450 --> 00:21:00,360
I can't count--

434
00:21:00,360 --> 00:21:03,920
steps to get to the end
and check things out.

435
00:21:03,920 --> 00:21:06,800
So quick implementation here.

436
00:21:06,800 --> 00:21:10,230
Rob went through a linked
list in his postmortem.

437
00:21:10,230 --> 00:21:11,600
So check that out.

438
00:21:11,600 --> 00:21:11,720
Sorry.

439
00:21:11,720 --> 00:21:13,240
Went through a trie in his postmortem.

440
00:21:13,240 --> 00:21:14,260
So check that out.

441
00:21:14,260 --> 00:21:24,410
>> But you basically have each node has 27
pointers to the next nodes and one

442
00:21:24,410 --> 00:21:27,050
Boolean for am I a word.

443
00:21:27,050 --> 00:21:31,530
Check out Rob's postmortem for how
this actually is implemented.

444
00:21:31,530 --> 00:21:34,750
Our final structure, our trees
and binary search trees.

445
00:21:34,750 --> 00:21:41,530
So looking at these, these were covered
most recently Week 8, Monday.

446
00:21:41,530 --> 00:21:46,960
A tree is similar to a trie, except you
don't necessarily have 27 nodes at

447
00:21:46,960 --> 00:21:47,500
each point.

448
00:21:47,500 --> 00:21:52,820
And you don't have this data at each
step that signifies whether the--

449
00:21:52,820 --> 00:21:54,030
the path doesn't matter.

450
00:21:54,030 --> 00:22:00,870
Whereas a trie, the path from top to
bottom, Maxwell, was important to us.

451
00:22:00,870 --> 00:22:05,270
>> But each node has multiple
children, perhaps.

452
00:22:05,270 --> 00:22:07,290
We have some more vocabulary.

453
00:22:07,290 --> 00:22:09,530
The root of the tree
is at the very top.

454
00:22:09,530 --> 00:22:12,520
And we say that the very bottommost
nodes that have no

455
00:22:12,520 --> 00:22:14,530
children are leaves.

456
00:22:14,530 --> 00:22:18,040
So like a trie, a tree is
a structure of nodes.

457
00:22:18,040 --> 00:22:21,490
A common type of tree that we're going
to talk about is a binary tree, where

458
00:22:21,490 --> 00:22:26,040
each node has no children or
one child or two children.

459
00:22:26,040 --> 00:22:28,890
So this picture here is not
a binary tree, because

460
00:22:28,890 --> 00:22:32,890
node 3 has three children.

461
00:22:32,890 --> 00:22:38,140
>> But if we were to ignore those, the rest
of it is a binary tree because it

462
00:22:38,140 --> 00:22:43,200
demonstrates the property that each node
has zero, one, or two children.

463
00:22:43,200 --> 00:22:46,400
So how could we express this in code?

464
00:22:46,400 --> 00:22:51,460
We could have a node where each node has
an integer inside of it, as well

465
00:22:51,460 --> 00:22:55,590
as a pointer to the tree on the left
and a pointer to the tree on the

466
00:22:55,590 --> 00:22:59,510
right, so the two children.

467
00:22:59,510 --> 00:23:00,880
How is this useful?

468
00:23:00,880 --> 00:23:05,740
Well, if we make rules about where we
put nodes, we can make search faster.

469
00:23:05,740 --> 00:23:10,630
>> So there's a concept of a binary search
tree, where all nodes on the

470
00:23:10,630 --> 00:23:14,420
left subtree have a smaller value
than the node we're looking at.

471
00:23:14,420 --> 00:23:17,880
And all nodes on the right subtree
have a greater value

472
00:23:17,880 --> 00:23:18,660
than the root node.

473
00:23:18,660 --> 00:23:20,670
Now, that looks like a lot of words.

474
00:23:20,670 --> 00:23:23,770
I'm going to put it inside of double
quotes and show you a picture.

475
00:23:23,770 --> 00:23:27,010
So here is an example of
a binary search tree.

476
00:23:27,010 --> 00:23:28,770
See that we start with 10.

477
00:23:28,770 --> 00:23:31,780
Everything to the left of
10 is smaller than it.

478
00:23:31,780 --> 00:23:33,130
And everything to the right
is bigger than it.

479
00:23:33,130 --> 00:23:37,620
>> But more so than that, each node in
the tree expresses this property.

480
00:23:37,620 --> 00:23:42,110
So the node 7 has a 3 to the
left and a 9 to the right.

481
00:23:42,110 --> 00:23:44,440
So all of those are smaller than 10.

482
00:23:44,440 --> 00:23:50,470
But looking at just those, the 7 has
3 to its left and 9 to its right.

483
00:23:50,470 --> 00:23:56,100
And similarly on the right, 15 has 14
to its left and 50 to its right.

484
00:23:56,100 --> 00:24:03,770
So the three nodes over there, 15, 14,
and 50, are also a valid binary tree

485
00:24:03,770 --> 00:24:05,480
or a valid binary search tree.

486
00:24:05,480 --> 00:24:07,250
And they're all bigger than 10.

487
00:24:07,250 --> 00:24:08,960
So they are allowed to be
on the right there.

488
00:24:08,960 --> 00:24:09,940
Is there a question?

489
00:24:09,940 --> 00:24:12,580
>> AUDIENCE: How do you deal when
you have two sevens?

490
00:24:12,580 --> 00:24:12,850
>> R.J. AQUINO: Yeah.

491
00:24:12,850 --> 00:24:16,550
How do you deal with two values
that are the same?

492
00:24:16,550 --> 00:24:21,465
Some binary search trees say that you
ignore duplicates, because the goal is

493
00:24:21,465 --> 00:24:24,280
just to say, I've seen
these things so far.

494
00:24:24,280 --> 00:24:28,120
Some binary search trees you could say
have a count inside of the node.

495
00:24:28,120 --> 00:24:32,000
Others might say that everything to
the left is less than or equal to.

496
00:24:32,000 --> 00:24:33,470
And everything to the right
is greater than.

497
00:24:33,470 --> 00:24:36,520
>> It just depends on what the
problem is you're solving.

498
00:24:36,520 --> 00:24:40,840
So in a dictionary, for instance, you
wouldn't care about duplicates.

499
00:24:40,840 --> 00:24:41,490
You would throw them out.

500
00:24:41,490 --> 00:24:44,719
But some other problem you might care.

501
00:24:44,719 --> 00:24:49,242
>> AUDIENCE: Is it possible to have
a 1 to the left of 15, which

502
00:24:49,242 --> 00:24:50,590
is less than 10?

503
00:24:50,590 --> 00:24:51,885
>> R.J. AQUINO: No.

504
00:24:51,885 --> 00:24:56,570
If the 14 here were a 1, this would
not be a valid binary search tree,

505
00:24:56,570 --> 00:25:00,840
because everything to the right of
10 has to be bigger than it.

506
00:25:00,840 --> 00:25:02,300
And we'll see why.

507
00:25:02,300 --> 00:25:07,960
If in the land of search my goal is
to find 14, I start at the root.

508
00:25:07,960 --> 00:25:08,500
So I look.

509
00:25:08,500 --> 00:25:08,710
OK.

510
00:25:08,710 --> 00:25:10,670
We're going to start at the root.

511
00:25:10,670 --> 00:25:12,500
Look at 10.

512
00:25:12,500 --> 00:25:16,050
>> Well, 14, our target,
is bigger than 10.

513
00:25:16,050 --> 00:25:17,370
So it must be on the right.

514
00:25:17,370 --> 00:25:21,780
This is very similar to the whole phone
book thing we did, the binary

515
00:25:21,780 --> 00:25:23,720
search there.

516
00:25:23,720 --> 00:25:26,430
But instead of binary searching
in an array, we're binary

517
00:25:26,430 --> 00:25:28,490
searching in this tree.

518
00:25:28,490 --> 00:25:31,260
So we're still looking for 14.

519
00:25:31,260 --> 00:25:32,480
Well, 14 is smaller than 15.

520
00:25:32,480 --> 00:25:36,430
So if it's in our tree, it must
be in this area here.

521
00:25:36,430 --> 00:25:39,680
It must be to the right of
10 and to the left of 15.

522
00:25:39,680 --> 00:25:42,250
>> And so we check this node.

523
00:25:42,250 --> 00:25:45,790
And yay, we've found 14.

524
00:25:45,790 --> 00:25:46,760
I'm not going to walk through it.

525
00:25:46,760 --> 00:25:48,090
But here's the code.

526
00:25:48,090 --> 00:25:49,690
It's actually relatively
straightforward,

527
00:25:49,690 --> 00:25:52,630
because this is recursive.

528
00:25:52,630 --> 00:25:55,420
What could we ask you to do on a quiz?

529
00:25:55,420 --> 00:25:57,000
We could ask you to write this code.

530
00:25:57,000 --> 00:25:59,170
We could ask you to look at this code
and modify this code and explain what

531
00:25:59,170 --> 00:26:00,210
it's doing.

532
00:26:00,210 --> 00:26:00,390
Yeah.

533
00:26:00,390 --> 00:26:00,770
Question?

534
00:26:00,770 --> 00:26:04,240
>> AUDIENCE: Are these slides going to be
made available as they were last time?

535
00:26:04,240 --> 00:26:04,740
>> R.J. AQUINO: Yes.

536
00:26:04,740 --> 00:26:06,460
So these slides will definitely
be posted.

537
00:26:06,460 --> 00:26:08,640
>> AUDIENCE: They're actually posted
right now on the website.

538
00:26:08,640 --> 00:26:10,020
David just did that.

539
00:26:10,020 --> 00:26:12,720
>> R.J. AQUINO: The slides are
right now on the website.

540
00:26:12,720 --> 00:26:16,420
I'll probably patch up a couple of
the typos I noted and fix them.

541
00:26:16,420 --> 00:26:19,940
But there's a current
version on the site.

542
00:26:19,940 --> 00:26:21,820
Others things we could ask you to do--

543
00:26:21,820 --> 00:26:23,790
write insert.

544
00:26:23,790 --> 00:26:27,490
Write an iterative version of the
recursive function we just showed you

545
00:26:27,490 --> 00:26:32,520
or talk about these things, like in
paragraphs, in words, in sentences.

546
00:26:32,520 --> 00:26:35,760
Comparing the run times and explaining
what you would want to use a binary

547
00:26:35,760 --> 00:26:39,200
search tree for instead of a
hash table, for instance.

548
00:26:39,200 --> 00:26:43,580
>> So understand these structures
at a pretty deep level.

549
00:26:43,580 --> 00:26:47,440
Understand how to write them, how to
use them, how to talk about them.

550
00:26:47,440 --> 00:26:50,270
And you'll be all set.

551
00:26:50,270 --> 00:26:50,630
Question?

552
00:26:50,630 --> 00:26:55,070
>> AUDIENCE: When you're writing the
binary search tree, how do you

553
00:26:55,070 --> 00:27:01,460
determine what value to
make it as the root?

554
00:27:01,460 --> 00:27:06,120
>> R.J. AQUINO: So the question was, what
value do you make as the root?

555
00:27:06,120 --> 00:27:08,760
Depending on your code, you
may have a global root.

556
00:27:08,760 --> 00:27:14,290
So you may have likely had in
pset6 a global hash table.

557
00:27:14,290 --> 00:27:18,640
Or you might pass the root
in as an argument.

558
00:27:18,640 --> 00:27:23,810
So this search function here
takes an argument a node*.

559
00:27:23,810 --> 00:27:27,420
And so whatever node you happen to be
looking at is the one you're treating

560
00:27:27,420 --> 00:27:31,510
as your root when you pass it in.

561
00:27:31,510 --> 00:27:32,320
And I'm all set.

562
00:27:32,320 --> 00:27:33,480
Those are my slides.

563
00:27:33,480 --> 00:27:35,940
The next person can come swap
in a laptop and mic.

564
00:27:35,940 --> 00:27:47,390

565
00:27:47,390 --> 00:27:49,760
>> ROB BOWDEN: I think I might have
interpreted that question differently.

566
00:27:49,760 --> 00:27:53,826
But I interpreted it as, if you have
the numbers 1, 2, and 3, how do we

567
00:27:53,826 --> 00:27:56,720
know to make 2 the root
as opposed to 1 or 3?

568
00:27:56,720 --> 00:27:59,480
If we make 2 the root, then it's nicely
1 and 3 to the left and right.

569
00:27:59,480 --> 00:28:04,610
But if 1 is the root, then it's 1 to the
top, 2 the right, 3 to the right.

570
00:28:04,610 --> 00:28:10,880
So by default, you don't know
what to make the root.

571
00:28:10,880 --> 00:28:15,400
And for any algorithm we're expecting to
give you, just the first thing you

572
00:28:15,400 --> 00:28:16,680
insert would be the root.

573
00:28:16,680 --> 00:28:19,890
Or we'd give you a binary tree that
already exists that has a root.

574
00:28:19,890 --> 00:28:24,760
But other algorithms exists such that
the root will update, so that if you

575
00:28:24,760 --> 00:28:28,370
end up in the situation where it's 1, 2,
3, it would automatically update to

576
00:28:28,370 --> 00:28:30,900
make 2 the new root, so that
it's still nicely balanced.

577
00:28:30,900 --> 00:28:33,750

578
00:28:33,750 --> 00:28:34,833
>> ANGELA LI: Cool.

579
00:28:34,833 --> 00:28:36,170
Hey, guys.

580
00:28:36,170 --> 00:28:37,810
I'm Angela.

581
00:28:37,810 --> 00:28:42,490
And I'm going to finish off our C
and then go into some of our web

582
00:28:42,490 --> 00:28:43,120
technologies--

583
00:28:43,120 --> 00:28:46,570
HTTP, HTML, and CSS.

584
00:28:46,570 --> 00:28:49,610
So the first thing is buffer
overflow attacks.

585
00:28:49,610 --> 00:28:53,070
So let's take a look at this code.

586
00:28:53,070 --> 00:28:54,260
It's pretty simple.

587
00:28:54,260 --> 00:28:55,460
There's a function foo.

588
00:28:55,460 --> 00:28:56,990
And it doesn't return anything.

589
00:28:56,990 --> 00:29:00,950
But it takes in a pointer
to a string called bar.

590
00:29:00,950 --> 00:29:04,920
>> And it's going to declare this
buffer, which is a character

591
00:29:04,920 --> 00:29:07,690
array that has 12 slots.

592
00:29:07,690 --> 00:29:11,730
And it uses memcpy, which is just a
function that copies from one address

593
00:29:11,730 --> 00:29:12,910
into another.

594
00:29:12,910 --> 00:29:19,400
So this is trying to copy into
our buffer from whatever

595
00:29:19,400 --> 00:29:21,140
bar is pointing to.

596
00:29:21,140 --> 00:29:24,640
So any idea what's wrong
with this code?

597
00:29:24,640 --> 00:29:27,568

598
00:29:27,568 --> 00:29:30,830
>> AUDIENCE: If bar is longer than
C, they will overwrite.

599
00:29:30,830 --> 00:29:31,520
>> ANGELA LI: Yeah, exactly.

600
00:29:31,520 --> 00:29:34,200
We have no guarantee that bar
is going to be less than 12.

601
00:29:34,200 --> 00:29:36,080
We just made some arbitrary number 12.

602
00:29:36,080 --> 00:29:38,380
And we were like, let's hope that
our user input is less than

603
00:29:38,380 --> 00:29:40,440
12 characters long.

604
00:29:40,440 --> 00:29:46,320
So in an ideal world, if our input is
always as expected, then we'll get

605
00:29:46,320 --> 00:29:47,550
something like, hello.

606
00:29:47,550 --> 00:29:48,920
That's less than 12 characters.

607
00:29:48,920 --> 00:29:51,870
It gets read into char c.

608
00:29:51,870 --> 00:29:53,280
And then we do something with it.

609
00:29:53,280 --> 00:29:54,800
It doesn't really matter.

610
00:29:54,800 --> 00:29:59,740
>> But a malicious person could do
something more like this, where they

611
00:29:59,740 --> 00:30:04,760
give us whatever bar is pointing to,
it's going to point to this huge array

612
00:30:04,760 --> 00:30:06,280
of just A's.

613
00:30:06,280 --> 00:30:10,680
And this is way longer than 12.

614
00:30:10,680 --> 00:30:13,830
So it's going to go all the way
down here to where the return

615
00:30:13,830 --> 00:30:15,420
address used to be.

616
00:30:15,420 --> 00:30:17,860
So let's say this function
is called foo.

617
00:30:17,860 --> 00:30:20,970
Maybe foo was called by some other
function, which was called by main.

618
00:30:20,970 --> 00:30:24,890
So when foo is running, it needs
to know where to return to.

619
00:30:24,890 --> 00:30:29,130
>> If foo was called by some function named
baz, it has to know that it's

620
00:30:29,130 --> 00:30:30,250
got to go back to baz.

621
00:30:30,250 --> 00:30:34,040
And that's what this return address
down here is telling us.

622
00:30:34,040 --> 00:30:38,340
But if we overwrite it with some other
address, in this case, this is a

623
00:30:38,340 --> 00:30:42,650
representation of the address at the
very beginning of this buffer, then

624
00:30:42,650 --> 00:30:45,240
what's actually going to happen is that
instead of returning back to baz,

625
00:30:45,240 --> 00:30:48,470
which called our function, it's just
going to go to the front of this code.

626
00:30:48,470 --> 00:30:53,930
>> And if this was there because a
malicious hacker dude came and

627
00:30:53,930 --> 00:30:56,820
injected this, then maybe this amount
of A's is not actually A's.

628
00:30:56,820 --> 00:31:02,030
And it's actually just code that breaks
your computer or something.

629
00:31:02,030 --> 00:31:05,930
So to be defensive about this sort of
thing, you have to never assume that

630
00:31:05,930 --> 00:31:09,120
user input is a certain
amount of characters.

631
00:31:09,120 --> 00:31:13,310
For example, when you were doing
speller, you were told that words were

632
00:31:13,310 --> 00:31:15,580
only going to be 40 characters
long maximum.

633
00:31:15,580 --> 00:31:16,570
And that was good.

634
00:31:16,570 --> 00:31:20,150
>> But if not, then you would have to
make sure to only read in 45

635
00:31:20,150 --> 00:31:21,520
characters at a time.

636
00:31:21,520 --> 00:31:24,430
Otherwise, you might overwrite
your buffer.

637
00:31:24,430 --> 00:31:26,140
Any questions on that.

638
00:31:26,140 --> 00:31:26,733
Yeah.

639
00:31:26,733 --> 00:31:28,850
>> AUDIENCE: Could you just talk
a little more about these?

640
00:31:28,850 --> 00:31:29,790
>> ANGELA LI: Sorry.

641
00:31:29,790 --> 00:31:31,040
Yes.

642
00:31:31,040 --> 00:31:32,813

643
00:31:32,813 --> 00:31:35,870
>> AUDIENCE: The mic is just for video.

644
00:31:35,870 --> 00:31:37,640
I will try and project.

645
00:31:37,640 --> 00:31:39,900
Hi, guys.

646
00:31:39,900 --> 00:31:40,920
Sup?

647
00:31:40,920 --> 00:31:45,330
So let's go over a few things in the
CS50 library, which you've been using

648
00:31:45,330 --> 00:31:49,072
all semester, mostly
to get user input.

649
00:31:49,072 --> 00:31:53,140
As you know, you include the CS50
library by just doing CS50.h, which

650
00:31:53,140 --> 00:31:55,660
contains all the prototypes of the
functions that you can use, like

651
00:31:55,660 --> 00:31:58,640
GetString and GetInt, and
GetFloat, et cetera.

652
00:31:58,640 --> 00:32:02,870
And there's this one line in the CS50
library which defines a string, which

653
00:32:02,870 --> 00:32:05,380
you guys all know by now
is just a char*.

654
00:32:05,380 --> 00:32:07,900
>> But let's take a peek at
how GetString works.

655
00:32:07,900 --> 00:32:10,010
This is a very abridged version.

656
00:32:10,010 --> 00:32:15,090
You can pull up the CS50 library files
from, I think, manuals.CS50.net.

657
00:32:15,090 --> 00:32:16,750
And you can read through
the actual function.

658
00:32:16,750 --> 00:32:19,330
But this covers some of
the important parts.

659
00:32:19,330 --> 00:32:23,870
So we've created some buffer
with some capacity.

660
00:32:23,870 --> 00:32:27,570
And what we do is we get one character
at a time from standard n.

661
00:32:27,570 --> 00:32:30,910
That's where the user inputs
text in the console.

662
00:32:30,910 --> 00:32:33,430
>> And so we're going to read in a
character so long as it's not a new

663
00:32:33,430 --> 00:32:37,220
line and it's not end of file, which
is the end of standard input.

664
00:32:37,220 --> 00:32:45,690
And for every character that we read in,
if that character ends up adding

665
00:32:45,690 --> 00:32:50,120
to the number of characters we've read
in and that is more than our capacity,

666
00:32:50,120 --> 00:32:53,490
then what we do is we just resize our
buffer so that it's twice as long.

667
00:32:53,490 --> 00:32:56,950
So again, this protect against buffer
overflow attacks, because you read in

668
00:32:56,950 --> 00:32:58,315
a character at a time.

669
00:32:58,315 --> 00:33:02,290
And if at any point you read in too
many, you just expand your buffer.

670
00:33:02,290 --> 00:33:03,330
You multiply it by two.

671
00:33:03,330 --> 00:33:05,510
And then you have more room.

672
00:33:05,510 --> 00:33:09,120
>> Otherwise, you just add
a character to buffer.

673
00:33:09,120 --> 00:33:15,080
And after you've read in all the
characters, it will shrink the buffer

674
00:33:15,080 --> 00:33:18,510
back down to the normal size, add a
null terminator, and then return.

675
00:33:18,510 --> 00:33:21,880

676
00:33:21,880 --> 00:33:24,960
Now, let's look at GetInt.

677
00:33:24,960 --> 00:33:27,700
Can you guys read this?

678
00:33:27,700 --> 00:33:30,710
I can zoom in a bit.

679
00:33:30,710 --> 00:33:33,410

680
00:33:33,410 --> 00:33:34,660
I don't know how computers work.

681
00:33:34,660 --> 00:33:40,840

682
00:33:40,840 --> 00:33:41,270
Never mind.

683
00:33:41,270 --> 00:33:42,520
I can't zoom in properly.

684
00:33:42,520 --> 00:33:47,500

685
00:33:47,500 --> 00:33:48,770
>> This is really hard.

686
00:33:48,770 --> 00:33:49,180
I'm sorry.

687
00:33:49,180 --> 00:33:51,490
Let's just look at this.

688
00:33:51,490 --> 00:33:57,140
So what GetInt does is it first reads
in a string from GetString, which

689
00:33:57,140 --> 00:33:59,250
we've implemented before.

690
00:33:59,250 --> 00:34:02,945
And the important part to note here
is if this sharing that it ends up

691
00:34:02,945 --> 00:34:06,400
reading is like not actually a string,
then we just return INT_MAX to

692
00:34:06,400 --> 00:34:09,409
represent failure.

693
00:34:09,409 --> 00:34:12,645
Why do we return INT_MAX instead
of negative 1 or 1?

694
00:34:12,645 --> 00:34:13,895
Any ideas?

695
00:34:13,895 --> 00:34:16,853

696
00:34:16,853 --> 00:34:19,350
>> AUDIENCE: [INAUDIBLE]
negative 1 on one.

697
00:34:19,350 --> 00:34:20,070
>> ANGELA LI: Yeah, exactly.

698
00:34:20,070 --> 00:34:24,560
So you're way more likely to just want
to input 1 or negative 1 when prompted

699
00:34:24,560 --> 00:34:27,469
for an nth and whatever nth maxes.

700
00:34:27,469 --> 00:34:27,969
It's huge.

701
00:34:27,969 --> 00:34:29,690
You're probably not going to use it.

702
00:34:29,690 --> 00:34:32,690
So this is like a design decision to
make sure that you don't accidentally

703
00:34:32,690 --> 00:34:38,540
return an error or you don't return
1, which might be parsed

704
00:34:38,540 --> 00:34:41,199
as a correct answer.

705
00:34:41,199 --> 00:34:45,110
So if a line doesn't exist,
we return INT-MAX.

706
00:34:45,110 --> 00:34:48,090
Otherwise, we use sscanf,
which is like scanf.

707
00:34:48,090 --> 00:34:49,449
But it reads from a string.

708
00:34:49,449 --> 00:34:54,310
>> And we have this formatted
string, which is %i %c.

709
00:34:54,310 --> 00:34:57,440
And we try and match that with
whatever the user gave us.

710
00:34:57,440 --> 00:35:01,420
We want the number of matched things
to be 1, which means that we only

711
00:35:01,420 --> 00:35:04,940
really want to match an integer
surrounded by maybe white

712
00:35:04,940 --> 00:35:06,840
space, maybe not.

713
00:35:06,840 --> 00:35:10,710
In this case, if you put in something
like bar, bar doesn't match at all,

714
00:35:10,710 --> 00:35:14,400
because there needs to be
an integer at the start.

715
00:35:14,400 --> 00:35:17,060
So sscan never turned 0.

716
00:35:17,060 --> 00:35:19,640
So you don't return that.

717
00:35:19,640 --> 00:35:23,850
>> Alternatively, if you put in something
like 1, 2, 3, A, B, C, that matches

718
00:35:23,850 --> 00:35:27,180
both the integer but also
the character after it.

719
00:35:27,180 --> 00:35:29,990
So sscanf will return 2, which
is also not ideal.

720
00:35:29,990 --> 00:35:34,620
You don't want 1, 2, 3, A,
B, C to be a valid int.

721
00:35:34,620 --> 00:35:36,990
So that also doesn't work.

722
00:35:36,990 --> 00:35:38,530
But say you put in something like 50.

723
00:35:38,530 --> 00:35:42,460
That will match the %i, which means
it will get read into n.

724
00:35:42,460 --> 00:35:44,790
And now, n will contain the number 50.

725
00:35:44,790 --> 00:35:46,110
And then you can return it.

726
00:35:46,110 --> 00:35:49,270
>> Otherwise, you hit Retry.

727
00:35:49,270 --> 00:35:55,790
And then it just goes over again until
you get a proper input from the user.

728
00:35:55,790 --> 00:35:56,891
Any questions on that?

729
00:35:56,891 --> 00:36:02,182
>> AUDIENCE: So if you were to print out
the value of the GetInt on [INAUDIBLE]

730
00:36:02,182 --> 00:36:06,250
would it be just the integer and max?

731
00:36:06,250 --> 00:36:07,440
>> ANGELA LI: Yeah.

732
00:36:07,440 --> 00:36:11,780
So if you use GetInt, you should assume
that you don't want nth max to

733
00:36:11,780 --> 00:36:15,328
be a valid input, because you're going
to assume that that was bad.

734
00:36:15,328 --> 00:36:27,740
>> AUDIENCE: If we didn't have char c and
someone put in 1, 2, 3, Sam, would it

735
00:36:27,740 --> 00:36:29,430
still work for 1, 2, 3?

736
00:36:29,430 --> 00:36:29,750
>> ANGELA LI: I think it would work.

737
00:36:29,750 --> 00:36:33,340
But you don't want 123Sam to
be a valid input by a user.

738
00:36:33,340 --> 00:36:34,670
That's not really an int.

739
00:36:34,670 --> 00:36:36,840
So it doesn't seem fair
to parse it as an int.

740
00:36:36,840 --> 00:36:40,910

741
00:36:40,910 --> 00:36:42,160
OK.

742
00:36:42,160 --> 00:36:45,800
In that case, let's move
on to the internet.

743
00:36:45,800 --> 00:36:49,120
So HTTP is not a language.

744
00:36:49,120 --> 00:36:56,060
HTTP is just the set of standards for
how you send things from clients,

745
00:36:56,060 --> 00:36:57,280
that's you, to servers.

746
00:36:57,280 --> 00:36:59,730
That's other people on the web.

747
00:36:59,730 --> 00:37:02,900
>> So HTTP stands for Hypertext
Transfer Protocol.

748
00:37:02,900 --> 00:37:04,610
It's the heart and soul
of the whole web.

749
00:37:04,610 --> 00:37:07,050
The hypertext part just
refers to HTML.

750
00:37:07,050 --> 00:37:10,690
The transfer is clients like
you will send requests to

751
00:37:10,690 --> 00:37:13,060
servers, which give responses.

752
00:37:13,060 --> 00:37:16,380
And the protocol is just, how do
you expect a server to behave?

753
00:37:16,380 --> 00:37:19,960
And how are you supposed to behave
such that you can streamline this

754
00:37:19,960 --> 00:37:21,920
communication process?

755
00:37:21,920 --> 00:37:26,650
>> So HTTP requests look a lot like this.

756
00:37:26,650 --> 00:37:28,070
GET is the type of request.

757
00:37:28,070 --> 00:37:31,220
You guys have seen GET requests
and POST requests.

758
00:37:31,220 --> 00:37:36,690
That second thing there, /me, that's
just the URI or the URL of where you

759
00:37:36,690 --> 00:37:38,140
want to go within the host.

760
00:37:38,140 --> 00:37:44,140
So this request is asking for the
page, like www.facebook.com/me.

761
00:37:44,140 --> 00:37:45,300
And it's a GET request.

762
00:37:45,300 --> 00:37:51,020
And then this HTTP/1.1, that's just
the version of HTTP you're using.

763
00:37:51,020 --> 00:37:55,020
It's almost always 1.1.

764
00:37:55,020 --> 00:37:56,880
>> And then there's a bunch
of other stuff too.

765
00:37:56,880 --> 00:38:02,510
You can actually see these if you
open up your console when you're

766
00:38:02,510 --> 00:38:03,770
browsing the web.

767
00:38:03,770 --> 00:38:07,290
Responses look something
more like this.

768
00:38:07,290 --> 00:38:09,620
The top part is, again, the
type of HTTP you're using

769
00:38:09,620 --> 00:38:12,310
followed by a status code.

770
00:38:12,310 --> 00:38:14,700
So 200 OK is everything worked out.

771
00:38:14,700 --> 00:38:16,200
Here is your content.

772
00:38:16,200 --> 00:38:17,390
Your content is going to follow.

773
00:38:17,390 --> 00:38:21,730
And then it will tell you what kind
of content and other stuff too.

774
00:38:21,730 --> 00:38:24,620
>> The status codes, there are a few
important ones that you should know.

775
00:38:24,620 --> 00:38:26,460
200 OK is like everything's golden.

776
00:38:26,460 --> 00:38:28,490
Everything works.

777
00:38:28,490 --> 00:38:29,710
403 Forbidden.

778
00:38:29,710 --> 00:38:32,910
This you've probably seen if you forgot
to chmod something properly.

779
00:38:32,910 --> 00:38:34,510
It means that you don't have
the right permissions to

780
00:38:34,510 --> 00:38:36,210
access that on the server.

781
00:38:36,210 --> 00:38:38,110
It's like, no, you can't see it.

782
00:38:38,110 --> 00:38:39,780
404 means that thing doesn't exist.

783
00:38:39,780 --> 00:38:40,400
Not found.

784
00:38:40,400 --> 00:38:41,640
You've probably seen that a lot.

785
00:38:41,640 --> 00:38:45,510
>> 500 Internal Server Error is usually
like something went wrong on the side

786
00:38:45,510 --> 00:38:46,460
of the server.

787
00:38:46,460 --> 00:38:50,830
So when you were implementing pset7,
if you had PHP errors, you could

788
00:38:50,830 --> 00:38:53,890
actually go to the page and see a
whole bunch of PHP error stuff.

789
00:38:53,890 --> 00:38:56,900
But that doesn't normally happen,
because websites don't really want to

790
00:38:56,900 --> 00:38:58,830
tell you why their site is broken.

791
00:38:58,830 --> 00:39:03,370
They'll probably just return a
500 Internal Server Error.

792
00:39:03,370 --> 00:39:06,120
>> And then there's 418 I'm a teapot.

793
00:39:06,120 --> 00:39:07,910
There's a whole story about
why that's a thing.

794
00:39:07,910 --> 00:39:09,860
But you can read about that
on your own time.

795
00:39:09,860 --> 00:39:11,450
There's a whole bunch of
other status codes too.

796
00:39:11,450 --> 00:39:12,700
But these are the ones
you should know.

797
00:39:12,700 --> 00:39:15,660

798
00:39:15,660 --> 00:39:18,610
So let's talk about HTML.

799
00:39:18,610 --> 00:39:22,180
HTML, remember, is not a
programming language.

800
00:39:22,180 --> 00:39:23,510
It's a markup language.

801
00:39:23,510 --> 00:39:25,210
That means it describes content.

802
00:39:25,210 --> 00:39:30,440
It tells you what an HTML document looks
like or not what it looks like

803
00:39:30,440 --> 00:39:32,230
but how it's structured.

804
00:39:32,230 --> 00:39:36,110
>> So it defines a structure and
semantics of web pages.

805
00:39:36,110 --> 00:39:37,830
It's like, this is a paragraph.

806
00:39:37,830 --> 00:39:40,060
This is an ordered list.

807
00:39:40,060 --> 00:39:42,360
This is like a section of my page.

808
00:39:42,360 --> 00:39:43,260
Here's the title.

809
00:39:43,260 --> 00:39:44,310
It does stuff like that.

810
00:39:44,310 --> 00:39:48,770
It doesn't style any of that, because
that's what you do in CSS.

811
00:39:48,770 --> 00:39:50,270
And it looks like a series
of nested tags.

812
00:39:50,270 --> 00:39:54,720
So to use an example of a really basic
HTML page, you have the DOCTYPE

813
00:39:54,720 --> 00:39:56,720
declaration up there.

814
00:39:56,720 --> 00:40:00,940
>> This DOCTYPE declaration is
saying, we're using HTML5.

815
00:40:00,940 --> 00:40:03,370
Then you have the big HTML tag.

816
00:40:03,370 --> 00:40:05,230
It contains a head and a body.

817
00:40:05,230 --> 00:40:06,970
Inside the head, you have the title.

818
00:40:06,970 --> 00:40:12,950
That's what goes in the title
bar of your browser.

819
00:40:12,950 --> 00:40:15,810
We have a link tag that links
in an external style sheet.

820
00:40:15,810 --> 00:40:19,880
And then we have a script that pulls
from an external JavaScript as well.

821
00:40:19,880 --> 00:40:23,750
>> And then inside our body is actually
what gets shown on the page.

822
00:40:23,750 --> 00:40:28,210
We've got a paragraph and then an
image inside that paragraph.

823
00:40:28,210 --> 00:40:32,000
This one is a picture of kittens.

824
00:40:32,000 --> 00:40:35,840
Notice that the image
tag closes itself.

825
00:40:35,840 --> 00:40:41,760
So instead of opening with an image
and then doing another /image, you

826
00:40:41,760 --> 00:40:47,500
just have this little slash
here, which closes it.

827
00:40:47,500 --> 00:40:53,670
And the image tag also has this key
value attribute called alt.

828
00:40:53,670 --> 00:40:56,970
That's the alternative text that
happens when you hover over it.

829
00:40:56,970 --> 00:41:03,170
>> Most HTML elements have some key value
things that you can give it, various

830
00:41:03,170 --> 00:41:04,420
customization.

831
00:41:04,420 --> 00:41:06,230

832
00:41:06,230 --> 00:41:08,705
Yeah.

833
00:41:08,705 --> 00:41:09,955
>> AUDIENCE: [INAUDIBLE].

834
00:41:09,955 --> 00:41:17,510

835
00:41:17,510 --> 00:41:19,680
>> ANGELA LI: Well, so it's an
attribute of the tag.

836
00:41:19,680 --> 00:41:25,320
So if you were using jQuery, you could
do select image.getAttribute.

837
00:41:25,320 --> 00:41:27,930
And then you can search for
get the alt attribute.

838
00:41:27,930 --> 00:41:31,040
And it will give you kittens.

839
00:41:31,040 --> 00:41:37,400
If you remember forms in HTML, input
elements will have name attributes.

840
00:41:37,400 --> 00:41:41,870
And that's what PHP uses to send
requests when a form is submitted.

841
00:41:41,870 --> 00:41:44,762

842
00:41:44,762 --> 00:41:50,064
>> AUDIENCE: Did you mention something
about how if you use kittens.jpg or

843
00:41:50,064 --> 00:41:54,410
something that has the missing
file folders or other files?

844
00:41:54,410 --> 00:41:54,750
>> ANGELA LI: Yes.

845
00:41:54,750 --> 00:41:57,010
So this is what's called a relative
path, because I'm not giving

846
00:41:57,010 --> 00:41:58,740
you the full path.

847
00:41:58,740 --> 00:42:05,160
This is like when in C if you do fopen
some file, if you fopen hi.txt, that

848
00:42:05,160 --> 00:42:09,190
hi.txt is expected to be in the same
directory, unless you give it a more

849
00:42:09,190 --> 00:42:11,530
complex path.

850
00:42:11,530 --> 00:42:14,900
>> AUDIENCE: So you could specify
which folder [INAUDIBLE]?

851
00:42:14,900 --> 00:42:17,660
>> ANGELA LI: Yeah.

852
00:42:17,660 --> 00:42:19,370
And you can look up how to do that.

853
00:42:19,370 --> 00:42:23,480
But if I wanted to get kittens.jpg out
of the parent directory, I would do

854
00:42:23,480 --> 00:42:24,730
../kittens.jpg.

855
00:42:24,730 --> 00:42:29,680

856
00:42:29,680 --> 00:42:30,930
Yeah.

857
00:42:30,930 --> 00:42:32,960

858
00:42:32,960 --> 00:42:33,760
Sorry.

859
00:42:33,760 --> 00:42:34,045
Yeah.

860
00:42:34,045 --> 00:42:35,700
Oh man, I forgot the question.

861
00:42:35,700 --> 00:42:36,460
What was the question?

862
00:42:36,460 --> 00:42:39,570
Oh, the question was is, kittens.jpg
expected to be in the same directory?

863
00:42:39,570 --> 00:42:40,630
And in this case, it is.

864
00:42:40,630 --> 00:42:44,030
But you can also give it a certain path
such that it doesn't have to be.

865
00:42:44,030 --> 00:42:47,100

866
00:42:47,100 --> 00:42:48,350
Good?

867
00:42:48,350 --> 00:42:50,190

868
00:42:50,190 --> 00:42:51,350
>> CSS.

869
00:42:51,350 --> 00:42:55,420
So CSS, like HTML, is not
a programming language.

870
00:42:55,420 --> 00:42:58,250
CSS is just a series of styling rules.

871
00:42:58,250 --> 00:43:00,130
It stands for Cascading Style Sheets.

872
00:43:00,130 --> 00:43:03,910
And you use it in conjunction
with HTML to style pages.

873
00:43:03,910 --> 00:43:08,140
So there are three ways
you can include it.

874
00:43:08,140 --> 00:43:11,950
One way you can do it is in the head
portion of your HTML, you can just

875
00:43:11,950 --> 00:43:15,410
open a style tag and then stick
some CSS rules in there.

876
00:43:15,410 --> 00:43:16,759
It's pretty OK.

877
00:43:16,759 --> 00:43:17,228
Yeah.

878
00:43:17,228 --> 00:43:21,449
>> AUDIENCE: Could you put those
style tags in between, let's

879
00:43:21,449 --> 00:43:22,860
say, body and /body.

880
00:43:22,860 --> 00:43:27,400
And then you would be styling
only in body.

881
00:43:27,400 --> 00:43:28,840
>> ANGELA LI: You could.

882
00:43:28,840 --> 00:43:29,590
It'll work.

883
00:43:29,590 --> 00:43:33,990
But you shouldn't, because styling is
kind of the metadata that should go in

884
00:43:33,990 --> 00:43:35,890
the head of your document.

885
00:43:35,890 --> 00:43:38,280
Body should really only contain
what's actually going to

886
00:43:38,280 --> 00:43:39,420
show up on your page.

887
00:43:39,420 --> 00:43:42,155
>> AUDIENCE: So you'd put style
in your head to style the

888
00:43:42,155 --> 00:43:43,930
entire web page, right?

889
00:43:43,930 --> 00:43:44,300
>> ANGELA LI: Yeah.

890
00:43:44,300 --> 00:43:50,470
So putting style here, these CSS rules
will apply to the whole page based on

891
00:43:50,470 --> 00:43:52,100
their selectors.

892
00:43:52,100 --> 00:43:57,090
So the better way to do it is instead
of having a style tag in your head,

893
00:43:57,090 --> 00:44:00,430
you have this link to an external style
sheet like I showed you in the

894
00:44:00,430 --> 00:44:01,980
previous example.

895
00:44:01,980 --> 00:44:05,920
What this does is it tries and finds the
file style.css and then pulls it

896
00:44:05,920 --> 00:44:08,470
in and uses that as the
styles for the page.

897
00:44:08,470 --> 00:44:10,500
And your style.css would
just look like this.

898
00:44:10,500 --> 00:44:13,330
It would just be a bunch of CSS.

899
00:44:13,330 --> 00:44:16,210
>> And finally, there's another way you
can include CSS, which you really

900
00:44:16,210 --> 00:44:17,480
shouldn't ever do.

901
00:44:17,480 --> 00:44:18,950
It's call inline styling.

902
00:44:18,950 --> 00:44:22,650
And so any HTML element can also
take a style attribute.

903
00:44:22,650 --> 00:44:26,320
And then in that style attribute,
you can give it CSS rules.

904
00:44:26,320 --> 00:44:29,140
So in this case, whatever div I'm
defining right here, it's going to

905
00:44:29,140 --> 00:44:32,580
have a black background and
a white text color.

906
00:44:32,580 --> 00:44:35,620
But you shouldn't do this, because what
this does is it puts your styling

907
00:44:35,620 --> 00:44:36,850
inside your HTML.

908
00:44:36,850 --> 00:44:40,530
>> And I know we've been talking about HTML
is structure and CSS is style.

909
00:44:40,530 --> 00:44:42,790
If you do this, it mixes
them together.

910
00:44:42,790 --> 00:44:44,550
And it's not very clean.

911
00:44:44,550 --> 00:44:45,800
So don't do that.

912
00:44:45,800 --> 00:44:47,690

913
00:44:47,690 --> 00:44:52,100
Using an example of CSS, up there, we
just select the body of the HTML

914
00:44:52,100 --> 00:44:52,380
documentary.

915
00:44:52,380 --> 00:44:55,110
And we're like, everything's
going to be Comic Sans.

916
00:44:55,110 --> 00:44:57,290
I also don't recommend that.

917
00:44:57,290 --> 00:44:59,940
But you could do that.

918
00:44:59,940 --> 00:45:03,140
>> The second rule right here, it's going
to select the element on the

919
00:45:03,140 --> 00:45:04,880
page with ID main.

920
00:45:04,880 --> 00:45:11,690
So whatever HTML element, I said ID
= main, I'm going to give that a

921
00:45:11,690 --> 00:45:16,020
20-pixel margin and align everything,
all the text, to the center.

922
00:45:16,020 --> 00:45:19,030
The last thing selects by CSS class.

923
00:45:19,030 --> 00:45:24,450
So any element on the page that I gave
a section class, I'm going to make it

924
00:45:24,450 --> 00:45:26,602
a background color of light blue.

925
00:45:26,602 --> 00:45:29,380

926
00:45:29,380 --> 00:45:30,040
Yep.

927
00:45:30,040 --> 00:45:30,700
That's all I got.

928
00:45:30,700 --> 00:45:30,890
Question?

929
00:45:30,890 --> 00:45:34,020
>> AUDIENCE: What does the hashtag
before main do?

930
00:45:34,020 --> 00:45:36,310
>> ANGELA LI: The question is, what does
the hashtag before main do?

931
00:45:36,310 --> 00:45:40,770
In this case, the hash in
CSS means select by ID.

932
00:45:40,770 --> 00:45:47,490
So if I had some HTML element, like
divID = main, this CSS rule selects

933
00:45:47,490 --> 00:45:49,260
the thing with ID main.

934
00:45:49,260 --> 00:45:53,940
And similarly, the period in front of
section is select by CSS class or

935
00:45:53,940 --> 00:45:56,558
select by HTML class.

936
00:45:56,558 --> 00:46:00,940
>> AUDIENCE: Why is there a has before
6 in background color?

937
00:46:00,940 --> 00:46:01,270
>> ANGELA LI: Yeah.

938
00:46:01,270 --> 00:46:03,360
So the question is, why is there
a hash before the 6?

939
00:46:03,360 --> 00:46:05,320
This is different than that hash.

940
00:46:05,320 --> 00:46:09,500
This means that you're giving
a hexadecimal color.

941
00:46:09,500 --> 00:46:14,260
So hex colors, this just
represents a color.

942
00:46:14,260 --> 00:46:17,860
And you remember RGB triples when
you did the forensics pset?

943
00:46:17,860 --> 00:46:18,770
This is similar.

944
00:46:18,770 --> 00:46:21,590
The first two digits represent
how much red is in the color.

945
00:46:21,590 --> 00:46:23,260
The second two represent
how much green.

946
00:46:23,260 --> 00:46:25,450
And the third represents
how much blue.

947
00:46:25,450 --> 00:46:30,060
And the hash is this is going
to represent a color.

948
00:46:30,060 --> 00:46:35,660
>> So anything from 0, 0, 0, 0, 0, 0
up to F, F, F, F, F, F is valid.

949
00:46:35,660 --> 00:46:39,550
It's some valid color that can
be displayed by your browser.

950
00:46:39,550 --> 00:46:39,790
Question?

951
00:46:39,790 --> 00:46:43,590
>> AUDIENCE: What's the difference between
using by ID and by class?

952
00:46:43,590 --> 00:46:46,470
>> ANGELA LI: The question is what's
the difference between

953
00:46:46,470 --> 00:46:48,950
using by ID and class?

954
00:46:48,950 --> 00:46:54,390
You can only have one element in an
HTML document that has a given ID.

955
00:46:54,390 --> 00:46:58,660
So only one thing on my page
is allowed to have ID main.

956
00:46:58,660 --> 00:47:02,580
So you use it for this is the header.

957
00:47:02,580 --> 00:47:03,850
This is the navigation.

958
00:47:03,850 --> 00:47:05,230
This is the footer.

959
00:47:05,230 --> 00:47:09,070
Classes are different, because you can
apply classes to as many HTML elements

960
00:47:09,070 --> 00:47:10,100
as you want.

961
00:47:10,100 --> 00:47:15,860
>> So for example, I did class section,
because there's probably more than one

962
00:47:15,860 --> 00:47:17,540
section on my page.

963
00:47:17,540 --> 00:47:20,200
You're just allowed to have as many
elements on the page with the same

964
00:47:20,200 --> 00:47:23,190
class but only one with a certain ID.

965
00:47:23,190 --> 00:47:25,600
>> AUDIENCE: So the dot represents
the class?

966
00:47:25,600 --> 00:47:26,090
>> ANGELA LI: Yeah.

967
00:47:26,090 --> 00:47:27,380
A dot represents a class.

968
00:47:27,380 --> 00:47:29,990

969
00:47:29,990 --> 00:47:31,540
Cool.

970
00:47:31,540 --> 00:47:32,370
That's all I've got, guys.

971
00:47:32,370 --> 00:47:33,544
Thank you.

972
00:47:33,544 --> 00:48:13,380
>> [APPLAUSE]

973
00:48:13,380 --> 00:48:14,290
>> ZAMYLA CHAN: Hi, everyone.

974
00:48:14,290 --> 00:48:14,880
I'm Zamyla.

975
00:48:14,880 --> 00:48:18,830
I'm going to be covering PHP,
MVC, and SQL today.

976
00:48:18,830 --> 00:48:22,350

977
00:48:22,350 --> 00:48:26,110
A lot of the material that I'll be
covering is going to be pretty much

978
00:48:26,110 --> 00:48:29,100
right out of pset7.

979
00:48:29,100 --> 00:48:29,700
All right.

980
00:48:29,700 --> 00:48:31,180
So what is PHP?

981
00:48:31,180 --> 00:48:35,150
PHP stands for PHP Hypertext
Preprocessor.

982
00:48:35,150 --> 00:48:38,740
So it, in itself, is a recursive
name, which is pretty cool.

983
00:48:38,740 --> 00:48:42,220
PHP is a server-side scripting language,
and it provides the backend

984
00:48:42,220 --> 00:48:44,610
and the logical underpinnings
of our website.

985
00:48:44,610 --> 00:48:48,520
>> So Angela talked a lot about the HTML
and CSS that will make the structure

986
00:48:48,520 --> 00:48:49,530
of the website.

987
00:48:49,530 --> 00:48:53,210
But what if you want to change that
content dynamically or if it varies

988
00:48:53,210 --> 00:48:55,240
based on the user or
certain conditions?

989
00:48:55,240 --> 00:48:57,060
That's where PHP comes in.

990
00:48:57,060 --> 00:49:02,610
Now, typically, PHP might take a few
less lines to implement the same thing

991
00:49:02,610 --> 00:49:07,380
in C. That's because PHP handles memory
management for the programmer,

992
00:49:07,380 --> 00:49:11,170
as opposed to us having to malloc
free, things like that.

993
00:49:11,170 --> 00:49:15,430
>> But since PHP is an interpretive
language, typically, it might execute

994
00:49:15,430 --> 00:49:19,540
a bit more slowly than C, which
is a compiled language.

995
00:49:19,540 --> 00:49:23,150
Because we're moving programming
languages, let's look at how the

996
00:49:23,150 --> 00:49:24,570
syntax will differ.

997
00:49:24,570 --> 00:49:28,770
Let's be very careful not to
get confused with this.

998
00:49:28,770 --> 00:49:33,750
So with PHP syntax, whether you are
embedding your PHP inside of an HTML

999
00:49:33,750 --> 00:49:40,430
file or within a .php file itself, you
need to enclose the code in the open

1000
00:49:40,430 --> 00:49:45,270
PHP and the closed PHP tags like
follows, like on the screen.

1001
00:49:45,270 --> 00:49:46,660
>> Variables in PHP.

1002
00:49:46,660 --> 00:49:51,490
Every single variable will start with
the $ sign followed by the name of

1003
00:49:51,490 --> 00:49:53,150
your variable.

1004
00:49:53,150 --> 00:49:56,530
Now, variables in PHP are loosely typed,
which means that you don't need

1005
00:49:56,530 --> 00:50:00,030
to indicate what the data type
is when you're declaring it.

1006
00:50:00,030 --> 00:50:03,505
However, this doesn't mean that they
don't have any types at all.

1007
00:50:03,505 --> 00:50:09,370
So if I declare a variable and just set
it equal to 1, and then I declare

1008
00:50:09,370 --> 00:50:15,140
another variable, set it equal to "1,"
and then another one 1.0, well,

1009
00:50:15,140 --> 00:50:19,410
depending on the type of equality
operators I use, if I want to compare

1010
00:50:19,410 --> 00:50:21,830
across all types, then
they'll be equal.

1011
00:50:21,830 --> 00:50:25,570
But if I want to make sure that the
types are equal, PHP can still do

1012
00:50:25,570 --> 00:50:28,690
that, even though we don't indicate
what type it is when we

1013
00:50:28,690 --> 00:50:31,170
first make the file.

1014
00:50:31,170 --> 00:50:33,990
>> Now, in PHP, even though we are
switching over from programming

1015
00:50:33,990 --> 00:50:39,360
languages from C, we still have our
trusty if condition, just like this.

1016
00:50:39,360 --> 00:50:43,270
We still have our while loops, just
like this, where you put in your

1017
00:50:43,270 --> 00:50:47,300
condition and then the
body of the loop.

1018
00:50:47,300 --> 00:50:50,360
And then we also have our for loop,
which typically looks like this.

1019
00:50:50,360 --> 00:50:55,330
So if I wanted to iterate over all
nine psets and submit and call a

1020
00:50:55,330 --> 00:50:58,960
function submitPset, then I can do that
here, which you guys have all

1021
00:50:58,960 --> 00:50:59,830
done by this point.

1022
00:50:59,830 --> 00:51:01,080
Congratulations, by the way.

1023
00:51:01,080 --> 00:51:04,560

1024
00:51:04,560 --> 00:51:07,550
>> For the camera, people
said, thank you.

1025
00:51:07,550 --> 00:51:11,220
Now, if you didn't want to just use this
for loop, then PHP actually also

1026
00:51:11,220 --> 00:51:13,580
has things called foreach loops.

1027
00:51:13,580 --> 00:51:22,210
So if I had an array of integers, 0
through 8, stored in the array psets,

1028
00:51:22,210 --> 00:51:27,290
then I could have a foreach loop that
iterates over every number in psets.

1029
00:51:27,290 --> 00:51:30,640
And then I could call the same
function eight times,

1030
00:51:30,640 --> 00:51:31,910
just like I did before.

1031
00:51:31,910 --> 00:51:36,480
So this for each loop is nice, because
you don't have to if you don't know

1032
00:51:36,480 --> 00:51:39,470
the exact length of the array that you
have, then using this foreach loop

1033
00:51:39,470 --> 00:51:42,800
will take care of that for you.

1034
00:51:42,800 --> 00:51:45,410
>> So I made psets as an array.

1035
00:51:45,410 --> 00:51:46,700
Let's look at that.

1036
00:51:46,700 --> 00:51:51,290
Arrays in PHP are typically the same as
the ones that we've had in C, where

1037
00:51:51,290 --> 00:51:52,960
you can declare an array.

1038
00:51:52,960 --> 00:51:59,200
And here, I can declare an empty array
and then build up dynamically by using

1039
00:51:59,200 --> 00:52:00,850
indices as integers.

1040
00:52:00,850 --> 00:52:04,140
So index 0, I'm going to store
an integer named 1.

1041
00:52:04,140 --> 00:52:09,210
At index 1 of my list, I'm going
to store the value 2.

1042
00:52:09,210 --> 00:52:12,670
And at the third index but the
second number, I'm going to

1043
00:52:12,670 --> 00:52:14,870
store the number 12.

1044
00:52:14,870 --> 00:52:17,250
>> Now, this is fine in that
works it works well.

1045
00:52:17,250 --> 00:52:21,310
But say it matters to me
what each index holds.

1046
00:52:21,310 --> 00:52:24,500
For me, index 0 means how
many cats I have.

1047
00:52:24,500 --> 00:52:27,400
And the index 1 means how
many owls I have.

1048
00:52:27,400 --> 00:52:29,450
And the next one means how many dogs.

1049
00:52:29,450 --> 00:52:34,140
Well, then it to specify that, instead
of having to remember 0 relates to

1050
00:52:34,140 --> 00:52:38,090
cats and 1 to owls, I can use
associative arrays, which means that

1051
00:52:38,090 --> 00:52:42,260
instead of integers as my indices,
I can actually use strings.

1052
00:52:42,260 --> 00:52:43,290
>> So this is quite useful.

1053
00:52:43,290 --> 00:52:47,130
And you've basically just replaced
the integers with strings.

1054
00:52:47,130 --> 00:52:50,074
And there you have an
associative array.

1055
00:52:50,074 --> 00:52:51,930
Yeah.

1056
00:52:51,930 --> 00:52:55,800
>> AUDIENCE: Is there a reason why there's
an underscore for the second

1057
00:52:55,800 --> 00:52:58,750
part, because my list has the array.

1058
00:52:58,750 --> 00:53:01,330
>> ZAMYLA CHAN: The question was, is
there a reason why there's an

1059
00:53:01,330 --> 00:53:03,320
underscore between my and list?

1060
00:53:03,320 --> 00:53:03,610
No.

1061
00:53:03,610 --> 00:53:06,878
That's just how I'm naming
my variable.

1062
00:53:06,878 --> 00:53:11,670
>> AUDIENCE: On the first
line, it's one word.

1063
00:53:11,670 --> 00:53:12,560
>> ZAMYLA CHAN: My apologies.

1064
00:53:12,560 --> 00:53:13,410
I'll fix that.

1065
00:53:13,410 --> 00:53:13,620
Yeah.

1066
00:53:13,620 --> 00:53:15,460
They should be the same variable name.

1067
00:53:15,460 --> 00:53:16,710
Good catch.

1068
00:53:16,710 --> 00:53:19,640

1069
00:53:19,640 --> 00:53:19,950
OK.

1070
00:53:19,950 --> 00:53:22,610
So let's move on to string
concatenation.

1071
00:53:22,610 --> 00:53:27,500
If I wanted to take two strings,
then I can concatenate them

1072
00:53:27,500 --> 00:53:28,550
with the dot operator.

1073
00:53:28,550 --> 00:53:32,440
So if I have Milo as a first name
and Banana as a last name, then

1074
00:53:32,440 --> 00:53:35,430
concatenating with the dot operator and
then putting a space in between

1075
00:53:35,430 --> 00:53:39,210
will make a string that contains Milo
Banana, which I can then echo or,

1076
00:53:39,210 --> 00:53:41,280
rather, print out.

1077
00:53:41,280 --> 00:53:44,465
>> Speaking of echo, let's talk
about a few useful--

1078
00:53:44,465 --> 00:53:44,920
oops.

1079
00:53:44,920 --> 00:53:46,030
I'm sorry.

1080
00:53:46,030 --> 00:53:52,920
A few useful PHP functions.

1081
00:53:52,920 --> 00:53:56,240
So we have the--

1082
00:53:56,240 --> 00:53:57,444
technical difficulties.

1083
00:53:57,444 --> 00:53:58,694
One second.

1084
00:53:58,694 --> 00:54:16,960

1085
00:54:16,960 --> 00:54:19,550
I sent it.

1086
00:54:19,550 --> 00:54:22,320
PowerPoint problems.

1087
00:54:22,320 --> 00:54:29,200
And we are back with PHP functions.

1088
00:54:29,200 --> 00:54:32,010

1089
00:54:32,010 --> 00:54:35,150
And we are back with PHP functions.

1090
00:54:35,150 --> 00:54:39,890
>> So we have the require function, where
if you pass in a file, here's is just

1091
00:54:39,890 --> 00:54:43,300
an example of a file that
I might pass in.

1092
00:54:43,300 --> 00:54:47,605
Then that will include the PHP code
from that file that I indicate.

1093
00:54:47,605 --> 00:54:49,940
And it will evaluate that in.

1094
00:54:49,940 --> 00:54:54,450
Then we also have echo, which
is a parallel to printf.

1095
00:54:54,450 --> 00:54:57,710
Exit is a parallel to break,
which exits the block of

1096
00:54:57,710 --> 00:54:58,570
code that you're in.

1097
00:54:58,570 --> 00:55:03,180
And then empty checks whether a given
variable is like null or zero or

1098
00:55:03,180 --> 00:55:08,482
whatever is equated with being empty.

1099
00:55:08,482 --> 00:55:09,438
Yeah.

1100
00:55:09,438 --> 00:55:15,341
>> AUDIENCE: For the string concatenation
dot operator one, in PHP, is that the

1101
00:55:15,341 --> 00:55:20,158
same as in JavaScript where it's using
the dot for concatenation means plus?

1102
00:55:20,158 --> 00:55:27,440
So for full name, you could have dollar
sign first + and then + last?

1103
00:55:27,440 --> 00:55:27,720
>> ZAMYLA CHAN: Yeah.

1104
00:55:27,720 --> 00:55:32,150
So the question was whether in PHP we
can use the same string concatenation

1105
00:55:32,150 --> 00:55:33,890
as in JavaScript with the pluses.

1106
00:55:33,890 --> 00:55:35,410
And Joseph will get into that later.

1107
00:55:35,410 --> 00:55:36,620
I think he has a slide on that.

1108
00:55:36,620 --> 00:55:37,570
Actually, it's different.

1109
00:55:37,570 --> 00:55:41,310
So in JavaScript, you need to use
the plus to concatenate strings.

1110
00:55:41,310 --> 00:55:43,280
And in PHP, you have to
use the dot operator.

1111
00:55:43,280 --> 00:55:44,530
So they're different.

1112
00:55:44,530 --> 00:55:46,680

1113
00:55:46,680 --> 00:55:46,910
>> OK.

1114
00:55:46,910 --> 00:55:49,500
So now that we've covered all
this PHP, where does it

1115
00:55:49,500 --> 00:55:50,490
really come in handy?

1116
00:55:50,490 --> 00:55:54,470
Well, it comes in handy when we
can combine it with our HTML.

1117
00:55:54,470 --> 00:55:59,550
So our PHP will give us the power to
alter a page's HTML content prior to

1118
00:55:59,550 --> 00:56:00,000
its loading.

1119
00:56:00,000 --> 00:56:04,270
So based on different conditions,
usually the specific user that's

1120
00:56:04,270 --> 00:56:07,520
logged in, we can display
different information.

1121
00:56:07,520 --> 00:56:08,800
Linda, did you have a question?

1122
00:56:08,800 --> 00:56:15,510
>> AUDIENCE: Can you concatenate
an integer also?

1123
00:56:15,510 --> 00:56:16,760
>> ZAMYLA CHAN: Yes, you can.

1124
00:56:16,760 --> 00:56:19,530

1125
00:56:19,530 --> 00:56:23,270
So the question was if you can
concatenate integers or other

1126
00:56:23,270 --> 00:56:28,920
variable.s now, we move on to MVC, which
is a paradigm that we used in

1127
00:56:28,920 --> 00:56:33,380
pset7 and a lot of web designers use for
organizing the code in the files

1128
00:56:33,380 --> 00:56:34,490
in their website.

1129
00:56:34,490 --> 00:56:35,870
M stands for Model.

1130
00:56:35,870 --> 00:56:41,450
And basically, model files will deal
with interactions with the database.

1131
00:56:41,450 --> 00:56:44,640
View files, they relate to the
aesthetics of the website.

1132
00:56:44,640 --> 00:56:47,550
And the Controller handles
user requests, parses

1133
00:56:47,550 --> 00:56:49,230
data, does other logic.

1134
00:56:49,230 --> 00:56:52,520
>> In pset7, we combined the model
and the controller.

1135
00:56:52,520 --> 00:56:55,880
And we just called them controllers and
put them in the public directory.

1136
00:56:55,880 --> 00:57:01,730
And the view files, we use them as
templates in the templates directory.

1137
00:57:01,730 --> 00:57:07,260
So this diagram here also represents
that same kind of division with the

1138
00:57:07,260 --> 00:57:10,510
model and the controller in purple
here on the left and

1139
00:57:10,510 --> 00:57:12,770
the view on the right.

1140
00:57:12,770 --> 00:57:16,020
So this is a schematic that some of you
may have seen at Office Hours or

1141
00:57:16,020 --> 00:57:19,130
diagrams that we were drawing as you
were figuring out your pset.

1142
00:57:19,130 --> 00:57:25,030
>> So here, in a given controller, a model
controller, we have functions

1143
00:57:25,030 --> 00:57:30,490
that relate to querying the SQL
database, executing PHP logic.

1144
00:57:30,490 --> 00:57:32,370
Maybe you would look up
a stock in Yahoo!

1145
00:57:32,370 --> 00:57:34,590
Finance.

1146
00:57:34,590 --> 00:57:37,390
Or perhaps, you would just check to
see whether a user had submitted a

1147
00:57:37,390 --> 00:57:40,250
form already before having
visited your page.

1148
00:57:40,250 --> 00:57:43,390
And then you would render
a form over here.

1149
00:57:43,390 --> 00:57:48,210
After that form had been submitted
by the user, the action that was

1150
00:57:48,210 --> 00:57:53,470
specified in the form's HTML tag
would indicate the page that it

1151
00:57:53,470 --> 00:57:55,620
returns that data to.

1152
00:57:55,620 --> 00:57:59,460
>> So all of that information would be
sent back to your controller.

1153
00:57:59,460 --> 00:58:02,620
Then you would probably do a bit more
logic on that and maybe execute a few

1154
00:58:02,620 --> 00:58:06,510
more queries in the SQL database and
then, finally, come up with a nicely

1155
00:58:06,510 --> 00:58:11,930
packed set of information that you would
pass in into some other template

1156
00:58:11,930 --> 00:58:13,950
that displayed that information.

1157
00:58:13,950 --> 00:58:17,030
Now, how do we actually package
that information up?

1158
00:58:17,030 --> 00:58:23,980
Well, we have a function called Render
that was in the functions.php file in

1159
00:58:23,980 --> 00:58:29,950
pset7, where you pass in the name of
a file, the name of a template.

1160
00:58:29,950 --> 00:58:32,160
>> And then you also pass in
an associative array.

1161
00:58:32,160 --> 00:58:37,150
And so that associative array represents
the different information

1162
00:58:37,150 --> 00:58:39,040
that you want to pass in.

1163
00:58:39,040 --> 00:58:43,460
Now, what's going to be constant in
these examples is that the keys or,

1164
00:58:43,460 --> 00:58:47,070
rather, the keys of the associative
arrays, those are what's going to be

1165
00:58:47,070 --> 00:58:51,050
expected to be constant by the template,
because it knows it needs

1166
00:58:51,050 --> 00:58:53,990
something called message
or called name.

1167
00:58:53,990 --> 00:58:56,940
And then the things on the right, the
actual values, so in this case, who's

1168
00:58:56,940 --> 00:59:00,750
a good boy and Milo, those are going
to be the values that are changing

1169
00:59:00,750 --> 00:59:05,610
that the controller changes every time
or based on a certain condition and

1170
00:59:05,610 --> 00:59:07,120
will pass that in.

1171
00:59:07,120 --> 00:59:12,790
>> So here in templates, we see that we
are using HTML special characters,

1172
00:59:12,790 --> 00:59:16,370
which just basically means that we want
to get the peer string that the

1173
00:59:16,370 --> 00:59:17,580
user put in.

1174
00:59:17,580 --> 00:59:20,880
And we want to substitute
message in there.

1175
00:59:20,880 --> 00:59:26,110
So then when we actually view
the file, the specific

1176
00:59:26,110 --> 00:59:28,700
information is passed in.

1177
00:59:28,700 --> 00:59:33,850
Note that the key how render works is
that the keys of the associative

1178
00:59:33,850 --> 00:59:37,170
arrays, those become variable
names here.

1179
00:59:37,170 --> 00:59:40,720
And so the values of that key in the
associative array then becomes the

1180
00:59:40,720 --> 00:59:41,970
value of the variable.

1181
00:59:41,970 --> 00:59:44,800

1182
00:59:44,800 --> 00:59:46,040
>> Now, let's move on to SQL.

1183
00:59:46,040 --> 00:59:48,010
It stands for Structured
Query Language.

1184
00:59:48,010 --> 00:59:50,460
And so this is just a programming
language designed

1185
00:59:50,460 --> 00:59:51,880
for managing databases.

1186
00:59:51,880 --> 00:59:56,860
And it came in handy for us in
our pset7 finance website.

1187
00:59:56,860 --> 01:00:00,510
Essentially, it's just an easy way to
track and manage objects and tables

1188
01:00:00,510 --> 01:00:02,070
and link them to each other.

1189
01:00:02,070 --> 01:00:06,860
Now, think of your SQL database
basically as an Excel file, perhaps,

1190
01:00:06,860 --> 01:00:10,040
with multiple tabbed sheets.

1191
01:00:10,040 --> 01:00:13,820
>> So you could have multiple tables,
perhaps, that linked to one another.

1192
01:00:13,820 --> 01:00:19,420
And much like Excel, we have a lot of
the functionality that we want.

1193
01:00:19,420 --> 01:00:22,300
For instance, we can select
certain rows.

1194
01:00:22,300 --> 01:00:24,110
We can insert information.

1195
01:00:24,110 --> 01:00:25,560
We can update rows.

1196
01:00:25,560 --> 01:00:27,440
And we can also delete things.

1197
01:00:27,440 --> 01:00:30,920

1198
01:00:30,920 --> 01:00:36,560
>> The SQL select works by selecting rows
or a row of specified columns from a

1199
01:00:36,560 --> 01:00:39,640
database that match a certain
criteria that you indicate.

1200
01:00:39,640 --> 01:00:44,930
So over here when I see select * from
wizards where house = Ravenclaw, then

1201
01:00:44,930 --> 01:00:48,340
I'm selecting *, which means I'm
selecting every single column in that

1202
01:00:48,340 --> 01:00:56,340
row from the wizards table but only if
the house column equals Ravenclaw.

1203
01:00:56,340 --> 01:00:57,840
Now, this is pure or SQL.

1204
01:00:57,840 --> 01:01:02,680
So if I went into PHPmyadmin, which is
the specific way that we use to manage

1205
01:01:02,680 --> 01:01:07,040
our SQL databases, then I could insert
that into the PHPmyadmin website.

1206
01:01:07,040 --> 01:01:08,290
And that would execute.

1207
01:01:08,290 --> 01:01:11,280
>> But we actually want to do
that on the PHP side.

1208
01:01:11,280 --> 01:01:12,580
So how do we do that?

1209
01:01:12,580 --> 01:01:20,180
Well, we use the query function, which
basically executes that SQL query.

1210
01:01:20,180 --> 01:01:21,830
Using ?

1211
01:01:21,830 --> 01:01:25,850
as a placeholder, we can pass in certain
values to our string that we

1212
01:01:25,850 --> 01:01:26,920
want to replace.

1213
01:01:26,920 --> 01:01:32,110
So perhaps I'm storing different
values in the curr_house, which

1214
01:01:32,110 --> 01:01:34,400
represents the current house
that I'm going through.

1215
01:01:34,400 --> 01:01:39,040
So I can pass that in as a placeholder
with the question mark.

1216
01:01:39,040 --> 01:01:43,290
And then I'll basically execute the
same thing as I did before, except

1217
01:01:43,290 --> 01:01:45,550
now, I'm in PHP.

1218
01:01:45,550 --> 01:01:51,300
>> And query will return an
associative array.

1219
01:01:51,300 --> 01:01:53,470
And I'm going to store it in rows.

1220
01:01:53,470 --> 01:01:56,880
Now, query can always fail.

1221
01:01:56,880 --> 01:02:02,870
Perhaps the SQL query couldn't execute
because the table didn't exist.

1222
01:02:02,870 --> 01:02:04,310
Or perhaps, the column didn't exist.

1223
01:02:04,310 --> 01:02:05,400
Something went wrong.

1224
01:02:05,400 --> 01:02:08,170
Well, in that case, you'll want to make
sure that you check whether the

1225
01:02:08,170 --> 01:02:09,700
query returned false.

1226
01:02:09,700 --> 01:02:15,590
And that's by using the triple
equals operation there.

1227
01:02:15,590 --> 01:02:19,660
>> And then I apologize, which is another
CS50 function, passing in a message.

1228
01:02:19,660 --> 01:02:23,435
And if you look into apologize, all it
really does is render apology.php.

1229
01:02:23,435 --> 01:02:26,100

1230
01:02:26,100 --> 01:02:26,410
Yeah.

1231
01:02:26,410 --> 01:02:29,630
>> AUDIENCE: Could you explain what that
star does between select and from?

1232
01:02:29,630 --> 01:02:30,710
>> ZAMYLA CHAN: Yeah, absolutely.

1233
01:02:30,710 --> 01:02:35,220
So the star in between select and from
means that I want to select the whole

1234
01:02:35,220 --> 01:02:37,440
entire row from my table.

1235
01:02:37,440 --> 01:02:41,900
I could've indicated select
name, year, house.

1236
01:02:41,900 --> 01:02:46,160
And I would only get those three
columns in my table.

1237
01:02:46,160 --> 01:02:51,560
But if I say select *, then I'll
get everything in that column.

1238
01:02:51,560 --> 01:02:53,760
Then I'm going to go you
in the back first.

1239
01:02:53,760 --> 01:02:57,656
>> AUDIENCE: So this is still
in SQL, right?

1240
01:02:57,656 --> 01:02:59,610
Is this query or is this PHP?

1241
01:02:59,610 --> 01:03:00,550
>> ZAMYLA CHAN: We're in a query.

1242
01:03:00,550 --> 01:03:01,940
So this is in PHP.

1243
01:03:01,940 --> 01:03:06,280
So using the PHP function query,
we're executing a SQL query.

1244
01:03:06,280 --> 01:03:11,988

1245
01:03:11,988 --> 01:03:15,364
>> AUDIENCE: Is anything in SQL
case-sensitive, like select

1246
01:03:15,364 --> 01:03:17,834
or wizards or house?

1247
01:03:17,834 --> 01:03:20,050
>> ZAMYLA CHAN: Is anything
in SQL case-sensitive?

1248
01:03:20,050 --> 01:03:21,760
I believe so, yes.

1249
01:03:21,760 --> 01:03:24,620
I believe that SELECT and FROM
and WHERE are case-sensitive.

1250
01:03:24,620 --> 01:03:25,535
No?

1251
01:03:25,535 --> 01:03:27,500
>> ROB BOWDEN: So, it's the opposite.

1252
01:03:27,500 --> 01:03:32,030
The column names and the table means,
all of those are case-sensitive.

1253
01:03:32,030 --> 01:03:35,470
But any of the MySQL key words, like
SELECT, FROM, and WHERE, those are not

1254
01:03:35,470 --> 01:03:36,140
case-sensitive.

1255
01:03:36,140 --> 01:03:36,420
OK.

1256
01:03:36,420 --> 01:03:37,780
So the opposite of what I said.

1257
01:03:37,780 --> 01:03:40,420
So all of the MySQL keywords--

1258
01:03:40,420 --> 01:03:42,670
select, from, where-- those
are not case-sensitive.

1259
01:03:42,670 --> 01:03:44,630
But everything else is.

1260
01:03:44,630 --> 01:03:45,210
OK.

1261
01:03:45,210 --> 01:03:46,500
You in the front.

1262
01:03:46,500 --> 01:03:52,041
>> AUDIENCE: If I have $ rows in terms of
more than one row, does that mean is

1263
01:03:52,041 --> 01:03:53,640
just becomes an associative array?

1264
01:03:53,640 --> 01:03:59,550
>> ZAMYLA CHAN: So the question was if rows
has more than one row in it, does

1265
01:03:59,550 --> 01:04:01,800
it become an associative array?

1266
01:04:01,800 --> 01:04:05,680
So it is an array of associative
arrays already.

1267
01:04:05,680 --> 01:04:10,730
So even if there's only one row
returned, then you'd have to go to

1268
01:04:10,730 --> 01:04:12,690
index 0 of that result.

1269
01:04:12,690 --> 01:04:15,316
And then you'd have that first row.

1270
01:04:15,316 --> 01:04:17,482
Yes, Belinda?

1271
01:04:17,482 --> 01:04:21,258
>> AUDIENCE: When you use ===,
is this the only instance?

1272
01:04:21,258 --> 01:04:22,210
Or are there others?

1273
01:04:22,210 --> 01:04:26,815
>> ZAMYLA CHAN: So in this case, ===
is a comparison across types.

1274
01:04:26,815 --> 01:04:29,870

1275
01:04:29,870 --> 01:04:34,050
Sorry. === is a comparison
that compares the types.

1276
01:04:34,050 --> 01:04:37,620
And then == compares across all types.

1277
01:04:37,620 --> 01:04:41,620
>> AUDIENCE: Can you explain what
rows is in this situation?

1278
01:04:41,620 --> 01:04:45,120
Is it row of data?

1279
01:04:45,120 --> 01:04:48,100
>> ZAMYLA CHAN: In the next slide, I'm
going to explain what rows is.

1280
01:04:48,100 --> 01:04:49,890
So if you don't mind holding
off on that.

1281
01:04:49,890 --> 01:04:50,620
And then you in the back?

1282
01:04:50,620 --> 01:04:54,699
>> AUDIENCE: For functions like query,
render and apologize [INAUDIBLE]?

1283
01:04:54,699 --> 01:04:59,050

1284
01:04:59,050 --> 01:05:03,050
>> ZAMYLA CHAN: The question was whether
these functions-- query, apologize,

1285
01:05:03,050 --> 01:05:04,510
and render--

1286
01:05:04,510 --> 01:05:05,930
are common across PHP.

1287
01:05:05,930 --> 01:05:09,460
These are ones that CS50
wrote for pset7.

1288
01:05:09,460 --> 01:05:09,910
And Jay?

1289
01:05:09,910 --> 01:05:15,333
>> AUDIENCE: When you need to say
$_session, is that only for IDs?

1290
01:05:15,333 --> 01:05:17,310
Or could you have said that here?

1291
01:05:17,310 --> 01:05:23,440
>> ZAMYLA CHAN: So the question was, when
we use $_session, that was a specific

1292
01:05:23,440 --> 01:05:25,290
global variable that we're using.

1293
01:05:25,290 --> 01:05:32,080
Here this variable is going to
be local to our function.

1294
01:05:32,080 --> 01:05:36,588
So we're just declaring
a new variable.

1295
01:05:36,588 --> 01:05:38,460
>> AUDIENCE: How is apologize
implemented?

1296
01:05:38,460 --> 01:05:40,960
>> ZAMYLA CHAN: The question was,
how is apologize implemented?

1297
01:05:40,960 --> 01:05:44,180
And I think this is actually a pretty
good practice for you guys to go into

1298
01:05:44,180 --> 01:05:49,260
the functions.php section and look at
apologize and see how you could have

1299
01:05:49,260 --> 01:05:50,670
done it yourself.

1300
01:05:50,670 --> 01:05:55,620
So I may leave that to you but just
say that if you look at apologize,

1301
01:05:55,620 --> 01:06:02,110
then it takes the message that you
submitted to apologize, and then it

1302
01:06:02,110 --> 01:06:06,570
renders that message.

1303
01:06:06,570 --> 01:06:08,240
Any more questions?

1304
01:06:08,240 --> 01:06:08,710
I love questions.

1305
01:06:08,710 --> 01:06:09,555
So keep them coming.

1306
01:06:09,555 --> 01:06:11,888
>> AUDIENCE: [INAUDIBLE]

1307
01:06:11,888 --> 01:06:13,840
echo or print there?

1308
01:06:13,840 --> 01:06:15,900
>> ZAMYLA CHAN: The question was,
could we not just have put

1309
01:06:15,900 --> 01:06:17,000
echo or print there.

1310
01:06:17,000 --> 01:06:19,710
So that would have done something
slightly different.

1311
01:06:19,710 --> 01:06:23,750
That would have printed query
failed into that--

1312
01:06:23,750 --> 01:06:27,420
well, right now, we're actually
in our controller.

1313
01:06:27,420 --> 01:06:30,350
So we don't actually have
HTML set up here.

1314
01:06:30,350 --> 01:06:34,946
Apologize by rendering apologize.php
actually redirects you to apology.php.

1315
01:06:34,946 --> 01:06:39,560

1316
01:06:39,560 --> 01:06:42,200
OK.

1317
01:06:42,200 --> 01:06:45,880
>> So now, let's go on to address the
question from earlier about what

1318
01:06:45,880 --> 01:06:47,330
really is rows.

1319
01:06:47,330 --> 01:06:51,960
Well, query will return
an array of rows.

1320
01:06:51,960 --> 01:06:55,020
And every row is represented
by an associative array.

1321
01:06:55,020 --> 01:07:02,840
So if I've executed some SQL query and
I've stored the result in rows, then

1322
01:07:02,840 --> 01:07:07,850
using a foreach loop, then the array
name is the first one there-- rows.

1323
01:07:07,850 --> 01:07:13,170
And then I'm going to call
every row in there $row.

1324
01:07:13,170 --> 01:07:20,060
>> So iterating over that, I can then
access the given row's name column,

1325
01:07:20,060 --> 01:07:22,340
year column, and house column.

1326
01:07:22,340 --> 01:07:28,010
Note that I wouldn't have been able to
do this with rows, because rows index

1327
01:07:28,010 --> 01:07:29,290
name doesn't exist.

1328
01:07:29,290 --> 01:07:31,970
Rows is just an array of
associative arrays.

1329
01:07:31,970 --> 01:07:34,870
So you have two levels there.

1330
01:07:34,870 --> 01:07:37,170
Once you have the array of rows,
you have to get into that.

1331
01:07:37,170 --> 01:07:39,110
And then you can access the columns.

1332
01:07:39,110 --> 01:07:41,636
Did that make it clear?

1333
01:07:41,636 --> 01:07:42,520
Yeah, in front?

1334
01:07:42,520 --> 01:07:45,490
>> AUDIENCE: [INAUDIBLE] open
brackets for [INAUDIBLE]?

1335
01:07:45,490 --> 01:07:46,220
>> ZAMYLA CHAN: Pardon me?

1336
01:07:46,220 --> 01:07:49,740
>> AUDIENCE: The open brackets.

1337
01:07:49,740 --> 01:07:52,420
>> ZAMYLA CHAN: These here?

1338
01:07:52,420 --> 01:07:58,520
That's allowing me to include
that variable.

1339
01:07:58,520 --> 01:07:58,670
Yeah.

1340
01:07:58,670 --> 01:08:01,900
>> AUDIENCE: When you print, are you
printing to the HTML code?

1341
01:08:01,900 --> 01:08:03,110
>> ZAMYLA CHAN: Yes.

1342
01:08:03,110 --> 01:08:07,720
When I print, this here is inside
my template now, so

1343
01:08:07,720 --> 01:08:10,310
my view of MVC method.

1344
01:08:10,310 --> 01:08:12,750
So I'm printing into the HTML.

1345
01:08:12,750 --> 01:08:16,670
>> AUDIENCE: So if we went into developer
tools after running this, we could

1346
01:08:16,670 --> 01:08:17,160
that actually in code?

1347
01:08:17,160 --> 01:08:18,410
>> ZAMYLA CHAN: That's a great
question, yeah.

1348
01:08:18,410 --> 01:08:22,359
So if you went into the developer tools
in Firefox using Firebug or

1349
01:08:22,359 --> 01:08:26,109
Chrome, then yeah, you could
see the specific HTML.

1350
01:08:26,109 --> 01:08:28,470
So it wouldn't show $row["Name"].

1351
01:08:28,470 --> 01:08:32,524
It would show whichever
name is in that row.

1352
01:08:32,524 --> 01:08:36,268
>> AUDIENCE: Just a general issue,
what are tr and td defined as?

1353
01:08:36,268 --> 01:08:37,672
Why would we [INAUDIBLE]?

1354
01:08:37,672 --> 01:08:41,850

1355
01:08:41,850 --> 01:08:44,814
>> ZAMYLA CHAN: Table row tr,
table then td column.

1356
01:08:44,814 --> 01:08:48,060

1357
01:08:48,060 --> 01:08:49,310
OK.

1358
01:08:49,310 --> 01:08:55,771

1359
01:08:55,771 --> 01:08:56,835
>> AUDIENCE: Yeah, it's table data.

1360
01:08:56,835 --> 01:08:58,770
>> ZAMYLA CHAN: Table data.

1361
01:08:58,770 --> 01:08:59,894
Yeah.

1362
01:08:59,894 --> 01:09:08,670
>> AUDIENCE: It's a row in which the
row is treated like a column?

1363
01:09:08,670 --> 01:09:08,910
>> ZAMYLA CHAN: Sorry.

1364
01:09:08,910 --> 01:09:10,570
Can you repeat that?

1365
01:09:10,570 --> 01:09:14,450
>> AUDIENCE: How would you
visualize rows?

1366
01:09:14,450 --> 01:09:16,310
>> ZAMYLA CHAN: How would you visualize
rows in what kind of way?

1367
01:09:16,310 --> 01:09:21,796
Are you talking about these
rows here or the tr rows?

1368
01:09:21,796 --> 01:09:22,630
>> AUDIENCE: The rows.

1369
01:09:22,630 --> 01:09:25,229
>> ZAMYLA CHAN: These rows here?

1370
01:09:25,229 --> 01:09:28,620
I'd visualize this as
I execute my query.

1371
01:09:28,620 --> 01:09:38,729
And it says, OK, I have either 0 to n
amount of rows that match the criteria

1372
01:09:38,729 --> 01:09:40,510
that you had queried.

1373
01:09:40,510 --> 01:09:43,740
So I have some number of rows.

1374
01:09:43,740 --> 01:09:51,450
So rows, the $rows, stores each
one of those rows in an array.

1375
01:09:51,450 --> 01:09:58,110
So even if it's just one of them, it's
still an array of rows that match it.

1376
01:09:58,110 --> 01:10:03,010
>> So then, for instance, this is
similar to when you fetched

1377
01:10:03,010 --> 01:10:05,390
the cache from users.

1378
01:10:05,390 --> 01:10:10,810
And the criteria there was where
ID equals the session ID.

1379
01:10:10,810 --> 01:10:14,250
There really only is one row
that could match that.

1380
01:10:14,250 --> 01:10:18,960
But still rows just returned one row.

1381
01:10:18,960 --> 01:10:22,620
So you'd have to go to rows,
index 0, index cache to

1382
01:10:22,620 --> 01:10:26,195
actually get to your cache.

1383
01:10:26,195 --> 01:10:29,650
>> AUDIENCE: Is the print function
in echo the same thing?

1384
01:10:29,650 --> 01:10:30,670
>> ZAMYLA CHAN: Yes.

1385
01:10:30,670 --> 01:10:31,190
Yes.

1386
01:10:31,190 --> 01:10:33,304
Print an echo of the same.

1387
01:10:33,304 --> 01:10:42,400
>> AUDIENCE: Is the foreach loop the
only way to index into rows?

1388
01:10:42,400 --> 01:10:46,110
>> ZAMYLA CHAN: Is a foreach loop
the only way that you can

1389
01:10:46,110 --> 01:10:47,030
iterate through rows?

1390
01:10:47,030 --> 01:10:47,180
No.

1391
01:10:47,180 --> 01:10:51,000
You can also use a for loop, provided
that you know the length

1392
01:10:51,000 --> 01:10:53,024
of the row's array.

1393
01:10:53,024 --> 01:10:58,500
>> AUDIENCE: Could you access it
using a row as [INAUDIBLE]?

1394
01:10:58,500 --> 01:11:01,640
>> ZAMYLA CHAN: So you can't access it
just using row if you don't have a

1395
01:11:01,640 --> 01:11:05,160
foreach loop provided that
you haven't declared row.

1396
01:11:05,160 --> 01:11:08,150

1397
01:11:08,150 --> 01:11:09,660
Yes.

1398
01:11:09,660 --> 01:11:10,810
Yeah, in the white.

1399
01:11:10,810 --> 01:11:13,990
>> AUDIENCE: So what do tr and td do?

1400
01:11:13,990 --> 01:11:16,790
>> ZAMYLA CHAN: So tr and
td are HTML tags.

1401
01:11:16,790 --> 01:11:19,590
tr indicates the beginning
of a table row.

1402
01:11:19,590 --> 01:11:26,625
And each td indicates a
new table data column.

1403
01:11:26,625 --> 01:11:32,275
>> AUDIENCE: For a visual of what a row
is like, just imagine the SQL, how

1404
01:11:32,275 --> 01:11:33,510
they have a row.

1405
01:11:33,510 --> 01:11:35,980
[INAUDIBLE].

1406
01:11:35,980 --> 01:11:36,390
>> ZAMYLA CHAN: Yeah.

1407
01:11:36,390 --> 01:11:37,630
That's a great point.

1408
01:11:37,630 --> 01:11:41,510
You can visualize rows as just
like in an Excel table, just

1409
01:11:41,510 --> 01:11:44,540
the list of the rows.

1410
01:11:44,540 --> 01:11:46,870
OK.

1411
01:11:46,870 --> 01:11:47,230
All right.

1412
01:11:47,230 --> 01:11:50,740
So now that we've gone over select, if
there aren't any more questions, we'll

1413
01:11:50,740 --> 01:11:52,970
go over onto insert.

1414
01:11:52,970 --> 01:11:58,220
So if I wanted to insert into some
table and insert certain column

1415
01:11:58,220 --> 01:12:02,320
values, I could insert myself
into Ravenclaw in year 7.

1416
01:12:02,320 --> 01:12:07,245
But sometimes there might be duplicate
values, as we saw in pset7 when we

1417
01:12:07,245 --> 01:12:09,240
were updating our portfolio.

1418
01:12:09,240 --> 01:12:15,610
>> So in this case, we want to use ON
DUPLICATE KEY UPDATE, so that we don't

1419
01:12:15,610 --> 01:12:18,280
store multiple rows with the same
value but rather update it.

1420
01:12:18,280 --> 01:12:21,780

1421
01:12:21,780 --> 01:12:25,560
Then we actually have update,
which isn't an insert.

1422
01:12:25,560 --> 01:12:30,100
It's just an update where you update
in a certain table with a given

1423
01:12:30,100 --> 01:12:34,403
criteria and then, finally, delete,
which does a very similar thing.

1424
01:12:34,403 --> 01:12:40,200
>> AUDIENCE: Could you briefly
go over the duplicate key?

1425
01:12:40,200 --> 01:12:40,850
>> ZAMYLA CHAN: Yeah.

1426
01:12:40,850 --> 01:12:48,340
Essentially here, I have INSERT INTO
gringotts, is, galleons, these values.

1427
01:12:48,340 --> 01:12:56,900
But ID, presumably, is a unique key
value set up in MySQL table.

1428
01:12:56,900 --> 01:13:06,750
So if I already have that ID set up,
then I can't insert a new row.

1429
01:13:06,750 --> 01:13:10,961
So if it doesn't exist already,
then I have to update it.

1430
01:13:10,961 --> 01:13:16,040

1431
01:13:16,040 --> 01:13:17,722
In the middle in the white.

1432
01:13:17,722 --> 01:13:21,570
>> AUDIENCE: So insert, update, delete,
and select, are those all available

1433
01:13:21,570 --> 01:13:22,875
locally [INAUDIBLE]?

1434
01:13:22,875 --> 01:13:27,320

1435
01:13:27,320 --> 01:13:29,630
>> ZAMYLA CHAN: So insert,
update, delete, and

1436
01:13:29,630 --> 01:13:31,120
select are all SQL queries.

1437
01:13:31,120 --> 01:13:34,970
So whenever you're using SQL,
you'll have those available.

1438
01:13:34,970 --> 01:13:38,226

1439
01:13:38,226 --> 01:13:40,706
>> AUDIENCE: Back to the past quizzes--

1440
01:13:40,706 --> 01:13:44,178
there was a question that dealt with
if you had a table and wanted to

1441
01:13:44,178 --> 01:13:52,610
insert test scores in one and you insert
your name so it won't let you

1442
01:13:52,610 --> 01:13:54,594
[INAUDIBLE] your friend's test score.

1443
01:13:54,594 --> 01:13:59,060
How would you do that with insert?

1444
01:13:59,060 --> 01:14:02,490
>> ZAMYLA CHAN: So the question was about
a previous mid-term question.

1445
01:14:02,490 --> 01:14:04,320
I'm not aware of which
one it is right now.

1446
01:14:04,320 --> 01:14:06,790
So maybe afterwards, if you want to
come up and show me, then I can

1447
01:14:06,790 --> 01:14:08,155
certainly give you tips.

1448
01:14:08,155 --> 01:14:11,250

1449
01:14:11,250 --> 01:14:15,460
But speaking of inserting things, like
taking someone's score when you

1450
01:14:15,460 --> 01:14:18,800
shouldn't, let's talk about
SQL injection attacks.

1451
01:14:18,800 --> 01:14:22,050
So a SQL injection attack is essentially
where someone takes

1452
01:14:22,050 --> 01:14:27,680
advantage of the low security of the
way that you are taking in data.

1453
01:14:27,680 --> 01:14:35,660
>> So in here, just like in CS50 finance,
when we logged in, we can enter in a

1454
01:14:35,660 --> 01:14:39,510
username in the login form,
the first text box, and

1455
01:14:39,510 --> 01:14:41,570
then enter in a password.

1456
01:14:41,570 --> 01:14:45,610
Perhaps our PHP code might look
something like this, where $ username

1457
01:14:45,610 --> 01:14:51,410
is the post-data username and password
is post-data password.

1458
01:14:51,410 --> 01:14:55,110
And then we just execute our query, say,
OK, well, our query is going to

1459
01:14:55,110 --> 01:14:57,990
select from our users, where
the username is the

1460
01:14:57,990 --> 01:14:58,970
one that they submitted.

1461
01:14:58,970 --> 01:15:06,870
And the password is the password,
meaning that the passwords match.

1462
01:15:06,870 --> 01:15:12,670
>> Now, what if instead of actually
submitting an actual password, like

1463
01:15:12,670 --> 01:15:16,850
12345 and guessing at prong that says
password and trying to hack their

1464
01:15:16,850 --> 01:15:21,460
account, what if instead
they submitted this.

1465
01:15:21,460 --> 01:15:24,380
They could type in maybe
a guess at a password.

1466
01:15:24,380 --> 01:15:30,480
And then they would finish the
quote then type in or 1 = 1.

1467
01:15:30,480 --> 01:15:35,160
That would pace directly into the SQL
query to look something like this.

1468
01:15:35,160 --> 01:15:38,200
Select from users where username
= prongs and password

1469
01:15:38,200 --> 01:15:41,450
equals lily or 1 = 1.

1470
01:15:41,450 --> 01:15:45,450
>> So either the password has
to be correct or 1 = 1,

1471
01:15:45,450 --> 01:15:47,870
which is always true.

1472
01:15:47,870 --> 01:15:52,780
So in this case, basically, a user can
take advantage of this and just log

1473
01:15:52,780 --> 01:15:55,240
themselves in and hack
someone's account.

1474
01:15:55,240 --> 01:15:58,950
So that's why we want to avoid
someone having to do this.

1475
01:15:58,950 --> 01:16:03,890
But luckily, the query function by
passing in the placeholders will take

1476
01:16:03,890 --> 01:16:04,800
care of this for you.

1477
01:16:04,800 --> 01:16:08,570
Also, you'll typically never want
to actually submit the passwords

1478
01:16:08,570 --> 01:16:09,310
themselves.

1479
01:16:09,310 --> 01:16:14,115
That's why we hashed or encrypted
them in CS50 finance.

1480
01:16:14,115 --> 01:16:17,553
>> AUDIENCE: The past quiz talked
about MySQL escape strings.

1481
01:16:17,553 --> 01:16:21,800
Do we have to worry about that?

1482
01:16:21,800 --> 01:16:22,670
>> ZAMYLA CHAN: That's a good question.

1483
01:16:22,670 --> 01:16:29,050
The MySQL escape strings is definitely a
function that was used in our query.

1484
01:16:29,050 --> 01:16:30,420
But definitely look into that.

1485
01:16:30,420 --> 01:16:32,950
I'd say that's fair game to know
that you'd need to call that

1486
01:16:32,950 --> 01:16:34,370
function on a string.

1487
01:16:34,370 --> 01:16:35,245
Yeah, Belinda?

1488
01:16:35,245 --> 01:16:38,606
>> AUDIENCE: How do you know when it's
single quotes or double quotes?

1489
01:16:38,606 --> 01:16:43,516
And also, I feel like in lecture you
mentioned something about not having

1490
01:16:43,516 --> 01:16:48,426
the [INAUDIBLE] or something or the
second single quote at the end.

1491
01:16:48,426 --> 01:16:56,407

1492
01:16:56,407 --> 01:17:00,825
I think he pointed out in lecture that
you're supposed to have apostrophe 1

1493
01:17:00,825 --> 01:17:04,290
and then not have apostrophes
or something.

1494
01:17:04,290 --> 01:17:05,540
>> AUDIENCE: [INAUDIBLE].

1495
01:17:05,540 --> 01:17:14,860

1496
01:17:14,860 --> 01:17:18,657
>> AUDIENCE: The thing is the last single
quote in there in that second box

1497
01:17:18,657 --> 01:17:19,391
shouldn't be there.

1498
01:17:19,391 --> 01:17:21,840
[INAUDIBLE]

1499
01:17:21,840 --> 01:17:26,730
Because when you take that last single
quote out and match these for content

1500
01:17:26,730 --> 01:17:30,522
where the password is, if you have that
query, there's a single quote at

1501
01:17:30,522 --> 01:17:31,340
the end already.

1502
01:17:31,340 --> 01:17:36,580
You want to use that single quote
as the one that faces the one

1503
01:17:36,580 --> 01:17:37,574
[INAUDIBLE].

1504
01:17:37,574 --> 01:17:42,820
So what's actually in that text
box should not have that.

1505
01:17:42,820 --> 01:17:44,070
>> ZAMYLA CHAN: I'll change that.

1506
01:17:44,070 --> 01:17:56,490

1507
01:17:56,490 --> 01:17:56,880
OK.

1508
01:17:56,880 --> 01:18:00,435
If there aren't any questions, then I'll
pass it over to Joseph to talk

1509
01:18:00,435 --> 01:18:04,332
about JavaScript, et cetera.

1510
01:18:04,332 --> 01:18:09,540
>> [APPLAUSE]

1511
01:18:09,540 --> 01:18:11,660
>> JOSEPH ONG: So we are running
a little bit behind.

1512
01:18:11,660 --> 01:18:13,280
So if you have to leave, that's OK.

1513
01:18:13,280 --> 01:18:16,540
But we ask that you keep your heads down
if you're in the middle, so you

1514
01:18:16,540 --> 01:18:19,560
don't block the camera and you use
the back exit if you have to.

1515
01:18:19,560 --> 01:18:25,010

1516
01:18:25,010 --> 01:18:26,306
I'm Joseph by the way.

1517
01:18:26,306 --> 01:18:27,556
Hi.

1518
01:18:27,556 --> 01:18:37,910

1519
01:18:37,910 --> 01:18:38,630
Test, test.

1520
01:18:38,630 --> 01:18:39,630
Dan, is that good?

1521
01:18:39,630 --> 01:18:40,880
Cool.

1522
01:18:40,880 --> 01:18:44,460

1523
01:18:44,460 --> 01:18:47,600
>> So the video will also be posted
online for those that

1524
01:18:47,600 --> 01:18:48,850
have to leave now.

1525
01:18:48,850 --> 01:18:51,350

1526
01:18:51,350 --> 01:18:52,600
Awkward.

1527
01:18:52,600 --> 01:18:56,300

1528
01:18:56,300 --> 01:18:57,510
OK.

1529
01:18:57,510 --> 01:18:59,730
So quiz review.

1530
01:18:59,730 --> 01:19:00,980
This is a cat.

1531
01:19:00,980 --> 01:19:03,150

1532
01:19:03,150 --> 01:19:07,170
Now, JavaScript, which is maybe not
as aww for some of you guys.

1533
01:19:07,170 --> 01:19:07,370
OK.

1534
01:19:07,370 --> 01:19:09,280
So that's first, recall from Zamyla.

1535
01:19:09,280 --> 01:19:11,270
Remember that PHP is
run on the server.

1536
01:19:11,270 --> 01:19:15,230
And lots of times, you guys wrote loops
in PHP to print out HTML, right?

1537
01:19:15,230 --> 01:19:19,170
So once that code executes, that
HTML output that you print out

1538
01:19:19,170 --> 01:19:20,500
gets sent to the user.

1539
01:19:20,500 --> 01:19:25,960
And once that happens, no more PHP can
be run, unless you reload the page, of

1540
01:19:25,960 --> 01:19:27,660
course, which reexecutes the PHP.

1541
01:19:27,660 --> 01:19:30,080
But once you print out that HTML,
you can't go anywhere.

1542
01:19:30,080 --> 01:19:33,580
>> So that HTML is sent over to the user,
which is the browser over here, where

1543
01:19:33,580 --> 01:19:35,470
Milo is using the computer.

1544
01:19:35,470 --> 01:19:40,340
And so well, there are several things
once we send HTML to the user.

1545
01:19:40,340 --> 01:19:43,350
Sometimes we want to do something like
when you click on something, we want

1546
01:19:43,350 --> 01:19:47,350
alert boxes to pop up, those sorts of
interactions, like when you press the

1547
01:19:47,350 --> 01:19:50,740
key, when you click something on the
page, I want something to happen.

1548
01:19:50,740 --> 01:19:54,550
Well, you can't reexecute PHP
code once that HTML is set.

1549
01:19:54,550 --> 01:19:55,900
So how do you do this?

1550
01:19:55,900 --> 01:19:59,930
>> We introduce a new language called
JavaScript, which runs in the browser

1551
01:19:59,930 --> 01:20:02,890
that allows you to do things
to HTML after you receive

1552
01:20:02,890 --> 01:20:04,020
them from the server.

1553
01:20:04,020 --> 01:20:07,050
And this is why we call it a client-side
programming language.

1554
01:20:07,050 --> 01:20:09,110
It works on your computer--

1555
01:20:09,110 --> 01:20:11,400
the client.

1556
01:20:11,400 --> 01:20:13,830
Any questions about that so far?

1557
01:20:13,830 --> 01:20:15,650
That paradigm makes sense to people?

1558
01:20:15,650 --> 01:20:16,010
OK.

1559
01:20:16,010 --> 01:20:16,790
Good.

1560
01:20:16,790 --> 01:20:17,340
All right.

1561
01:20:17,340 --> 01:20:20,850
So the first thing to note
is JavaScript is not PHP.

1562
01:20:20,850 --> 01:20:23,010
>> They have some different syntax,
which we'll go into.

1563
01:20:23,010 --> 01:20:24,530
And they have very different uses.

1564
01:20:24,530 --> 01:20:26,640
JavaScript, again, for your
browser, for the client.

1565
01:20:26,640 --> 01:20:30,640
Server runs somewhere on someone else's
computer that sends information

1566
01:20:30,640 --> 01:20:31,720
to you, correct?

1567
01:20:31,720 --> 01:20:36,730
So if we ask you to write PHP code
on an exam question, don't write

1568
01:20:36,730 --> 01:20:38,710
JavaScript and vice versa.

1569
01:20:38,710 --> 01:20:41,710
You'll just lose points,
and it won't be right.

1570
01:20:41,710 --> 01:20:43,690
>> So let's get into some
syntax differences--

1571
01:20:43,690 --> 01:20:46,140
JavaScript on the left
and PHP on the right.

1572
01:20:46,140 --> 01:20:48,670
The first thing you'll notice with
JavaScript, we declare variables with

1573
01:20:48,670 --> 01:20:49,440
the var keyword--

1574
01:20:49,440 --> 01:20:53,590
V-A-R. PHP used the dollar sign,
as Zamyla discussed earlier.

1575
01:20:53,590 --> 01:20:57,570
If you want to declare an associative
array, we see the familiar syntax on

1576
01:20:57,570 --> 01:20:59,030
the right side with PHP.

1577
01:20:59,030 --> 01:21:01,630
On the left side, instead
you use curly braces.

1578
01:21:01,630 --> 01:21:03,280
And then your keys are on the left.

1579
01:21:03,280 --> 01:21:04,670
Then you have a colon.

1580
01:21:04,670 --> 01:21:06,560
And then you have the values
that you want.

1581
01:21:06,560 --> 01:21:09,840
>> So this is how you would do it in PHP
on the right side with that second

1582
01:21:09,840 --> 01:21:10,955
line that starts at Milo.

1583
01:21:10,955 --> 01:21:14,540
And that is how you would do it on the
left side in JavaScript if you want

1584
01:21:14,540 --> 01:21:16,110
what we call an object.

1585
01:21:16,110 --> 01:21:19,340
And objects in JavaScript are
just associative arrays.

1586
01:21:19,340 --> 01:21:23,710
So if you want to access fields, in
PHP you use this bracket syntax.

1587
01:21:23,710 --> 01:21:27,170
And this way, you can reassign
this owner field to Lauren.

1588
01:21:27,170 --> 01:21:30,150
>> Well, in JavaScript, if one want to
access a field and change it, you can

1589
01:21:30,150 --> 01:21:31,370
use the dot syntax.

1590
01:21:31,370 --> 01:21:33,860
You can also use the bracket syntax.

1591
01:21:33,860 --> 01:21:35,550
But you can't use the
dot syntax in PHP.

1592
01:21:35,550 --> 01:21:36,525
That won't work.

1593
01:21:36,525 --> 01:21:38,910
It only works in PHP.

1594
01:21:38,910 --> 01:21:42,060
And finally, to print things to the
console, you use console.log, which

1595
01:21:42,060 --> 01:21:43,720
you guys use a lot in pset8.

1596
01:21:43,720 --> 01:21:45,260
You can console.log that.

1597
01:21:45,260 --> 01:21:50,030
>> If you want to print an array in
PHP, you have to use print r.

1598
01:21:50,030 --> 01:21:53,240
And on the right side, you see i hash
string concatenation over there.

1599
01:21:53,240 --> 01:21:54,070
Someone asked earlier.

1600
01:21:54,070 --> 01:21:55,850
I use a plus in JavaScript.

1601
01:21:55,850 --> 01:21:59,800
If I want to concatenate something
in PHP, I use the dot.

1602
01:21:59,800 --> 01:22:00,800
These are different.

1603
01:22:00,800 --> 01:22:03,420
If you're writing PHP code,
don't use a plus.

1604
01:22:03,420 --> 01:22:06,810
If you're writing JavaScript
code, don't write a dot.

1605
01:22:06,810 --> 01:22:08,230
It will be wrong.

1606
01:22:08,230 --> 01:22:09,640
And you'll be sad.

1607
01:22:09,640 --> 01:22:11,000
>> So syntax differences.

1608
01:22:11,000 --> 01:22:14,710
Know your syntax, because if you have to
write a question and you use syntax

1609
01:22:14,710 --> 01:22:16,890
from the wrong language,
it will not work.

1610
01:22:16,890 --> 01:22:19,520
And it will be wrong.

1611
01:22:19,520 --> 01:22:22,420
So let's talk about some control
flow differences, how you use

1612
01:22:22,420 --> 01:22:24,040
loops in each of them.

1613
01:22:24,040 --> 01:22:26,610
Zamyla went over the right side.

1614
01:22:26,610 --> 01:22:27,780
Stuff on the right side
should be familiar.

1615
01:22:27,780 --> 01:22:29,670
Let's look at on the left side.

1616
01:22:29,670 --> 01:22:34,830
>> When you use for n loop in JavaScript,
your loop variable, var i over there,

1617
01:22:34,830 --> 01:22:37,210
loop over the keys of the array.

1618
01:22:37,210 --> 01:22:39,150
So you see name, house, and role.

1619
01:22:39,150 --> 01:22:42,440
If I console.log i, I get
name, house, and role.

1620
01:22:42,440 --> 01:22:44,720
Those are the keys.

1621
01:22:44,720 --> 01:22:50,080
In JavaScript, a foreach loop goes
over the values of this array.

1622
01:22:50,080 --> 01:22:52,040
So you notice they're both i.

1623
01:22:52,040 --> 01:22:56,000
But in here on the PHP side, it prints
out Milo, CS50, and Mascot.

1624
01:22:56,000 --> 01:22:57,390
Those are the values in PHP.

1625
01:22:57,390 --> 01:23:00,180
>> So these are how these two are different
in the different languages.

1626
01:23:00,180 --> 01:23:02,970
So if you're using a foreach
loop, don't assume that it

1627
01:23:02,970 --> 01:23:03,760
gives you the keys.

1628
01:23:03,760 --> 01:23:08,010
And if you're using a for n loop, don't
assume it gives you the values.

1629
01:23:08,010 --> 01:23:10,420
Does this make sense so far?

1630
01:23:10,420 --> 01:23:13,630
The next slide is going to show you
how you can access the opposite in

1631
01:23:13,630 --> 01:23:15,160
each of them.

1632
01:23:15,160 --> 01:23:18,550
>> Well, if you have the key in JavaScript
and you want the value out,

1633
01:23:18,550 --> 01:23:20,440
you just index into the
array with that.

1634
01:23:20,440 --> 01:23:23,640
So Milo of i will get you what
you want-- the values.

1635
01:23:23,640 --> 01:23:25,260
There's this different syntax in PHP.

1636
01:23:25,260 --> 01:23:28,140
If you really want to know it, I don't
think we've showed it to you yet.

1637
01:23:28,140 --> 01:23:31,740
But if you're interested, you can use
this additional syntax on the right

1638
01:23:31,740 --> 01:23:35,130
side that will actually let you get the
keys in PHP when you're using a

1639
01:23:35,130 --> 01:23:35,880
foreach loop.

1640
01:23:35,880 --> 01:23:39,640
So just a little bit of trivia
if you're interested.

1641
01:23:39,640 --> 01:23:42,630
>> So that's just to demonstrate the
differences between these two loops.

1642
01:23:42,630 --> 01:23:47,060
Don't mix them up when you're
programming a question.

1643
01:23:47,060 --> 01:23:49,774
Any questions about that.

1644
01:23:49,774 --> 01:23:50,770
Cool.

1645
01:23:50,770 --> 01:23:51,310
All right.

1646
01:23:51,310 --> 01:23:52,250
JavaScript objects.

1647
01:23:52,250 --> 01:23:53,040
I talked about them.

1648
01:23:53,040 --> 01:23:54,650
They're like associative arrays.

1649
01:23:54,650 --> 01:23:57,730
The one thing I would like you to note
here is that a value in an associative

1650
01:23:57,730 --> 01:23:59,540
array can be anything in JavaScript.

1651
01:23:59,540 --> 01:24:02,240
>> It can even be a function,
like over there.

1652
01:24:02,240 --> 01:24:04,600
I have a function that
is a value of a key.

1653
01:24:04,600 --> 01:24:07,030
And if I want to call that function,
I just access bark.

1654
01:24:07,030 --> 01:24:08,490
And then I put the parentheses
after that.

1655
01:24:08,490 --> 01:24:09,790
And that works.

1656
01:24:09,790 --> 01:24:15,570
So any questions?

1657
01:24:15,570 --> 01:24:16,370
No?

1658
01:24:16,370 --> 01:24:16,650
OK.

1659
01:24:16,650 --> 01:24:18,190
Good.

1660
01:24:18,190 --> 01:24:20,130
JavaScript, like PHP,
is loosely typed.

1661
01:24:20,130 --> 01:24:20,970
What does that mean?

1662
01:24:20,970 --> 01:24:22,380
>> It does have types.

1663
01:24:22,380 --> 01:24:24,800
But when you declare a JavaScript
variable, you say var i.

1664
01:24:24,800 --> 01:24:25,850
You don't say it.

1665
01:24:25,850 --> 01:24:26,460
That's not a thing.

1666
01:24:26,460 --> 01:24:28,000
You just say it's a variable.

1667
01:24:28,000 --> 01:24:31,590
And then JavaScript will handle the
types under the hood for you.

1668
01:24:31,590 --> 01:24:34,390
We can freely convert between
types because of this.

1669
01:24:34,390 --> 01:24:37,360
So i starts out as a number
in this case.

1670
01:24:37,360 --> 01:24:38,790
And then I have a string.

1671
01:24:38,790 --> 01:24:39,710
And I add i to it.

1672
01:24:39,710 --> 01:24:41,810
And I reassign it back into i.

1673
01:24:41,810 --> 01:24:43,640
>> So on that first line,
i is the number.

1674
01:24:43,640 --> 01:24:47,310
On the second line, i now becomes a
string after I do the reassignment.

1675
01:24:47,310 --> 01:24:52,820
And here, I'm just concatenating
that number onto the string.

1676
01:24:52,820 --> 01:24:56,210
So you see that even though i was an
integer in the first part, it's sort

1677
01:24:56,210 --> 01:24:58,500
of like being converted to a
string and then being added

1678
01:24:58,500 --> 01:25:00,510
onto that hello string.

1679
01:25:00,510 --> 01:25:02,340
And so that's what I mean
by the loose typing.

1680
01:25:02,340 --> 01:25:04,760
That's that you convert between
types very easily.

1681
01:25:04,760 --> 01:25:08,840
And it doesn't throw warnings
at you like C does.

1682
01:25:08,840 --> 01:25:12,420
>> So i now contains hello
123 to the string.

1683
01:25:12,420 --> 01:25:13,270
Next.

1684
01:25:13,270 --> 01:25:15,610
We can also freely compare
between types.

1685
01:25:15,610 --> 01:25:19,310
So if you just use ==, very much
like in PHP, JavaScript

1686
01:25:19,310 --> 01:25:20,480
does a similar thing.

1687
01:25:20,480 --> 01:25:24,780
The string 123 is the same as the number
123 when you use double equals.

1688
01:25:24,780 --> 01:25:27,820
When used triple equals, it also
wants to make sure that

1689
01:25:27,820 --> 01:25:29,240
the type is the same.

1690
01:25:29,240 --> 01:25:31,960
So because that is a string and that is
a number, even though they're both

1691
01:25:31,960 --> 01:25:35,280
123, when you use triple
equals, you get false.

1692
01:25:35,280 --> 01:25:39,330
In the double equals case, you get true,
because double equals doesn't

1693
01:25:39,330 --> 01:25:40,020
care about type.

1694
01:25:40,020 --> 01:25:42,500
Triple equals does care about type.

1695
01:25:42,500 --> 01:25:43,750
Questions?

1696
01:25:43,750 --> 01:25:45,790

1697
01:25:45,790 --> 01:25:48,390
OK.

1698
01:25:48,390 --> 01:25:52,870
>> And another thing about JavaScript is
scope is kind of global unless you're

1699
01:25:52,870 --> 01:25:53,460
in a function.

1700
01:25:53,460 --> 01:25:55,660
And it works the same
way in PHP actually.

1701
01:25:55,660 --> 01:25:57,500
So let's go through this example.

1702
01:25:57,500 --> 01:25:59,110
I set i to 999.

1703
01:25:59,110 --> 01:26:00,900
And then I go into this for loop.

1704
01:26:00,900 --> 01:26:06,110
So if I'm printing i out in this for
loop, I expect 0, 1, 2, 3, 4.

1705
01:26:06,110 --> 01:26:07,390
I get to i = 4.

1706
01:26:07,390 --> 01:26:10,770
It increments i now to 5 at
the end of the for loop.

1707
01:26:10,770 --> 01:26:13,300
And then it breaks out of the loop,
because it doesn't fulfill the

1708
01:26:13,300 --> 01:26:14,550
condition anymore.

1709
01:26:14,550 --> 01:26:16,650
>> What do you think that next
console.log prints out?

1710
01:26:16,650 --> 01:26:20,570

1711
01:26:20,570 --> 01:26:24,790
So that's what it would do in C. In C,
because if you have like var i outside

1712
01:26:24,790 --> 01:26:28,855
and you have var i inside a loop, like a
for loop, then it makes it such that

1713
01:26:28,855 --> 01:26:30,690
it's scoped that the two
i's are different.

1714
01:26:30,690 --> 01:26:34,630
In JavaScript, it will just
treat it as the same i.

1715
01:26:34,630 --> 01:26:37,870
I get 5, because that was the value
after it exited out of the loop.

1716
01:26:37,870 --> 01:26:39,120
So those i's are the same i.

1717
01:26:39,120 --> 01:26:42,620

1718
01:26:42,620 --> 01:26:44,130
Does that make sense?

1719
01:26:44,130 --> 01:26:45,720
>> Well, it makes sense from
a JavaScript standpoint.

1720
01:26:45,720 --> 01:26:49,510
But the same paradigm doesn't
carry over to C. They have

1721
01:26:49,510 --> 01:26:50,310
different scoping rules.

1722
01:26:50,310 --> 01:26:50,630
Yes.

1723
01:26:50,630 --> 01:26:52,566
>> AUDIENCE: [INAUDIBLE]

1724
01:26:52,566 --> 01:26:54,990
outside the function [INAUDIBLE]?

1725
01:26:54,990 --> 01:26:58,340
>> JOSEPH ONG: So outside which function?

1726
01:26:58,340 --> 01:27:00,810
So I'll get to that in just a second.

1727
01:27:00,810 --> 01:27:02,580
So we call foo(i).

1728
01:27:02,580 --> 01:27:06,920
This passes i into foo, increments
it, and then logs it.

1729
01:27:06,920 --> 01:27:07,750
So it was 5.

1730
01:27:07,750 --> 01:27:09,210
So it becomes 6.

1731
01:27:09,210 --> 01:27:11,670
But what I'm talking about is
that i in that function.

1732
01:27:11,670 --> 01:27:14,430
Because it's a parameter, it's
scoped to that function.

1733
01:27:14,430 --> 01:27:17,800
So once I actually get out of that
function, it's now going to go

1734
01:27:17,800 --> 01:27:19,860
back to the old i.

1735
01:27:19,860 --> 01:27:23,000
That i is only scoped because
it's in a function.

1736
01:27:23,000 --> 01:27:24,200
And we have scope and functions.

1737
01:27:24,200 --> 01:27:29,080
But we don't have scope outside
of functions in JavaScript.

1738
01:27:29,080 --> 01:27:31,170
Does that make sense?

1739
01:27:31,170 --> 01:27:31,320
Yes.

1740
01:27:31,320 --> 01:27:31,660
Question.

1741
01:27:31,660 --> 01:27:34,030
>> AUDIENCE: Same [INAUDIBLE]?

1742
01:27:34,030 --> 01:27:34,500
>> JOSEPH ONG: So yeah.

1743
01:27:34,500 --> 01:27:36,260
In PHP, it's the same type of thing.

1744
01:27:36,260 --> 01:27:37,680
There's a slight subtlety actually.

1745
01:27:37,680 --> 01:27:40,130
But you can ask me about
that after the review.

1746
01:27:40,130 --> 01:27:43,930
You don't really need to know
that subtlety for the quiz.

1747
01:27:43,930 --> 01:27:47,600
For all intents and purposes, like
variables, global and PHP, unless

1748
01:27:47,600 --> 01:27:49,150
they're in a function, same
thing in JavaScript.

1749
01:27:49,150 --> 01:27:49,480
Yes.

1750
01:27:49,480 --> 01:27:52,890
>> AUDIENCE: Why is this allowed in
JavaScript and no where else?

1751
01:27:52,890 --> 01:27:55,010
>> JOSEPH ONG: So why is it allowed
in JavaScript and not in C?

1752
01:27:55,010 --> 01:27:58,180
It's just whoever came up with
JavaScript decided that this was OK in

1753
01:27:58,180 --> 01:27:59,510
JavaScript.

1754
01:27:59,510 --> 01:28:02,430
So it's just like a programming language
convention as we would say.

1755
01:28:02,430 --> 01:28:02,580
Yes.

1756
01:28:02,580 --> 01:28:04,480
>> AUDIENCE: So why did
it go from 6 to 5?

1757
01:28:04,480 --> 01:28:08,280
>> JOSEPH ONG: So it went from 6 to 5,
because when I passed i into foo, that

1758
01:28:08,280 --> 01:28:13,420
i inside of foo is now scoped to foo,
because scope exists in functions in

1759
01:28:13,420 --> 01:28:15,050
JavaScript.

1760
01:28:15,050 --> 01:28:18,720
But once I get out of here, because it
was scoped to the function, I'm just

1761
01:28:18,720 --> 01:28:24,300
using the regular i that was inside
the rest of the control flow.

1762
01:28:24,300 --> 01:28:25,210
Make sense?

1763
01:28:25,210 --> 01:28:26,910
Can I move on?

1764
01:28:26,910 --> 01:28:27,320
All right.

1765
01:28:27,320 --> 01:28:29,180
Cool.

1766
01:28:29,180 --> 01:28:31,890
>> The acceptance of this is objects
are passed by reference.

1767
01:28:31,890 --> 01:28:33,990
You know how when you pass
an array into C you could

1768
01:28:33,990 --> 01:28:35,810
actually modify the array?

1769
01:28:35,810 --> 01:28:37,350
It's the same thing in JavaScript.

1770
01:28:37,350 --> 01:28:42,960
If I pass an object, in this case, I
passed Milo into this catify function.

1771
01:28:42,960 --> 01:28:44,490
Milo starts out.

1772
01:28:44,490 --> 01:28:46,310
His name is Milo Banana.

1773
01:28:46,310 --> 01:28:51,670
I pass that object into a function
because it's an object, an associative

1774
01:28:51,670 --> 01:28:53,730
array in JavaScript.

1775
01:28:53,730 --> 01:28:57,110
>> When I perform an operation
in that function, it will

1776
01:28:57,110 --> 01:28:58,560
actually change the object.

1777
01:28:58,560 --> 01:29:01,720
So this will only happen for objects
in JavaScript, just like it happens

1778
01:29:01,720 --> 01:29:09,230
for arrays inside of C. So Milo's name
will actually become cat now.

1779
01:29:09,230 --> 01:29:10,090
Does that make sense?

1780
01:29:10,090 --> 01:29:11,840
So this only works for objects.

1781
01:29:11,840 --> 01:29:13,090
Objects are passed by reference.

1782
01:29:13,090 --> 01:29:13,400
Yes.

1783
01:29:13,400 --> 01:29:16,850
>> AUDIENCE: So you're saying that
in contrast to variable i.

1784
01:29:16,850 --> 01:29:17,240
>> JOSEPH ONG: Yeah.

1785
01:29:17,240 --> 01:29:20,330
Which variable i was just
a number, right?

1786
01:29:20,330 --> 01:29:25,280
It's like in C when you pass an
integer a, it makes a copy.

1787
01:29:25,280 --> 01:29:28,130
And when you pass an array, it actually
changes the actual array in

1788
01:29:28,130 --> 01:29:32,870
C. The same thing happens with
JavaScript in this case.

1789
01:29:32,870 --> 01:29:34,070
All right.

1790
01:29:34,070 --> 01:29:38,134
And next, Milo is sad because
he's now a cat.

1791
01:29:38,134 --> 01:29:40,790

1792
01:29:40,790 --> 01:29:45,060
That was actually Milo after
some trip to the vet.

1793
01:29:45,060 --> 01:29:46,860
>> So how do we use JavaScript
in a web page?

1794
01:29:46,860 --> 01:29:47,790
We can include it.

1795
01:29:47,790 --> 01:29:50,090
This is HTML code with the strip tags.

1796
01:29:50,090 --> 01:29:51,300
So I have strip tags there.

1797
01:29:51,300 --> 01:29:54,820
And then I put some JavaScript
code within the script tags.

1798
01:29:54,820 --> 01:29:56,390
And then it executes this.

1799
01:29:56,390 --> 01:29:58,830
When I just do it like this, it's
called inline JavaScript.

1800
01:29:58,830 --> 01:30:02,400
It's kind of messy, because the
JavaScript is actually in the HTML.

1801
01:30:02,400 --> 01:30:07,010
>> A better way to do this, much nicer,
is to write your JavaScript in an

1802
01:30:07,010 --> 01:30:10,920
external file and then provide
the script tag with a source.

1803
01:30:10,920 --> 01:30:14,920
And this will go to that JavaScript file
and read the JavaScript code from

1804
01:30:14,920 --> 01:30:15,960
that file instead.

1805
01:30:15,960 --> 01:30:18,820
And this way, you don't have a lot of
JavaScript at the beginning of your

1806
01:30:18,820 --> 01:30:20,760
HTML file, which makes
it really messy.

1807
01:30:20,760 --> 01:30:21,860
You just put it somewhere else.

1808
01:30:21,860 --> 01:30:24,310
And then it will read it from there.

1809
01:30:24,310 --> 01:30:25,560
Did that makes sense?

1810
01:30:25,560 --> 01:30:27,990

1811
01:30:27,990 --> 01:30:29,640
>> Placement matters.

1812
01:30:29,640 --> 01:30:32,620
In this particular case, the
script is before the body.

1813
01:30:32,620 --> 01:30:36,090
So when I execute that, there's
nothing in the body yet.

1814
01:30:36,090 --> 01:30:39,920
Maybe this will make a little bit more
sense when I show this next part.

1815
01:30:39,920 --> 01:30:43,210
In this case, the script
comes after the div.

1816
01:30:43,210 --> 01:30:46,620
So the div actually appears
on the page first.

1817
01:30:46,620 --> 01:30:49,470
Right here in this little red circle,
you see the text appears.

1818
01:30:49,470 --> 01:30:51,810
And then the alert shows up.

1819
01:30:51,810 --> 01:30:54,890
>> In the first case, because the script
was before the div, the

1820
01:30:54,890 --> 01:30:56,170
alert shows up first.

1821
01:30:56,170 --> 01:30:59,250
And then the div shows up after
you dismiss the box.

1822
01:30:59,250 --> 01:31:01,330
So the execution matters.

1823
01:31:01,330 --> 01:31:02,290
So we'll keep this in mind.

1824
01:31:02,290 --> 01:31:03,640
This will be important
in a little bit.

1825
01:31:03,640 --> 01:31:08,730

1826
01:31:08,730 --> 01:31:09,540
OK.

1827
01:31:09,540 --> 01:31:12,750
So well, how do you wait until the
entire page is loaded then before you

1828
01:31:12,750 --> 01:31:13,580
execute some code?

1829
01:31:13,580 --> 01:31:15,540
We'll get into this a little
bit later too.

1830
01:31:15,540 --> 01:31:19,016
But just keep this placement
matters in mind for when we

1831
01:31:19,016 --> 01:31:20,570
come to another slide.

1832
01:31:20,570 --> 01:31:22,030
>> So we get to DOM now.

1833
01:31:22,030 --> 01:31:23,550
And what is DOM?

1834
01:31:23,550 --> 01:31:26,830
So if you look at HTML code, it's just
a bunch of text on the screen.

1835
01:31:26,830 --> 01:31:30,560
So how does JavaScript know that
this is an HTML element?

1836
01:31:30,560 --> 01:31:33,410
So we have to have some memory
representation of this

1837
01:31:33,410 --> 01:31:34,930
structure that we have.

1838
01:31:34,930 --> 01:31:40,240
And whenever we have this in memory
representation in JavaScript, we call

1839
01:31:40,240 --> 01:31:41,750
that the DOM.

1840
01:31:41,750 --> 01:31:45,130
And it's just a way that people decided
that we should represent this

1841
01:31:45,130 --> 01:31:46,400
HTML structure as.

1842
01:31:46,400 --> 01:31:47,940
>> And what does this DOM look like?

1843
01:31:47,940 --> 01:31:50,460
Well, in memory representation,
we take this text.

1844
01:31:50,460 --> 01:31:52,870
And we turn it into memory
representation.

1845
01:31:52,870 --> 01:31:54,400
So this is the HTML.

1846
01:31:54,400 --> 01:31:58,090
So we first find out that every
DOM tree has a document.

1847
01:31:58,090 --> 01:31:59,400
It looks like a tree.

1848
01:31:59,400 --> 01:32:03,550
And the document contains the HTML
tag, actually everything

1849
01:32:03,550 --> 01:32:05,150
inside of this now.

1850
01:32:05,150 --> 01:32:06,970
The HTML tag has two children.

1851
01:32:06,970 --> 01:32:08,630
It has a head.

1852
01:32:08,630 --> 01:32:12,380
That head, if you look at indentation
over there at how it's structured

1853
01:32:12,380 --> 01:32:14,960
between the close tags,
head has a child.

1854
01:32:14,960 --> 01:32:17,130
The child is title.

1855
01:32:17,130 --> 01:32:18,370
Exactly.

1856
01:32:18,370 --> 01:32:21,000
>> Now, we have a body child.

1857
01:32:21,000 --> 01:32:24,870
And then that body has a
child called family.

1858
01:32:24,870 --> 01:32:27,950
And that family has three children--

1859
01:32:27,950 --> 01:32:29,550
oldest, middle, and youngest.

1860
01:32:29,550 --> 01:32:31,960
So you should know how to draw a diagram
like this when we ask you how

1861
01:32:31,960 --> 01:32:34,270
to draw a diagram when we give
you the HTML on the left.

1862
01:32:34,270 --> 01:32:36,350
Know how to produce the DOM tree.

1863
01:32:36,350 --> 01:32:38,930
And inside of these things, there's just
some text, which I've represented

1864
01:32:38,930 --> 01:32:40,180
as little boxes.

1865
01:32:40,180 --> 01:32:42,750

1866
01:32:42,750 --> 01:32:47,980
>> Does this DOM tree structure make
sense and what the DOM is?

1867
01:32:47,980 --> 01:32:49,300
So what does the p stand for?

1868
01:32:49,300 --> 01:32:51,850
Over here, the p over there
in that tag represents a

1869
01:32:51,850 --> 01:32:54,510
paragraph tag in HTML.

1870
01:32:54,510 --> 01:32:57,080
So you can look it up.

1871
01:32:57,080 --> 01:32:59,290
But it just means it's some
space for some text.

1872
01:32:59,290 --> 01:33:02,910
And it has some default CSS styling,
because it's a paragraph tag.

1873
01:33:02,910 --> 01:33:05,130
But don't really worry about
that part too much.

1874
01:33:05,130 --> 01:33:07,510
Just know it's a placeholder
for some text.

1875
01:33:07,510 --> 01:33:08,480
Yes.

1876
01:33:08,480 --> 01:33:10,200
Question?

1877
01:33:10,200 --> 01:33:11,021
Yes.

1878
01:33:11,021 --> 01:33:12,374
>> AUDIENCE: You just mentioned CSS.

1879
01:33:12,374 --> 01:33:15,492
The hash family and the hash
all that stuff is basically

1880
01:33:15,492 --> 01:33:17,400
representing IDs in CSS?

1881
01:33:17,400 --> 01:33:18,440
>> JOSEPH ONG: Yeah, exactly.

1882
01:33:18,440 --> 01:33:20,380
I'll get to what these hashes
mean in a second.

1883
01:33:20,380 --> 01:33:23,480
When Angela went over CSS, she
talked about CSS selectors.

1884
01:33:23,480 --> 01:33:26,770
These are the CSS selectors that
she was talking about.

1885
01:33:26,770 --> 01:33:28,268
Yes, Rob?

1886
01:33:28,268 --> 01:33:32,060
>> ROB BOWDEN: I would also comment
that DOM inside of title tag

1887
01:33:32,060 --> 01:33:35,385
is also a text node.

1888
01:33:35,385 --> 01:33:36,070
>> JOSEPH ONG: Right.

1889
01:33:36,070 --> 01:33:38,370
So inside the title tag,
I have some text DOM.

1890
01:33:38,370 --> 01:33:42,730
So really, this title should have like a
little box coming off of it as well.

1891
01:33:42,730 --> 01:33:45,740
But it doesn't really matter
too much in this case.

1892
01:33:45,740 --> 01:33:49,620
We don't really care about text nodes,
as we call them, too much.

1893
01:33:49,620 --> 01:33:50,270
OK, we do.

1894
01:33:50,270 --> 01:33:51,520
Apparently, we do.

1895
01:33:51,520 --> 01:33:54,260

1896
01:33:54,260 --> 01:33:57,100
And I will fix that when
I upload it again.

1897
01:33:57,100 --> 01:33:59,830
Does that make sense?

1898
01:33:59,830 --> 01:34:01,160
>> So how do we work with the DOM?

1899
01:34:01,160 --> 01:34:03,790
Whenever you deal with the DOM in
JavaScript, there are two steps.

1900
01:34:03,790 --> 01:34:05,030
You select a DOM element.

1901
01:34:05,030 --> 01:34:06,580
And then you do things to it.

1902
01:34:06,580 --> 01:34:11,480
So in this case, abstractly, I've
selected the middle element.

1903
01:34:11,480 --> 01:34:14,530
And then an example of doing stuff
to it would be changing the text.

1904
01:34:14,530 --> 01:34:16,020
That used to be Bob.

1905
01:34:16,020 --> 01:34:19,930
Now, what I did to it was I changed
Bob to Milo in this case.

1906
01:34:19,930 --> 01:34:22,130
>> So how do we actually do this?

1907
01:34:22,130 --> 01:34:23,440
How do we do the selecting?

1908
01:34:23,440 --> 01:34:26,560
And how do we do the doing stuff to
the thing once we've taken it?

1909
01:34:26,560 --> 01:34:30,470
Well, the way you guys have learned it
in this class is by using something we

1910
01:34:30,470 --> 01:34:32,420
called jQuery.

1911
01:34:32,420 --> 01:34:33,910
So what is jQuery?

1912
01:34:33,910 --> 01:34:37,220
jQuery is a library that makes
JavaScript easier to write.

1913
01:34:37,220 --> 01:34:39,500
So someone took the time
and wrote jQuery.

1914
01:34:39,500 --> 01:34:41,530
>> jQuery is actually written
in JavaScript.

1915
01:34:41,530 --> 01:34:44,550
And then because they did this, we now
have a whole bunch of functions that

1916
01:34:44,550 --> 01:34:46,020
we can use that make our
lives really easy.

1917
01:34:46,020 --> 01:34:48,580

1918
01:34:48,580 --> 01:34:50,030
So what are some of the
things it does?

1919
01:34:50,030 --> 01:34:51,650
It makes selecting elements easier.

1920
01:34:51,650 --> 01:34:54,020
It makes changing HTML,
adding classes easier.

1921
01:34:54,020 --> 01:34:55,360
It makes Ajax easier.

1922
01:34:55,360 --> 01:34:58,230
We'll get to that in a second.

1923
01:34:58,230 --> 01:35:00,630
>> And it's analogous to C libraries.

1924
01:35:00,630 --> 01:35:03,090
So you include string.h,
you get strlen.

1925
01:35:03,090 --> 01:35:04,680
You get strcpy, all of these things.

1926
01:35:04,680 --> 01:35:09,650
When you include jQuery, you get nice
ways to select elements to change

1927
01:35:09,650 --> 01:35:10,390
things, et cetera.

1928
01:35:10,390 --> 01:35:12,990
You get extra functionality that
JavaScript doesn't give you.

1929
01:35:12,990 --> 01:35:15,310
So jQuery is not JavaScript.

1930
01:35:15,310 --> 01:35:18,660
jQuery is a library that's written in
JavaScript that makes JavaScript

1931
01:35:18,660 --> 01:35:20,440
easier to write.

1932
01:35:20,440 --> 01:35:23,170
>> So jQuery is not a programming
language.

1933
01:35:23,170 --> 01:35:24,540
But JavaScript is.

1934
01:35:24,540 --> 01:35:27,420

1935
01:35:27,420 --> 01:35:27,590
make.

1936
01:35:27,590 --> 01:35:30,420
Sure you get your terminology right.

1937
01:35:30,420 --> 01:35:32,490
Any questions?

1938
01:35:32,490 --> 01:35:33,882
Yes.

1939
01:35:33,882 --> 01:35:35,132
Is that a question?

1940
01:35:35,132 --> 01:35:37,910

1941
01:35:37,910 --> 01:35:38,350
All right.

1942
01:35:38,350 --> 01:35:40,080
So how do you use jQuery?

1943
01:35:40,080 --> 01:35:42,390
Well, when you're writing some
JavaScript code and you include a

1944
01:35:42,390 --> 01:35:45,570
jQuery at the top of your file as a
script file, you use the dollar sign

1945
01:35:45,570 --> 01:35:47,310
now to get access to jQuery.

1946
01:35:47,310 --> 01:35:49,860
And this is different from
the dollar sign in PHP.

1947
01:35:49,860 --> 01:35:51,590
>> It's the same symbol you
type on your keyboard.

1948
01:35:51,590 --> 01:35:52,780
But they mean very different things.

1949
01:35:52,780 --> 01:35:56,090
Dollar sign in PHP means this
is how I declare a variable.

1950
01:35:56,090 --> 01:35:59,120
In JavaScript, when you've included
jQuery, it stands for jQuery.

1951
01:35:59,120 --> 01:36:01,280
So keep that in mind.

1952
01:36:01,280 --> 01:36:03,420
So how might we select DOM elements?

1953
01:36:03,420 --> 01:36:06,500
Well, when you do it the ugly JavaScript
way, you access the

1954
01:36:06,500 --> 01:36:08,240
document global variable.

1955
01:36:08,240 --> 01:36:11,170
And then you get element by ID family.

1956
01:36:11,170 --> 01:36:15,270
This is really long and wordy
and not very nice.

1957
01:36:15,270 --> 01:36:18,190
>> Or you can get all elements
that are a p tag.

1958
01:36:18,190 --> 01:36:20,080
That works too in JavaScript.

1959
01:36:20,080 --> 01:36:22,470
But we never really showed
you the syntax too much.

1960
01:36:22,470 --> 01:36:24,620
What we showed you was jQuery.

1961
01:36:24,620 --> 01:36:28,720
So that entire selector up there that
was expressed in JavaScript just gets

1962
01:36:28,720 --> 01:36:33,320
condensed to this very nice dollar
sign hashtag family.

1963
01:36:33,320 --> 01:36:38,480
And $ p, just where it's like that.

1964
01:36:38,480 --> 01:36:41,690
If you want to select all p tags
inside a family, we put a space

1965
01:36:41,690 --> 01:36:42,890
between the two.

1966
01:36:42,890 --> 01:36:44,815
And now, we get all the p
tags inside a family.

1967
01:36:44,815 --> 01:36:48,740

1968
01:36:48,740 --> 01:36:50,380
>> And look familiar?

1969
01:36:50,380 --> 01:36:52,880
Well, Angela talked about
CSS selectors.

1970
01:36:52,880 --> 01:36:54,200
Give me one second.

1971
01:36:54,200 --> 01:36:57,230
And so in order to select an element,
you just use the same thing as you

1972
01:36:57,230 --> 01:36:58,530
would do with a CSS selector.

1973
01:36:58,530 --> 01:37:00,910
If you put a hash in front
of it, it selects by ID.

1974
01:37:00,910 --> 01:37:02,220
A dot selects by classes.

1975
01:37:02,220 --> 01:37:06,230
If you just have the thing without
hashes or dots, it selects those tags.

1976
01:37:06,230 --> 01:37:07,140
Questions.

1977
01:37:07,140 --> 01:37:07,470
Yes?

1978
01:37:07,470 --> 01:37:11,510
>> AUDIENCE: When we use dot in our
HTML, is that not jQuery?

1979
01:37:11,510 --> 01:37:13,520
>> JOSEPH ONG: Dot in our HTML
is a JavaScript thing.

1980
01:37:13,520 --> 01:37:14,750
It's not a jQuery thing.

1981
01:37:14,750 --> 01:37:17,620
The way you guys learned it with
jQuery is to use .html.

1982
01:37:17,620 --> 01:37:21,510
And then you passed it whatever
the HTML is going to be.

1983
01:37:21,510 --> 01:37:23,480
So I'll get to that in just
a second actually.

1984
01:37:23,480 --> 01:37:27,800
So how do we do stuff to element
once we've selected it?

1985
01:37:27,800 --> 01:37:30,130
So that's an example of
selecting an element.

1986
01:37:30,130 --> 01:37:32,280
So now, we want to do stuff to it.

1987
01:37:32,280 --> 01:37:35,730
>> So in this case, let me go back
to the previous slide.

1988
01:37:35,730 --> 01:37:37,360
It was Bob before.

1989
01:37:37,360 --> 01:37:40,660
And I want to change that
inside HTML to Milo.

1990
01:37:40,660 --> 01:37:43,240
So I call the HTML function
from the element.

1991
01:37:43,240 --> 01:37:45,580
That HTML function is
a method of element.

1992
01:37:45,580 --> 01:37:47,430
And then I give it what
I want the HTML to be.

1993
01:37:47,430 --> 01:37:50,560
And it just replaces what's inside of
that tag with whatever I give it.

1994
01:37:50,560 --> 01:37:50,830
Yes.

1995
01:37:50,830 --> 01:37:51,170
Question?

1996
01:37:51,170 --> 01:37:57,397
>> AUDIENCE: The hashtag is used
for only the jQuery.

1997
01:37:57,397 --> 01:37:59,313
[INAUDIBLE]

1998
01:37:59,313 --> 01:38:01,230
we wouldn't use that.

1999
01:38:01,230 --> 01:38:01,960
>> JOSEPH ONG: Yeah, exactly.

2000
01:38:01,960 --> 01:38:03,750
But don't worry too much
about pure JavaScript.

2001
01:38:03,750 --> 01:38:06,670
I just want you guys to focus on how you
would do it with jQuery, because

2002
01:38:06,670 --> 01:38:10,020
that's going to be the important
part on the quiz.

2003
01:38:10,020 --> 01:38:10,400
Right.

2004
01:38:10,400 --> 01:38:10,880
Exactly.

2005
01:38:10,880 --> 01:38:16,025
So you see that hashtag, so that
corresponds to select the element with

2006
01:38:16,025 --> 01:38:18,310
the ID middle because of that hashtag.

2007
01:38:18,310 --> 01:38:19,670
Hashtag means ID.

2008
01:38:19,670 --> 01:38:22,870
And this element has an ID of middle.

2009
01:38:22,870 --> 01:38:24,366
So that's the element we select.

2010
01:38:24,366 --> 01:38:27,160
>> AUDIENCE: [INAUDIBLE].

2011
01:38:27,160 --> 01:38:31,090
dollar sign hashtag [INAUDIBLE]?

2012
01:38:31,090 --> 01:38:31,710
>> JOSEPH ONG: So no.

2013
01:38:31,710 --> 01:38:33,280
The question is can you use .value.

2014
01:38:33,280 --> 01:38:36,445
And .value only works on elements
that are inputs.

2015
01:38:36,445 --> 01:38:40,950

2016
01:38:40,950 --> 01:38:45,495
In jQuery, it would be
.val, not .value.

2017
01:38:45,495 --> 01:38:49,080
So I'll get to a small example that
demonstrates all this in combination

2018
01:38:49,080 --> 01:38:49,850
in a second.

2019
01:38:49,850 --> 01:38:53,130
But I think this serves a little snippet
makes sense to people so far.

2020
01:38:53,130 --> 01:38:55,450
Want to change the HTML,
call the HTML method.

2021
01:38:55,450 --> 01:38:56,432
Yes.

2022
01:38:56,432 --> 01:38:58,200
>> AUDIENCE: Can you explain
the method again?

2023
01:38:58,200 --> 01:39:01,900
>> JOSEPH ONG: So a method is just a
function that belongs to one, in this

2024
01:39:01,900 --> 01:39:04,590
case, one of these DOM elements,
because you see I

2025
01:39:04,590 --> 01:39:05,940
selected the element first.

2026
01:39:05,940 --> 01:39:07,320
Actually, let me use the mouse.

2027
01:39:07,320 --> 01:39:09,330
I selected the element first.

2028
01:39:09,330 --> 01:39:12,310
And then I called this HTML
function that it had.

2029
01:39:12,310 --> 01:39:15,710
And because this function belongs to
this thing, we call it a method.

2030
01:39:15,710 --> 01:39:19,480
That's just a fancy name for it.

2031
01:39:19,480 --> 01:39:20,730
Say that again.

2032
01:39:20,730 --> 01:39:22,880

2033
01:39:22,880 --> 01:39:25,170
So remember, we selected
the element now.

2034
01:39:25,170 --> 01:39:27,810
And we've put it inside of
the element variable.

2035
01:39:27,810 --> 01:39:28,600
Correct?

2036
01:39:28,600 --> 01:39:34,380
>> So when we want to change the HTML on
inside, because it was Bob before, you

2037
01:39:34,380 --> 01:39:36,420
want to change that text to Milo.

2038
01:39:36,420 --> 01:39:37,920
So we call HTML.

2039
01:39:37,920 --> 01:39:41,610
And we tell it what the HTML inside
that element should be now.

2040
01:39:41,610 --> 01:39:44,107
And so it changes it to Milo,
because I gave it Milo.

2041
01:39:44,107 --> 01:39:46,542
>> AUDIENCE: So they're working together.

2042
01:39:46,542 --> 01:39:47,030
[INAUDIBLE]

2043
01:39:47,030 --> 01:39:47,390
>> JOSEPH ONG: Yeah, yeah.

2044
01:39:47,390 --> 01:39:48,180
They're working together.

2045
01:39:48,180 --> 01:39:50,210
So one of them selects
the element first.

2046
01:39:50,210 --> 01:39:52,863
And the second one does
something to it.

2047
01:39:52,863 --> 01:39:53,790
Yes.

2048
01:39:53,790 --> 01:39:56,168
>> AUDIENCE: [INAUDIBLE].

2049
01:39:56,168 --> 01:40:01,280
If this method is different from in HTML
you have the method equal actual.

2050
01:40:01,280 --> 01:40:01,560
>> JOSEPH ONG: Yeah.

2051
01:40:01,560 --> 01:40:03,370
That is a different method.

2052
01:40:03,370 --> 01:40:04,670
That is a different method.

2053
01:40:04,670 --> 01:40:07,860
And we can cover that in just a second
when we get to an example.

2054
01:40:07,860 --> 01:40:12,000
I want to make sure that we speed up
because we're running out of time.

2055
01:40:12,000 --> 01:40:15,360
But we've run way over time now.

2056
01:40:15,360 --> 01:40:15,490
OK.

2057
01:40:15,490 --> 01:40:16,430
Cool.

2058
01:40:16,430 --> 01:40:20,130
So if you want to add a class, there's
also an add class method.

2059
01:40:20,130 --> 01:40:24,300
This is just an example of what
you can do with jQuery.

2060
01:40:24,300 --> 01:40:25,950
That just adds a class.

2061
01:40:25,950 --> 01:40:28,660
>> If you want to remove it,
you can call remove.

2062
01:40:28,660 --> 01:40:32,280
That's just another thing you can do.

2063
01:40:32,280 --> 01:40:35,680
So more examples of things you can do.

2064
01:40:35,680 --> 01:40:37,510
So can I just put it at
the top like this?

2065
01:40:37,510 --> 01:40:38,760
Youngest remove.

2066
01:40:38,760 --> 01:40:42,470

2067
01:40:42,470 --> 01:40:45,930
If I just execute that JavaScript at
the top of my file, will that work?

2068
01:40:45,930 --> 01:40:48,540

2069
01:40:48,540 --> 01:40:48,920
Right.

2070
01:40:48,920 --> 01:40:50,530
Because middle doesn't exist yet.

2071
01:40:50,530 --> 01:40:51,840
So this is not going to work.

2072
01:40:51,840 --> 01:40:52,800
Execution order.

2073
01:40:52,800 --> 01:40:55,040
It goes to the top first.

2074
01:40:55,040 --> 01:40:55,540
What?

2075
01:40:55,540 --> 01:40:57,450
>> AUDIENCE: Youngest doesn't exist yet?

2076
01:40:57,450 --> 01:40:57,810
>> JOSEPH ONG: Yeah.

2077
01:40:57,810 --> 01:40:58,710
Youngest doesn't exist yet.

2078
01:40:58,710 --> 01:40:59,600
Exactly.

2079
01:40:59,600 --> 01:41:01,320
>> AUDIENCE: You said middle.

2080
01:41:01,320 --> 01:41:01,510
>> JOSEPH ONG: Sorry.

2081
01:41:01,510 --> 01:41:02,720
Youngest doesn't exist yet.

2082
01:41:02,720 --> 01:41:04,510
And the other thing is I haven't
included the jQuery

2083
01:41:04,510 --> 01:41:06,580
file ask script src.

2084
01:41:06,580 --> 01:41:07,960
So that's not going to work.

2085
01:41:07,960 --> 01:41:09,580
Actually, I didn't do that in
the next slide, which is

2086
01:41:09,580 --> 01:41:10,700
supposed to fix that either.

2087
01:41:10,700 --> 01:41:14,120
But the way we do this is JavaScript
is event driven.

2088
01:41:14,120 --> 01:41:17,410
So what we do is we use an event
handler to make this happen.

2089
01:41:17,410 --> 01:41:19,510
And so I select the document
set first.

2090
01:41:19,510 --> 01:41:23,810
I say, OK, when the document is
ready, let me run a function.

2091
01:41:23,810 --> 01:41:25,470
So that's all that syntax means.

2092
01:41:25,470 --> 01:41:27,100
>> I selected the document.

2093
01:41:27,100 --> 01:41:29,530
Now, when the document is
ready, run the function.

2094
01:41:29,530 --> 01:41:32,970
And so over here when the document is
ready, which means all the HTML has

2095
01:41:32,970 --> 01:41:36,140
loaded, then I run the function
that removes that element.

2096
01:41:36,140 --> 01:41:40,270
And so now, when I run this function
that I passed into ready, I'm

2097
01:41:40,270 --> 01:41:43,780
guaranteed that all the HTML on the
page is going to exist first.

2098
01:41:43,780 --> 01:41:44,100
Yes.

2099
01:41:44,100 --> 01:41:44,425
Question?

2100
01:41:44,425 --> 01:41:48,200
>> AUDIENCE: What is the event keyword
within the function?

2101
01:41:48,200 --> 01:41:51,750
>> JOSEPH ONG: So that event keyword in the
function is just a parameter that

2102
01:41:51,750 --> 01:41:53,490
gets passed to the function
for any event.

2103
01:41:53,490 --> 01:41:55,470
It's just something that
you get for free.

2104
01:41:55,470 --> 01:41:59,320
When you are using key handlers in
pset8, that event could tell you, for

2105
01:41:59,320 --> 01:42:01,350
example, which key you pressed on.

2106
01:42:01,350 --> 01:42:05,540
In this case, for a ready event,
it's actually not super useful.

2107
01:42:05,540 --> 01:42:08,640
But for a key down event, it's more
useful, because you get to know which

2108
01:42:08,640 --> 01:42:12,330
key you pressed by accessing key
code off that event object.

2109
01:42:12,330 --> 01:42:13,530
Correct?

2110
01:42:13,530 --> 01:42:15,160
Does that make sense?

2111
01:42:15,160 --> 01:42:16,280
OK.

2112
01:42:16,280 --> 01:42:16,580
Yes.

2113
01:42:16,580 --> 01:42:17,150
Question?

2114
01:42:17,150 --> 01:42:19,290
>> AUDIENCE: So can you put the
script tag lower down?

2115
01:42:19,290 --> 01:42:19,940
>> JOSEPH ONG: So yeah.

2116
01:42:19,940 --> 01:42:21,500
You could put the script
tag lower down.

2117
01:42:21,500 --> 01:42:23,090
But then it just becomes really messy.

2118
01:42:23,090 --> 01:42:26,590
And we like to centralize all
of our code in one place.

2119
01:42:26,590 --> 01:42:28,290
And this will allow us to do it.

2120
01:42:28,290 --> 01:42:32,010
Remember earlier I said there's a nicer
way to ensure that elements are

2121
01:42:32,010 --> 01:42:33,880
on the page before you execute code?

2122
01:42:33,880 --> 01:42:36,079
And this is just a nice way
you would accomplish that.

2123
01:42:36,079 --> 01:42:37,329
>> AUDIENCE: [INAUDIBLE].

2124
01:42:37,329 --> 01:42:41,710

2125
01:42:41,710 --> 01:42:42,230
>> JOSEPH ONG: Yeah.

2126
01:42:42,230 --> 01:42:43,460
You would still have to, right?

2127
01:42:43,460 --> 01:42:46,930
Because remember, you included the
file at the top of the page.

2128
01:42:46,930 --> 01:42:49,890
So it's going to execute first before
you get to the bottom of the page.

2129
01:42:49,890 --> 01:42:54,600

2130
01:42:54,600 --> 01:42:55,180
OK.

2131
01:42:55,180 --> 01:42:59,210
So you can also add a different
type of event handler.

2132
01:42:59,210 --> 01:43:00,640
This one just processes clicks.

2133
01:43:00,640 --> 01:43:03,910
When I click on youngest, then
it will pop up with an alert.

2134
01:43:03,910 --> 01:43:05,440
This is just a different
type of event.

2135
01:43:05,440 --> 01:43:08,840
As opposed to the ready event, you now
use the click event when you receive

2136
01:43:08,840 --> 01:43:10,190
clicks on an element.

2137
01:43:10,190 --> 01:43:13,860
>> And so in this case, remember, the click
handler is attached to youngest.

2138
01:43:13,860 --> 01:43:16,080
So it only happens when
I click on youngest.

2139
01:43:16,080 --> 01:43:19,510
And in the other one, the ready event
was attached to the document.

2140
01:43:19,510 --> 01:43:23,750
So it waits for the document
to be ready.

2141
01:43:23,750 --> 01:43:25,120
Make sense?

2142
01:43:25,120 --> 01:43:26,190
I think I can move on.

2143
01:43:26,190 --> 01:43:26,610
Yes.

2144
01:43:26,610 --> 01:43:26,980
Question?

2145
01:43:26,980 --> 01:43:28,230
>> AUDIENCE: [INAUDIBLE].

2146
01:43:28,230 --> 01:43:31,676

2147
01:43:31,676 --> 01:43:33,620
in this case you use [INAUDIBLE].

2148
01:43:33,620 --> 01:43:36,650
>> JOSEPH ONG: Oh, yeah, because in this
case, I have to wait for the youngest

2149
01:43:36,650 --> 01:43:40,740
element to appear on the screen first
before I can attach a click handler to

2150
01:43:40,740 --> 01:43:43,062
it, which is why I put it inside
of a document ready.

2151
01:43:43,062 --> 01:43:45,780

2152
01:43:45,780 --> 01:43:46,840
OK.

2153
01:43:46,840 --> 01:43:52,390
And next, so this is a big example of
how you would combine everything.

2154
01:43:52,390 --> 01:43:55,930
This is just a form validation example
you've seen in lecture.

2155
01:43:55,930 --> 01:43:58,410
So take it step by step as
you go through this.

2156
01:43:58,410 --> 01:43:59,590
And it will be totally OK.

2157
01:43:59,590 --> 01:44:01,400
Just read it from top to bottom.

2158
01:44:01,400 --> 01:44:03,030
>> I have a form at the bottom.

2159
01:44:03,030 --> 01:44:07,590
When the document is ready, I attach a
submit handler to the form, such that

2160
01:44:07,590 --> 01:44:12,910
when I submit the form, I get the values
inside each of those inputs.

2161
01:44:12,910 --> 01:44:14,560
And I check if it's blank.

2162
01:44:14,560 --> 01:44:17,090
If it's blank, I return false, because
I don't want to submit the form,

2163
01:44:17,090 --> 01:44:18,950
because the form is wrong.

2164
01:44:18,950 --> 01:44:22,040
If the password is blank or it's less
than eight characters, I don't submit

2165
01:44:22,040 --> 01:44:24,470
the form, because that's also wrong.

2166
01:44:24,470 --> 01:44:28,150
And the return false just prevents
the form from submitting and

2167
01:44:28,150 --> 01:44:30,150
going to a new page.

2168
01:44:30,150 --> 01:44:31,310
>> And hopefully, this makes sense.

2169
01:44:31,310 --> 01:44:34,650
I think you guys should walk through
this code step by step on your own.

2170
01:44:34,650 --> 01:44:38,350
And once you understand what the select
elements and do stuff to it

2171
01:44:38,350 --> 01:44:40,520
actually entails, this will make
a lot of sense to you.

2172
01:44:40,520 --> 01:44:41,295
Yes?

2173
01:44:41,295 --> 01:44:44,150
>> AUDIENCE: What does the
name = username mean?

2174
01:44:44,150 --> 01:44:48,530
>> JOSEPH ONG: So the name = username and
name = password just means look at the

2175
01:44:48,530 --> 01:44:50,730
attribute of whatever
you're selecting.

2176
01:44:50,730 --> 01:44:51,790
And then that has to match.

2177
01:44:51,790 --> 01:44:53,870
So we go into registration.

2178
01:44:53,870 --> 01:44:56,240
And then we look at all inputs
and registration.

2179
01:44:56,240 --> 01:44:59,990
And then we pick the one where the name
attribute is equal to username.

2180
01:44:59,990 --> 01:45:04,040
So that first selector only selects
the username input.

2181
01:45:04,040 --> 01:45:08,220
And that second selector only selects
the password one, because those have

2182
01:45:08,220 --> 01:45:12,240
their name attributes set as what
they're supposed to be.

2183
01:45:12,240 --> 01:45:12,575
Question?

2184
01:45:12,575 --> 01:45:17,030
>> AUDIENCE: On submission, how does the
bottom part resolve the top part?

2185
01:45:17,030 --> 01:45:19,350
>> JOSEPH ONG: So that's because
of the event handler.

2186
01:45:19,350 --> 01:45:23,000
So we're waiting for a submit event
that gets fired from the form.

2187
01:45:23,000 --> 01:45:24,730
And that's all that is submit.

2188
01:45:24,730 --> 01:45:26,080
Why do I call submit up there?

2189
01:45:26,080 --> 01:45:28,870
It says, when the form is submitted,
I get a submit event.

2190
01:45:28,870 --> 01:45:33,480
So let me just intercept that and
then run this code instead.

2191
01:45:33,480 --> 01:45:33,823
Yes?

2192
01:45:33,823 --> 01:45:35,866
>> AUDIENCE: Why do you have
to have function event?

2193
01:45:35,866 --> 01:45:38,580
Why can't you just [INAUDIBLE]?

2194
01:45:38,580 --> 01:45:41,140
>> JOSEPH ONG: Because in JavaScript, you
have to declare the functions.

2195
01:45:41,140 --> 01:45:42,910
That's just how it works
in JavaScript.

2196
01:45:42,910 --> 01:45:44,800
You have to say it's going
to run a function.

2197
01:45:44,800 --> 01:45:47,290
So you're telling it that you're
expecting a function here instead of

2198
01:45:47,290 --> 01:45:48,260
just curly braces.

2199
01:45:48,260 --> 01:45:50,460
>> AUDIENCE: And the function
is whatever follows?

2200
01:45:50,460 --> 01:45:50,650
>> JOSEPH ONG: Yeah.

2201
01:45:50,650 --> 01:45:52,790
The function is whatever is inside
the curly braces after

2202
01:45:52,790 --> 01:45:53,630
that function keyword.

2203
01:45:53,630 --> 01:45:54,045
Yes?

2204
01:45:54,045 --> 01:45:55,295
>> AUDIENCE: [INAUDIBLE].

2205
01:45:55,295 --> 01:46:00,180

2206
01:46:00,180 --> 01:46:00,660
>> JOSEPH ONG: For submit?

2207
01:46:00,660 --> 01:46:03,520
>> AUDIENCE: No, for function
without the event.

2208
01:46:03,520 --> 01:46:03,770
>> JOSEPH ONG: Yeah.

2209
01:46:03,770 --> 01:46:05,610
So without the event,
you can have that.

2210
01:46:05,610 --> 01:46:08,480
If you don't need the event,
then you can just omit it.

2211
01:46:08,480 --> 01:46:12,220
But if you do, then you
just put it there.

2212
01:46:12,220 --> 01:46:12,520
Yes.

2213
01:46:12,520 --> 01:46:13,190
Quick question?

2214
01:46:13,190 --> 01:46:14,440
>> AUDIENCE: [INAUDIBLE].

2215
01:46:14,440 --> 01:46:21,170

2216
01:46:21,170 --> 01:46:21,440
>> JOSEPH ONG: Yeah.

2217
01:46:21,440 --> 01:46:24,550
Because what you need to do, the
document.ready just says wait for all

2218
01:46:24,550 --> 01:46:26,540
the HTML on the page to load first.

2219
01:46:26,540 --> 01:46:30,510
And usually, you want your elements
in place before you run any code.

2220
01:46:30,510 --> 01:46:31,030
All right.

2221
01:46:31,030 --> 01:46:32,180
We have to get to Ajax.

2222
01:46:32,180 --> 01:46:33,110
We don't have much time.

2223
01:46:33,110 --> 01:46:35,170
So pros and cons.

2224
01:46:35,170 --> 01:46:37,450
JavaScript is easier try
write with jQuery.

2225
01:46:37,450 --> 01:46:38,930
But jQuery is kind of slow.

2226
01:46:38,930 --> 01:46:42,290
>> It's like PHP is slower than C,
because it's interpreted.

2227
01:46:42,290 --> 01:46:45,690
And jQuery is a bit slower than
JavaScript, because it does a lot of

2228
01:46:45,690 --> 01:46:46,630
things under the hood.

2229
01:46:46,630 --> 01:46:48,660
And so if you're using jQuery, it's
just a little bit slower than

2230
01:46:48,660 --> 01:46:51,630
JavaScript, even though it
gives you nice elegance.

2231
01:46:51,630 --> 01:46:53,970
And finally, Ajax.

2232
01:46:53,970 --> 01:46:59,170
So far with Ajax, you haven't seen Ajax
in terms of pset7 yet, because

2233
01:46:59,170 --> 01:47:01,150
when you do, you submit
a form to quote.

2234
01:47:01,150 --> 01:47:02,350
It loads a new page.

2235
01:47:02,350 --> 01:47:04,440
So you get this big white flash
on the page while that

2236
01:47:04,440 --> 01:47:06,820
second page loads, correct?

2237
01:47:06,820 --> 01:47:08,780
>> It would be really nice if you
didn't have this flash.

2238
01:47:08,780 --> 01:47:11,600
Like Facebook, if you just scroll to
the bottom, it adds new content

2239
01:47:11,600 --> 01:47:13,490
without refreshing the entire page.

2240
01:47:13,490 --> 01:47:15,420
So something like this would be nice.

2241
01:47:15,420 --> 01:47:17,370
This is JavaScript code
on the left side.

2242
01:47:17,370 --> 01:47:19,390
You get what is inside of that input.

2243
01:47:19,390 --> 01:47:21,340
You get the stock info from Yahoo!

2244
01:47:21,340 --> 01:47:27,440
And then you make a big string that
says, OK, this is the message I want

2245
01:47:27,440 --> 01:47:28,400
to show on the screen.

2246
01:47:28,400 --> 01:47:32,280
And then you put that message inside
of some HTML element that gets

2247
01:47:32,280 --> 01:47:33,970
displayed on the screen.

2248
01:47:33,970 --> 01:47:35,540
>> So that's all that's happening here.

2249
01:47:35,540 --> 01:47:39,410
So basically, because this is all
JavaScript and you don't need to run

2250
01:47:39,410 --> 01:47:42,980
anymore PHP, this will make sure
that the page doesn't refresh.

2251
01:47:42,980 --> 01:47:47,470
So this is just an abstract idea
that I'm saying here for now.

2252
01:47:47,470 --> 01:47:50,630
The abstract idea is that if you do it
all in JavaScript, you don't have a

2253
01:47:50,630 --> 01:47:52,330
page refresh.

2254
01:47:52,330 --> 01:47:53,800
But how do you actually do this?

2255
01:47:53,800 --> 01:47:56,230
>> Well, actually, let's talk about
a problem with this first.

2256
01:47:56,230 --> 01:47:59,340
A problem is in JavaScript,
execution is synchronous.

2257
01:47:59,340 --> 01:48:02,000
So you have to wait for one
line to finish before you

2258
01:48:02,000 --> 01:48:03,370
execute the next line.

2259
01:48:03,370 --> 01:48:06,130
And what if I'm going over to Yahoo!,
and their servers are really slow, and

2260
01:48:06,130 --> 01:48:08,790
it takes them three seconds to
give me back that stock info?

2261
01:48:08,790 --> 01:48:12,620
When I hit that price line, if the
execution is synchronous, as it is by

2262
01:48:12,620 --> 01:48:15,390
default, what it's just going to do is
your browser is going to stall for

2263
01:48:15,390 --> 01:48:15,930
three seconds.

2264
01:48:15,930 --> 01:48:18,900
And you're not going to be able to do
anything while it gets that data.

2265
01:48:18,900 --> 01:48:20,010
It's going to be frozen.

2266
01:48:20,010 --> 01:48:20,800
>> And that's bad.

2267
01:48:20,800 --> 01:48:23,390
You don't want a user to
have a frozen web page.

2268
01:48:23,390 --> 01:48:24,170
Correct?

2269
01:48:24,170 --> 01:48:25,480
That's just bad.

2270
01:48:25,480 --> 01:48:26,770
Everyone agrees?

2271
01:48:26,770 --> 01:48:29,270
If you're browsing Facebook and it
freezes and you can't do anything, you

2272
01:48:29,270 --> 01:48:31,920
get really frustrated.

2273
01:48:31,920 --> 01:48:34,960
So the solution is we make something
asynchronous instead.

2274
01:48:34,960 --> 01:48:38,910
So all this asynchronous thing says
is, I'm going to ask this

2275
01:48:38,910 --> 01:48:40,280
URL for some data.

2276
01:48:40,280 --> 01:48:41,610
And then I'm going to keep going.

2277
01:48:41,610 --> 01:48:45,330
I'm just going to keep executing
whatever code that was after that.

2278
01:48:45,330 --> 01:48:49,290
>> And then whenever that data is ready,
then I will process it.

2279
01:48:49,290 --> 01:48:50,540
That's all it is saying.

2280
01:48:50,540 --> 01:48:52,795

2281
01:48:52,795 --> 01:48:56,710
>> AUDIENCE: Ajax just makes
code asynchronous?

2282
01:48:56,710 --> 01:48:58,560
>> JOSEPH ONG: It's an asynchronous
way of fetching data.

2283
01:48:58,560 --> 01:49:01,230
So the first thing about Ajax
is it lets me get data

2284
01:49:01,230 --> 01:49:03,170
from an external website.

2285
01:49:03,170 --> 01:49:07,045
And the second thing is it makes sure
that my page doesn't stall while I'm

2286
01:49:07,045 --> 01:49:07,970
fetching that data.

2287
01:49:07,970 --> 01:49:09,600
That's the asynchronous part of it.

2288
01:49:09,600 --> 01:49:13,040
Because it goes off somewhere else,
because I say I keep going on while

2289
01:49:13,040 --> 01:49:14,900
it's fetching that data, that
makes it asynchronous.

2290
01:49:14,900 --> 01:49:17,170
I keep executing.

2291
01:49:17,170 --> 01:49:18,960
>> So keep that asynchronous
idea in mind.

2292
01:49:18,960 --> 01:49:22,010
And I'll show you what
the difference is.

2293
01:49:22,010 --> 01:49:23,920
The synchronous version
is on the left side.

2294
01:49:23,920 --> 01:49:26,240
The asynchronous version
is on the right side.

2295
01:49:26,240 --> 01:49:29,170
Look at the numbers to see which
steps correspond to what

2296
01:49:29,170 --> 01:49:30,610
executes at each line.

2297
01:49:30,610 --> 01:49:32,730
Over there, the alert shows up first.

2298
01:49:32,730 --> 01:49:34,590
Because getting stock info from Yahoo!

2299
01:49:34,590 --> 01:49:37,250
takes three seconds, it stalls
for three seconds.

2300
01:49:37,250 --> 01:49:39,880
And then it alerts the price
after those three seconds.

2301
01:49:39,880 --> 01:49:43,690
>> So now, that alert shows
up at that time--

2302
01:49:43,690 --> 01:49:44,610
three seconds in.

2303
01:49:44,610 --> 01:49:47,670
And then it alerts by after that.

2304
01:49:47,670 --> 01:49:48,930
So it just goes step by step.

2305
01:49:48,930 --> 01:49:51,200
It's like what you guys
would accept, correct?

2306
01:49:51,200 --> 01:49:54,170
With asynchronous execution,
you alert first.

2307
01:49:54,170 --> 01:49:57,410
Then you go off to this URL.

2308
01:49:57,410 --> 01:49:59,530
And you say, I'm going to
just ask for the data.

2309
01:49:59,530 --> 01:50:01,170
And then I'm going to
process it later.

2310
01:50:01,170 --> 01:50:04,230
So it immediately executes the
next line after I make that

2311
01:50:04,230 --> 01:50:05,710
asynchronous request.

2312
01:50:05,710 --> 01:50:08,920
>> So a 0.001 seconds, you see alert hi.

2313
01:50:08,920 --> 01:50:10,960
Execute that function, alert bye.

2314
01:50:10,960 --> 01:50:14,240
And because I made a promise that I
would process the data later, what

2315
01:50:14,240 --> 01:50:17,920
happens is when that data comes back
three seconds later, then I run that

2316
01:50:17,920 --> 01:50:21,380
function that I have over there.

2317
01:50:21,380 --> 01:50:21,870
Yes?

2318
01:50:21,870 --> 01:50:25,750
>> AUDIENCE: Could you specify or
clarify what Ajax means?

2319
01:50:25,750 --> 01:50:30,460
>> JOSEPH ONG: So Ajax is a way that if I
need data when I'm on a website and I

2320
01:50:30,460 --> 01:50:34,690
don't want to refresh the page, then
I use this technology called Ajax.

2321
01:50:34,690 --> 01:50:40,630
That essentially just means, go fetch
data from another website.

2322
01:50:40,630 --> 01:50:43,724
And do it in a way that just
doesn't stall my web page.

2323
01:50:43,724 --> 01:50:46,650
>> AUDIENCE: So is that an inherent
part of JavaScript or jQuery?

2324
01:50:46,650 --> 01:50:50,590
>> JOSEPH ONG: So someone wrote a way to do
this in JavaScript a long time ago.

2325
01:50:50,590 --> 01:50:52,050
At one point, it didn't exist.

2326
01:50:52,050 --> 01:50:56,630
And so someone invented this technique
to allow people to request this data

2327
01:50:56,630 --> 01:50:57,680
in this fashion.

2328
01:50:57,680 --> 01:50:59,550
And they wrote some stuff
to do it for you.

2329
01:50:59,550 --> 01:51:01,605
And jQuery just gives you this
very nice way to do it

2330
01:51:01,605 --> 01:51:03,112
with this $.get function.

2331
01:51:03,112 --> 01:51:07,200

2332
01:51:07,200 --> 01:51:09,480
questions?

2333
01:51:09,480 --> 01:51:11,560
I can answer questions about
Ajax afterward too.

2334
01:51:11,560 --> 01:51:13,870
I'll be here.

2335
01:51:13,870 --> 01:51:16,390
>> So it let's us fetch data without
refreshing the page.

2336
01:51:16,390 --> 01:51:18,200
And it let's us do this in
an asynchronous way that

2337
01:51:18,200 --> 01:51:19,450
doesn't freeze the page.

2338
01:51:19,450 --> 01:51:22,680

2339
01:51:22,680 --> 01:51:27,310
Too long, didn't read if that
explanation was too long for you.

2340
01:51:27,310 --> 01:51:29,430
So finally, cross-site
scripting attacks.

2341
01:51:29,430 --> 01:51:30,910
We saw this with Zamyla.

2342
01:51:30,910 --> 01:51:34,285
If in my database someone has this name,
which is this script tag, and I

2343
01:51:34,285 --> 01:51:38,280
have some code on my page that prints
out people's names in a row, or I have

2344
01:51:38,280 --> 01:51:43,310
some JavaScript code that inserts
this name into the page,

2345
01:51:43,310 --> 01:51:45,680
what HTML gets produced?

2346
01:51:45,680 --> 01:51:47,290
Well, I print out the HTML tag.

2347
01:51:47,290 --> 01:51:48,390
I print out all these tags.

2348
01:51:48,390 --> 01:51:50,740
>> I get to the part where I'm printing
out with my friends.

2349
01:51:50,740 --> 01:51:52,980
I print Lauren out.

2350
01:51:52,980 --> 01:51:54,200
It print Milo out.

2351
01:51:54,200 --> 01:51:56,810
And then my name in the database
is script post

2352
01:51:56,810 --> 01:51:58,060
unflattering Facebook status.

2353
01:51:58,060 --> 01:52:00,740

2354
01:52:00,740 --> 01:52:04,330
Because I inserted this into the page
because it looks like JavaScript, when

2355
01:52:04,330 --> 01:52:07,930
this page gets sent to the user,
it gets executed as JavaScript.

2356
01:52:07,930 --> 01:52:10,800
And so this is what we call a
cross-site scripting attack.

2357
01:52:10,800 --> 01:52:14,570
Someone puts malicious information in
your database that could correspond to

2358
01:52:14,570 --> 01:52:17,080
some additional string or
some JavaScript string.

2359
01:52:17,080 --> 01:52:20,130
>> And when it gets printed out to the
page in this fashion, then what

2360
01:52:20,130 --> 01:52:22,580
happens is that bad code gets executed
that I didn't intend

2361
01:52:22,580 --> 01:52:25,110
for it to get executed.

2362
01:52:25,110 --> 01:52:28,230
And that's all a cross-site
scripting attack is.

2363
01:52:28,230 --> 01:52:31,790
And the way you get around
this is like Zamyla said.

2364
01:52:31,790 --> 01:52:34,340
You just wrap things in
HTML special chars.

2365
01:52:34,340 --> 01:52:39,460
And this HTML special chars is a PHP
function that will prevent this sort

2366
01:52:39,460 --> 01:52:42,000
of thing from happening to you
if you have a malicious

2367
01:52:42,000 --> 01:52:43,830
string in your database.

2368
01:52:43,830 --> 01:52:47,650
It just escapes it, so that it doesn't
get interpreted as HTML.

2369
01:52:47,650 --> 01:52:50,960
It replaces the little brackets
with what we call entities.

2370
01:52:50,960 --> 01:52:52,250
And we went over this in lecture too.

2371
01:52:52,250 --> 01:52:55,800
So I think you guys should have
a good grasp on that.

2372
01:52:55,800 --> 01:52:57,420
Questions?

2373
01:52:57,420 --> 01:52:57,820
Yes.

2374
01:52:57,820 --> 01:53:00,860
>> AUDIENCE: So how would
the [INAUDIBLE]?

2375
01:53:00,860 --> 01:53:01,555
>> JOSEPH ONG: Say that again.

2376
01:53:01,555 --> 01:53:02,500
>> AUDIENCE: How would the monitor--

2377
01:53:02,500 --> 01:53:02,860
>> JOSEPH ONG: Right.

2378
01:53:02,860 --> 01:53:06,080
So you have something that says, when
I register, type in my name.

2379
01:53:06,080 --> 01:53:09,390
I just type in that field, my name is
stript post unflattering Facebook

2380
01:53:09,390 --> 01:53:11,570
status close script tag.

2381
01:53:11,570 --> 01:53:15,690
And that just gets put into the
database, because I can't say someone

2382
01:53:15,690 --> 01:53:18,260
in the world doesn't have a name
with a left arrow in it or the

2383
01:53:18,260 --> 01:53:19,036
word script in it.

2384
01:53:19,036 --> 01:53:21,330
That doesn't really make sense.

2385
01:53:21,330 --> 01:53:24,560
So I just have to make sure that I
sanitize the stuff before I print it

2386
01:53:24,560 --> 01:53:25,420
out to the page.

2387
01:53:25,420 --> 01:53:27,140
>> AUDIENCE: So the HTML special cards
prevents the script tags?

2388
01:53:27,140 --> 01:53:28,710
>> JOSEPH ONG: Yeah.

2389
01:53:28,710 --> 01:53:29,960
So it doesn't prevent the script tags.

2390
01:53:29,960 --> 01:53:32,320
It just makes sure that the
script tags don't get

2391
01:53:32,320 --> 01:53:35,120
interpreted as HTML or--

2392
01:53:35,120 --> 01:53:35,400
yeah.

2393
01:53:35,400 --> 01:53:38,470
It just comes up as what
it actually is.

2394
01:53:38,470 --> 01:53:39,220
All right.

2395
01:53:39,220 --> 01:53:40,930
So that was the quiz review.

2396
01:53:40,930 --> 01:53:41,830
Cool.

2397
01:53:41,830 --> 01:53:45,088
>> [APPLAUSE]