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[ Silence ]

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>> This is CS50,
Friday of Week 0.

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Welcome back for those
returning, welcome to CS50

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for those joining us
for the first time.

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Typically, we will not in
fact have classes on Fridays,

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just this week and next for
shopping week and Labor Day.

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But thereafter we will
indeed be Monday, Wednesday.

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So, one of the side effects
perhaps of a course like this is

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that it begins to open
your eyes to all sorts

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of technical curiosities
in society

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and technical inaccuracies
in society.

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And so what I did this morning,

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this is literally how I spent
my morning, well, it was troll

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around YouTube looking for
nice incarnations of things

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that perhaps yesterday you
might not have blinked that

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but 10 weeks from now you
should absolutely poke fun

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at these people.

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So with that said, I give you a
little something from YouTube.

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>> For weeks I've been
investigating the [inaudible]

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murders with a certain
morbid fascination.

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>> This isn't real-time.

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>> I'll create a GUI
interface using Visual Basic,

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see if I can track
an IP address.

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>> So there we have, I'll create
a GUI interface using Visual

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Basic, see if I can
track an IP address.

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So, a lot of technical
words independently

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that are very much accurate
and used in technical context.

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But GUI, graphical
user interface,

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kind of [inaudible] said,

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a graphical user
interface interface,

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two using Visual Basic.

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So this is kind of a
modern show and that is kind

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of a retro language and
it's also as will be one

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of the takeaways of this
course completely irrelevant

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to what problem you are solving,

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the fact that this
engineer is using--

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proclaiming to use Visual Basic
really is not a compelling part

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of the story.

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And finally, see if I
can track an IP address.

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Well, all of that might in
fact be the goal but it's

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when you start hearing things
like this hopefully that you

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at least start to
grimace a little bit.

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And let me pull up a second one,
it turns out it's also from CSI.

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Let me make sure that the user
annotations are disabled this

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time and let's take a little
look at this one here.

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[ Music ]

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>> This is the vacation shot, it
gives us something to work with.

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>> [Inaudible] was taking
a picture of this girl.

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So, margin the reflection
in her eye.

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Just slide the image over.

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>> And there is our
victim, our program.

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>> Okay, so when you are working
with source material that looks

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like this, there is
in fact no such thing

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as infinite zoom in
computer science.

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You can not just tell the
computer to enhance that

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or can you clean that up
and then magically use zoom

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in not only on the image
but also on the reflection

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of the victim in
someone's eyeball.

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So here too are they
taking some liberties.

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But thankfully I also found in
my trolling around this morning,

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something that is perhaps
technically motivating

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and at least remains for those
of you which is many of you

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who actually use Gmail, you
might have seen a little advert

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in the top right hand corner
recently saying new beta version

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of a little something
called Gmail Priority Inbox.

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So this one is not a spoof, it's
actually a pretty cute video

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and it offers us the opportunity
to just take a look at some

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of the kinds of ideas we'll
be looking at in this course.

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So I give you your
Gmail Priority Inbox.

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If you haven't turned
it on or don't know what

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that thing is [background
music] here we go.

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[ Music ]

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>> So, if you've ever wondered,

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that is in fact how Gmail
works underneath the hood?

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So, what are the
takeaways though for us?

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So there're actually some really
interesting computer science

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ideas underlying that
we promised on Wednesday

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that computer science.

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And this course in particular
isn't at the end of the day

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about programming
even though that is

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in fact how you will spend
a good amount of your time

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and the skill that you will
in fact walk away with.

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But it's about approaching
something

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that you actually care
about, for instance, just--

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it's even something work a
day like managing your email

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and getting through the day
more efficiently and trying

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to answer a lot more cleverly,
a lot more effectively questions

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like which things
do I care about.

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I mean these are the kinds of
things the computers can in fact

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if programmed effectively
help us with.

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So this little video suggested
there's a couple of heuristics

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that a computer or that a
programmer could use to figure

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out what emails are
important to someone like me.

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We looked at what emails you
reply to if you reply in theory

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that suggest this user must care
about whoever sent this email

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or the topic they are in.

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If you actually read it,
that's a useful indicator.

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Are there other indicators
or tricks or rules of thumb

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that just off the top
of your head come to you

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as useful mechanisms
for doings this as well?

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Yeah.

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[ Inaudible Remark ]

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>> So that's a good one.

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So, whether or not the email is
addressed specifically to you

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in like the two field or
if maybe you were BCC'd

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or it's a big email list
of which you're just part,

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that could signify that this
is in fact useful for you.

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Yeah, in the back.

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[ Inaudible Remark ]

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>> So what the subject is,
okay, so if it's something like,

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you know, buy such and
such, a whole bunch of spam.

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I mean buy could actually
be a negative indicator

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that this is probably not
of interest to the user.

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Whereas please help
or dinner or something

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like that might actually be
more compelling to a human.

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Other thoughts that
come to mind.

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Yeah, in the middle.

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[ Inaudible Remark ]

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>> Okay, good.

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So in Gmail context most
of yo know, you can not--

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you can delete things
but most people just save

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or archive emails.

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So if you archive it,
that's maybe a little hint

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that either this is just
your habit or perhaps

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that you might actually care
about this later in the future,

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whereas if you delete it,
no, you really don't care.

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Yeah.

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[ Inaudible Remark ]

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>> So who the sender is, so
maybe you could keep track

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of just who your most
frequent correspondents are

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and if you keep seeing the
same person from person X

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or person Y coming into
your inbox and clearly I

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like having a dialog with these
people or have to for work,

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that might in fact
bubble up in your inbox.

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Other thoughts?

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[ Inaudible Remark ]

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>> Speed of response, right, so
I actually do tend to respond

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to some people's emails more
quickly than others and that

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in fact might be a
useful indicator.

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And if I'm already in this
habit, let my software,

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let my computer do that thinking
for me and bubble up to the top

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of my attention what I
should address first.

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And another idea?

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[ Inaudible Remark ]

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>> So how long you
leave the window open.

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So this one too is an
interesting heuristic

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and these kinds of ideas are
actually really prevalent

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in like the advertising
world, and I'm sure of sites

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like Facebook and the like
actually do keep track

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of how much time you spend
using this application

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or how much time you spend
on this particular page

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so that they can infer from that
behavior what you actually care

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about and what you might
actually look at more intensely.

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So feel free to play
with this if you like.

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I just turned it on literally
this morning to start playing

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and I'm on myself a
couple of email list,

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one of which had an email
this morning saying, "Hey,

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has anyone tried this,
it really kind of sucks."

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Whereas the other email list
said completely the opposite.

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So it's this interesting
scenario where it's probably hit

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or miss, certainly
early on and as one

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of the clips they are indicated,
these are the kinds of things

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where computers ideally
have to learn overtime.

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A priori, Google has
probably not going

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to know what you care about.

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But if watches you over
some amount of time just

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like it's been for years with
advertisements, can this kind

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of algorithm, this kind of
process presumably improve?

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So that's perhaps a hint of
where we're going in the future.

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So for those of you joining
us for the last time,

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know that the video of Wednesday

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as will generally be the
case is already online.

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You can go to cs50.net/lectures
and that will lead you to a link

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where the video is
nicely embedded in HDV.

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Thanks to our videographers
this year.

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What you'll see here is a
little clip of what I showed

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on Wednesday whereby
this semester

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for the first time we'll be
including after a few days,

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it takes a few days
of human labor to--

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we will be transcribing the
lectures in their entirety word

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for word, awkward
as this is for me

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to actually see the words
I'm saying in retrospect.

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But you will then be able
to read what's there.

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Do keyword search based on
anything that I happen to say,

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if you search for this
in a few days' time.

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And similarly can you click on
any sentence in the transcript,

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jump right to that
point in the video.

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So this is very much
online education CS50 style.

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>> And if you speak other
languages that I don't,

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you can nonetheless
compel me to speak them

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at least in subtitle form.

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So I wanted to take
a few excerpts

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from the course's syllabus since
it's not an interesting document

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to walk through line by line,
but something you should read

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at some point very soon.

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What ultimately matters
in this course is not--

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is not so much where you end
up relative to your classmates

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but where you end up in week 12
relative to yourself in week 0.

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And so this really is
the mantra of the course.

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One of the concerns out
of the gate for students

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in this course, particularly
those we dub less comfortable is

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that you're already starting
off at a disadvantage.

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My god, this student sitting
next to me was programming

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since he or she was 6.

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This person next to me
took AP computer science,

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and that may actually
be the case.

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And statistically it probably
is the case to some extents

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but it really doesn't matter.

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I mean unlike a lot
of the courses here

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and at universities, we
really do take a much more

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individualistic approach and
that is to say at the end

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of the day do we consider
both upward trending,

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both your initial experience

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and also how you approach the
course's projects throughout the

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course of the semester.

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You can nonetheless take
the course pass fail.

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I mentioned this on Wednesday.

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This is actually something
very near and dear to my heart

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because I literally would
not be doing what I do

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in life today had I not taken
a couple of courses here,

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this one in particular,
pass fail.

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It helped me in my mind
sort of take that step

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that I was otherwise just
a little too hesitant

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to take my freshman year.

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And then for a lot
of students too,

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what this mechanism does is it
just takes the edge off a course

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like this.

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Odds are if you're
an upperclassman,

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you've had that experience
where it's 12 AM, 1 AM, 2 AM,

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and you're really starting to
stress over the incompletion

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of some problems
that for some course,

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well frankly what this
mechanism allows you to do

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if you're already 90
percent of the way there,

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you can call it a night
and you can put it down

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and you could feel
good nonetheless

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about where you got to.

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And that in my mind is one of
the upsides of taking a course

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like this, pass fail, in
order to take the edge off.

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The course is not FYI
graded on the curve.

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There is no pre-determined
cutoffs between letter grades

241
00:12:07,196 --> 00:12:08,486
for those who do
take letter grades,

242
00:12:08,676 --> 00:12:11,606
and those less comfortable and
somewhere in between are not

243
00:12:11,746 --> 00:12:12,546
at a disadvantage

244
00:12:12,546 --> 00:12:14,356
as the syllabus promises
vis-a-vis those

245
00:12:14,636 --> 00:12:15,896
more comfortable.

246
00:12:15,896 --> 00:12:18,906
We do in fact-- I do have
conversations with all 30 plus

247
00:12:18,906 --> 00:12:21,406
of the teaching fellows about
all 300 plus of the students

248
00:12:21,576 --> 00:12:22,526
at the end of the semester,

249
00:12:22,866 --> 00:12:25,596
and that is in the end what
determines the outcome.

250
00:12:25,826 --> 00:12:27,626
So some of the mechanisms
that are in place now

251
00:12:27,626 --> 00:12:31,556
to make the next several
weeks among the most memorable

252
00:12:31,556 --> 00:12:32,376
of experiences.

253
00:12:32,376 --> 00:12:33,536
So with the problems, that is

254
00:12:33,536 --> 00:12:35,406
where you really will
get your hands dirty

255
00:12:35,546 --> 00:12:37,686
and truly immerse
yourself in this world

256
00:12:37,686 --> 00:12:39,186
of computer science
and programming.

257
00:12:39,436 --> 00:12:42,036
But life happens, stuff
comes up with sports,

258
00:12:42,036 --> 00:12:43,626
with student groups, with
elements and the like.

259
00:12:43,856 --> 00:12:46,516
So the course very
generously offers 9 late days

260
00:12:46,516 --> 00:12:48,346
that you can spend accurate
discretion throughout

261
00:12:48,346 --> 00:12:48,906
the semester.

262
00:12:49,126 --> 00:12:50,826
You can take one late
day on one piece set,

263
00:12:50,826 --> 00:12:52,576
9 late days on one piece set.

264
00:12:52,916 --> 00:12:56,496
It really is up to you to
manage throughout the term.

265
00:12:56,496 --> 00:13:00,266
We also drop the lowest score
on a problem set and also lest

266
00:13:00,476 --> 00:13:02,896
in my appeal in the last lecture

267
00:13:02,896 --> 00:13:05,376
in this sounds a
little too tailored

268
00:13:05,376 --> 00:13:08,516
to only those less comfortable
realize too those more

269
00:13:08,516 --> 00:13:09,856
comfortable as we'll see today.

270
00:13:10,146 --> 00:13:13,146
For those do we offer what
we call hacker editions

271
00:13:13,146 --> 00:13:14,256
of each of the problem sets.

272
00:13:14,256 --> 00:13:16,406
We do expect the majority
of the students to tackle,

273
00:13:16,606 --> 00:13:19,316
as you'll see tonight when
the PDFs go online at 7 PM,

274
00:13:19,536 --> 00:13:21,776
the so called standard
edition of each problem set.

275
00:13:22,046 --> 00:13:24,426
But if you are so inclined
whether for this problem set

276
00:13:24,426 --> 00:13:26,096
or any other throughout
the term,

277
00:13:26,096 --> 00:13:27,586
you can change your
mind week to week.

278
00:13:27,906 --> 00:13:31,186
Do you have available to you
what we call the hacker editions

279
00:13:31,186 --> 00:13:34,866
in more details on those
in the courses syllabus?

280
00:13:35,416 --> 00:13:38,166
And besides that, lest
that not be enough

281
00:13:38,566 --> 00:13:41,546
to make sure everyone can
succeed in a course like this,

282
00:13:41,756 --> 00:13:44,736
we have a very diligent teaching
fellow who not only likes

283
00:13:44,776 --> 00:13:47,096
to scribe the courses lectures.

284
00:13:47,096 --> 00:13:50,496
It's not much fun to read 20
pages of my verbal text written.

285
00:13:50,756 --> 00:13:54,586
But having a few pages of
authoritative note do exist.

286
00:13:54,666 --> 00:13:56,656
The teaching fellow for each
lecture takes what we call

287
00:13:56,656 --> 00:13:57,306
scribe notes.

288
00:13:57,306 --> 00:13:59,166
These are posted within
a few days online.

289
00:13:59,406 --> 00:14:01,116
He likes to include
some snarky comments

290
00:14:01,116 --> 00:14:02,966
on the footnotes generally
about me at my expense.

291
00:14:02,966 --> 00:14:04,536
So keep an eye up for those.

292
00:14:04,536 --> 00:14:07,406
And the point of this is so that
you don't come to class heads

293
00:14:07,406 --> 00:14:10,286
down the whole time thinking
everything I say is absolutely

294
00:14:10,286 --> 00:14:11,476
worthy of being written down.

295
00:14:11,786 --> 00:14:14,066
Ideally like we started
today, it will be a bit more

296
00:14:14,316 --> 00:14:16,606
like a conversation to
the extent that's possible

297
00:14:16,826 --> 00:14:17,686
in a theater like this.

298
00:14:17,986 --> 00:14:21,106
One hundred fifty plus office
hours is the current tally

299
00:14:21,106 --> 00:14:23,326
that you'll see starting to
appear on the course's website,

300
00:14:23,326 --> 00:14:24,956
thanks to our 60
plus person staff

301
00:14:25,236 --> 00:14:28,356
and you'll also see online soon
once you have your accounts

302
00:14:28,356 --> 00:14:31,626
as per problem set 0 spec.
For the course's website,

303
00:14:31,626 --> 00:14:35,276
there'll be support forms and
an email address, help@cs50.net,

304
00:14:35,576 --> 00:14:38,406
that you can turn to and frankly
this is more a commentary

305
00:14:38,406 --> 00:14:41,146
on our lack of social
lives during the term.

306
00:14:41,446 --> 00:14:44,646
But we pride ourselves generally
on 5 or so minute response times

307
00:14:44,646 --> 00:14:45,906
at most hours during the day.

308
00:14:47,046 --> 00:14:50,486
So, it's a sick obsession with
some of us, myself included.

309
00:14:50,696 --> 00:14:53,256
So with that said,
what can you do?

310
00:14:53,256 --> 00:14:56,396
So this is such a silly
thing, but 15 years ago,

311
00:14:56,396 --> 00:14:59,686
I took this course followed by a
course called CS51 and then sat

312
00:14:59,876 --> 00:15:02,916
down one night and then
I'm spending the next week

313
00:15:02,916 --> 00:15:06,196
on this project even though
I lived in Mather House,

314
00:15:06,196 --> 00:15:08,746
never really was the shuttling
type on a lot of quad friends

315
00:15:08,746 --> 00:15:10,696
who did, and this was
kind of interesting to me.

316
00:15:10,696 --> 00:15:12,746
There was this big printout
of the shuttle schedule

317
00:15:13,006 --> 00:15:15,066
and this was not a very
navigable document.

318
00:15:15,066 --> 00:15:18,116
Certainly this is like mid
1990s, we did have the web

319
00:15:18,116 --> 00:15:19,876
and the internet
but things were slow

320
00:15:19,876 --> 00:15:21,276
and it just wasn't
really conducive

321
00:15:21,276 --> 00:15:24,456
to actually finding things
quickly unless you really gave

322
00:15:24,456 --> 00:15:25,596
some thought to the problem.

323
00:15:25,766 --> 00:15:27,516
So I sat down over
the course of a week

324
00:15:27,516 --> 00:15:31,176
and implement what I probably
shouldn't have called Shuttleboy

325
00:15:31,176 --> 00:15:33,646
at the time 'cause this
jargon has lived on.

326
00:15:34,036 --> 00:15:37,256
And this is now a program that's
web based, it's SMS based,

327
00:15:37,256 --> 00:15:41,756
it's voice based and it used
to be Linux based as you'll see

328
00:15:41,756 --> 00:15:44,036
in a couple of weeks
and it's just a program

329
00:15:44,036 --> 00:15:46,346
that let's you click where you
are, click where you wanna go

330
00:15:46,346 --> 00:15:48,776
and voila, here are the
next several shuttles

331
00:15:48,826 --> 00:15:49,666
that you can take.

332
00:15:49,896 --> 00:15:52,356
I thought I'd give you a little
glimpse at what you can do

333
00:15:52,666 --> 00:15:54,986
for your final project
toward terms end.

334
00:15:54,986 --> 00:15:59,626
We own our own phone
number, that allows you

335
00:15:59,626 --> 00:16:02,916
to input key strokes
like you would any system

336
00:16:02,996 --> 00:16:07,446
and this is an example of
what we call CS50 Voice.

337
00:16:07,556 --> 00:16:10,586
And that's an example
of someone else's phone.

338
00:16:10,586 --> 00:16:11,206
[ Laughter ]

339
00:16:11,206 --> 00:16:12,426
>> This is CS50.

340
00:16:12,736 --> 00:16:14,546
For Shuttleboy, press 1.

341
00:16:14,906 --> 00:16:17,516
To start over, Press 0.

342
00:16:17,516 --> 00:16:20,696
What is your origin?

343
00:16:20,926 --> 00:16:27,296
For Quad, press 1; Mather, press
2; Boylston, press 3; Lamont,

344
00:16:27,666 --> 00:16:33,036
press 4; Mem Hall, press
5; to start over, press 0.

345
00:16:33,036 --> 00:16:34,906
>> So press 1 for Quad.

346
00:16:35,536 --> 00:16:40,016
>> The next shuttle leaves in
2 minutes at 1:30 PM and then

347
00:16:40,016 --> 00:16:46,846
at 1:40 PM, 1:50 PM and 2 PM.

348
00:16:46,846 --> 00:16:47,786
This is CS50.

349
00:16:47,786 --> 00:16:48,916
>> So that is only--

350
00:16:48,916 --> 00:16:49,236
[ Laughter ]

351
00:16:49,236 --> 00:16:53,656
>> That is only option 1.

352
00:16:53,656 --> 00:16:56,036
We have like 9 more
slots to fill.

353
00:16:56,036 --> 00:16:58,316
So that's 9 potential final
projects that can live

354
00:16:58,316 --> 00:17:01,246
on in telephone form
toward semester's end.

355
00:17:01,246 --> 00:17:03,896
And we will show you exactly
how you can write programs

356
00:17:03,896 --> 00:17:07,226
that actually interact
with the user in that way.

357
00:17:07,466 --> 00:17:11,396
And just as a fun aside, we also
realized as computer scientists

358
00:17:11,396 --> 00:17:13,686
that sometimes technology
is not the answer.

359
00:17:13,686 --> 00:17:16,946
This is an unfortunate thing
you see throughout education

360
00:17:16,946 --> 00:17:19,146
frankly, certainly
at the secondary

361
00:17:19,146 --> 00:17:21,386
and primary school level where
you just insert technology

362
00:17:21,386 --> 00:17:23,226
into the classroom and
expect to solve problems,

363
00:17:23,456 --> 00:17:25,636
that is not in life
always the case.

364
00:17:25,676 --> 00:17:28,626
Sometimes the simplest
solutions are in fact the best

365
00:17:28,626 --> 00:17:32,176
and thus consistent with
this constant reminder

366
00:17:32,176 --> 00:17:35,536
to ourselves was born last year
what we dubbed Shuttleboy Cards.

367
00:17:35,536 --> 00:17:37,886
Now granted we're now starting
to take this all shuttle theme

368
00:17:37,886 --> 00:17:40,536
to an extreme perhaps, but
we realized you know what,

369
00:17:40,536 --> 00:17:42,706
it's actually not that
efficient sometimes

370
00:17:42,706 --> 00:17:44,706
to call a phone number,
it's not that efficient

371
00:17:44,706 --> 00:17:45,956
to pull up the mobile web.

372
00:17:46,226 --> 00:17:48,126
Sometimes it's a lot easier
to reach into your wallet,

373
00:17:48,316 --> 00:17:50,046
pull out a piece of
plastic and voila,

374
00:17:50,246 --> 00:17:51,736
there are the next
several shuttle times.

375
00:17:51,736 --> 00:17:54,486
We mention this only because
we're just going to press

376
00:17:54,486 --> 00:17:56,856
with the latest incarnation
of this this year.

377
00:17:57,096 --> 00:17:59,556
So if you go back to
shuttleboy.cs50.net,

378
00:17:59,836 --> 00:18:02,446
you can pre-order for free
your own CS50 shuttle card,

379
00:18:02,446 --> 00:18:05,836
particularly those of you in
the Quad, Mather and the like.

380
00:18:06,076 --> 00:18:09,916
So, what does that mean as
you exit a course like this?

381
00:18:09,916 --> 00:18:12,486
Well really, the things we've
looked at thus far can you do

382
00:18:12,486 --> 00:18:15,816
after just a semester of this
kind of material and exposure

383
00:18:15,816 --> 00:18:17,246
to this particular world.

384
00:18:17,476 --> 00:18:18,976
I just wanted to draw
your attention to some

385
00:18:18,976 --> 00:18:21,386
of the handouts we had Wednesday
which are still available today

386
00:18:21,386 --> 00:18:22,926
if you didn't yet get them.

387
00:18:22,926 --> 00:18:26,186
This is targeted for the
most part at students

388
00:18:26,186 --> 00:18:28,576
who might want a major or
minor in computer science,

389
00:18:28,616 --> 00:18:30,516
but there's also some
interesting detail toward the

390
00:18:30,516 --> 00:18:33,146
end of this document, if and
when you get a chance to peruse,

391
00:18:33,146 --> 00:18:35,646
including this chart that
shows you the pathways

392
00:18:35,646 --> 00:18:37,776
that you can take throughout
computer science here,

393
00:18:38,006 --> 00:18:40,626
sort of recommend it or popular
courses that students tend

394
00:18:40,626 --> 00:18:43,526
to take even if they are in
the humanities or life sciences

395
00:18:43,526 --> 00:18:47,106
or social sciences but want to
round out their education here

396
00:18:47,106 --> 00:18:50,236
with something in the computing
sciences do we have quads

397
00:18:50,366 --> 00:18:54,536
of courses recommended in the
back that you can take after 50

398
00:18:54,536 --> 00:18:56,376
in order to get a
minor in this stuff.

399
00:18:56,556 --> 00:18:59,426
And frankly this kind of stuff,
this visualization of data,

400
00:18:59,426 --> 00:19:02,966
this is one of the themes of
a course here called CS171,

401
00:19:02,966 --> 00:19:05,126
visualizations, which
we'll actually learn--

402
00:19:05,156 --> 00:19:07,726
hear a bit about toward the end
of the semester and it's all

403
00:19:07,726 --> 00:19:09,896
about taking data that
might very well have come

404
00:19:09,896 --> 00:19:12,196
out of a computer and
trying to decide what is

405
00:19:12,196 --> 00:19:14,556
in fact the best way
to present this data.

406
00:19:14,836 --> 00:19:17,466
To be honest, especially those
of you less comfortable here,

407
00:19:17,466 --> 00:19:21,076
if you've ever struggled with
some program or struggled

408
00:19:21,076 --> 00:19:22,326
with some piece of software,

409
00:19:22,616 --> 00:19:24,336
oh it's alright,
it's not your fault.

410
00:19:24,336 --> 00:19:26,486
It just really wasn't
designed very well.

411
00:19:26,706 --> 00:19:29,586
So actually thinking about how
humans interact with machines

412
00:19:29,586 --> 00:19:32,866
and how you can optimize
your so called user interface

413
00:19:33,146 --> 00:19:36,496
or GUI interface is
also one of the themes

414
00:19:36,576 --> 00:19:38,246
in this particular field.

415
00:19:38,246 --> 00:19:42,746
So, we ended Wednesday with
some quotes, particularly

416
00:19:42,746 --> 00:19:45,206
from isawyouharvard.com,
we thought it would be fun

417
00:19:45,206 --> 00:19:46,556
to just give you a
couple of teasers

418
00:19:46,556 --> 00:19:49,616
from another favorite
website of ours,

419
00:19:49,616 --> 00:19:52,826
a little something called
Harvard FML, search for CS50

420
00:19:52,866 --> 00:19:55,446
and you see some
choice quotes like this.

421
00:19:56,706 --> 00:19:58,576
I talk about CS50 so much

422
00:19:58,576 --> 00:20:00,956
that my teammates
implemented a CS50 talk tax.

423
00:20:01,226 --> 00:20:03,206
>> I get charged one dollar
for each mention of the class.

424
00:20:03,696 --> 00:20:06,086
I already owed 20 dollars
and haven't been able to talk

425
00:20:06,086 --> 00:20:07,106
for more than five minutes

426
00:20:07,206 --> 00:20:10,256
without getting called
out on it FML.

427
00:20:11,086 --> 00:20:13,726
This is what happens over
time as does-- whoops!

428
00:20:13,726 --> 00:20:14,386
Did I miss one there?

429
00:20:14,756 --> 00:20:17,756
This is what may, in fact,
happen to you over time.

430
00:20:17,846 --> 00:20:19,606
We are a recurring
theme it seems

431
00:20:19,606 --> 00:20:20,996
on that particular website.

432
00:20:21,386 --> 00:20:23,606
So now, let's make things
a little more technical

433
00:20:23,606 --> 00:20:24,056
for a moment.

434
00:20:24,056 --> 00:20:26,346
Those of you more comfortable,
rest assured we'll start filling

435
00:20:26,346 --> 00:20:27,596
in some gaps before long

436
00:20:27,596 --> 00:20:28,786
if you're already
familiar with this.

437
00:20:29,096 --> 00:20:32,186
But all of you in this room
probably remember what we

438
00:20:32,246 --> 00:20:35,956
technically call the base
10 system or decimal system

439
00:20:35,956 --> 00:20:39,146
because back in grade school or
the like, you learned to count

440
00:20:39,146 --> 00:20:42,126
and add and subtract
numbers by way of columns.

441
00:20:42,156 --> 00:20:45,516
The so called ones column on
the right, the tens column next

442
00:20:45,516 --> 00:20:48,886
to it, the hundreds column, the
thousands column, and so forth.

443
00:20:48,886 --> 00:20:53,756
So if you had some number back
in grade school like 123, well,

444
00:20:53,756 --> 00:20:56,956
why do we express this as 123?

445
00:20:56,996 --> 00:20:59,386
When really I just see
on the board 1, 2, 3?

446
00:20:59,666 --> 00:21:00,686
Well, we have what?

447
00:21:01,016 --> 00:21:03,766
One in the so called
hundreds columns, a two in the

448
00:21:03,766 --> 00:21:07,476
so called tens column so that's
10 times 1 plus 10 times 2 is

449
00:21:07,476 --> 00:21:11,806
120, plus 3 times
1, 123 alright,

450
00:21:11,806 --> 00:21:15,296
so completely uninteresting, if
this is something long familiar

451
00:21:15,296 --> 00:21:17,136
to you, but it turns
out computers work

452
00:21:17,136 --> 00:21:18,466
in exactly the same way.

453
00:21:18,686 --> 00:21:21,646
The so called binary
system, these zeros and ones

454
00:21:21,906 --> 00:21:24,576
that you'd probably have
some vague sense underlie all

455
00:21:24,576 --> 00:21:26,676
computers today actually reduced

456
00:21:26,676 --> 00:21:30,336
to the exact same grade school
kind of math and system except

457
00:21:30,546 --> 00:21:32,946
with computers, because
it's a lot easier

458
00:21:32,946 --> 00:21:36,146
to use just two digits,
zero and one.

459
00:21:36,436 --> 00:21:39,726
Do they use only columns
like with the ones column,

460
00:21:39,906 --> 00:21:43,046
the twos columns, the fours
column, and the eights column?

461
00:21:43,276 --> 00:21:46,506
And if you noticed earlier that
these are what we'd call powers

462
00:21:46,506 --> 00:21:48,896
of ten, these are
just powers of two.

463
00:21:49,066 --> 00:21:50,466
So if you need more--

464
00:21:50,466 --> 00:21:53,116
bigger numbers, you
just have a 16's column

465
00:21:53,116 --> 00:21:55,406
and a 32's column, and so forth.

466
00:21:55,446 --> 00:21:56,726
So what's the takeaway here?

467
00:21:56,916 --> 00:21:59,346
Well, if we actually want
to represent something

468
00:21:59,346 --> 00:22:03,466
like the number 7,
well in binary,

469
00:22:03,516 --> 00:22:05,896
not actually in decimal
like this.

470
00:22:06,166 --> 00:22:07,046
Well, what do we do?

471
00:22:07,046 --> 00:22:11,046
If we need a 7, this is the ones
column, the twos, fours, eights,

472
00:22:11,786 --> 00:22:14,506
so here are our columns and this
was our decimal system a moment

473
00:22:14,506 --> 00:22:16,626
ago, and I wanna
express the number 7

474
00:22:16,626 --> 00:22:20,976
and the only digits I now have
access to are zero and one,

475
00:22:21,156 --> 00:22:23,046
whereas, in decimal,
dec meaning 10.

476
00:22:23,046 --> 00:22:25,676
I had zero to 9, which
was 10, zero and 1,

477
00:22:25,676 --> 00:22:27,676
bi meaning two, high
binary numbers.

478
00:22:28,156 --> 00:22:30,846
I can only put a zero or
1 in each of these slides.

479
00:22:31,056 --> 00:22:33,486
But after that, the math is
the same, eight times whatever,

480
00:22:33,486 --> 00:22:36,026
four times whatever, two
times whatever, and so forth.

481
00:22:36,106 --> 00:22:36,776
So what's 7?

482
00:22:38,356 --> 00:22:40,986
>> Right, one, one,
one and then a zero.

483
00:22:41,156 --> 00:22:42,646
And just like in
the decimal system,

484
00:22:42,646 --> 00:22:45,066
you can put as many
leading zeros, any--

485
00:22:45,066 --> 00:22:46,606
as many zeros on the
left that you want.

486
00:22:46,606 --> 00:22:48,056
They're completely
inconsequential

487
00:22:48,316 --> 00:22:49,676
and so we now have the number 7.

488
00:22:49,676 --> 00:22:50,956
Meanwhile, the number zero is?

489
00:22:51,846 --> 00:22:54,856
Alright, it's all zeros so
I'll-- just for consistency,

490
00:22:54,856 --> 00:22:57,496
use four of them here, but
that's just an aesthetic.

491
00:22:57,696 --> 00:23:00,116
And I mentioned on Wednesday
that computer scientists do

492
00:23:00,116 --> 00:23:02,506
in fact start counting
from zero not one.

493
00:23:02,786 --> 00:23:04,006
Well, this is pretty much why.

494
00:23:04,006 --> 00:23:06,546
Because if you ever hold--
the numbering system reduces

495
00:23:06,546 --> 00:23:10,076
to this basic idea of zeros
and ones, each of which is

496
00:23:10,076 --> 00:23:11,316
in a certain column, each

497
00:23:11,316 --> 00:23:13,496
of which has some
weights associated

498
00:23:13,496 --> 00:23:15,026
with it, it's just natural.

499
00:23:15,026 --> 00:23:16,936
It's obvious to start
counting at zero.

500
00:23:16,936 --> 00:23:20,496
Otherwise, you're wasting a
bit, so to speak, unnecessarily.

501
00:23:20,496 --> 00:23:24,256
And a bit, B-I-T, just a
shorthand for binary digit,

502
00:23:24,576 --> 00:23:26,706
so a digit of zero over
a one, bi means two,

503
00:23:26,706 --> 00:23:27,686
you only have two digits.

504
00:23:27,686 --> 00:23:30,166
Hence, the word bit was born.

505
00:23:30,406 --> 00:23:32,136
So just as a little
sanity check,

506
00:23:32,256 --> 00:23:35,356
number 15 in binary would be?

507
00:23:36,096 --> 00:23:42,026
Just four ones, and how
about the number 65,535?

508
00:23:42,836 --> 00:23:46,156
Anyone? You know
you're a nerd if--

509
00:23:46,516 --> 00:23:49,516
so 16 of those ones will give
you that particular number.

510
00:23:49,516 --> 00:23:51,136
And I can't, myself,
go higher than that

511
00:23:51,326 --> 00:23:53,086
but you could certainly
do it mathematically.

512
00:23:53,086 --> 00:23:54,276
Now this is not the
kind of stuff

513
00:23:54,276 --> 00:23:56,036
that computer scientists
spend all their time doing,

514
00:23:56,306 --> 00:23:57,496
but it will germane.

515
00:23:57,496 --> 00:23:59,696
In fact, midway through
the semester, when we look

516
00:23:59,696 --> 00:24:03,296
at the notion of digital
forensics and recovering things

517
00:24:03,326 --> 00:24:06,476
like JPEGS and representing
files like JPEGS and JIPS

518
00:24:06,476 --> 00:24:08,616
and the like on disk
we're actually gonna have

519
00:24:08,616 --> 00:24:12,696
to revisit this idea of actually
storing information in zero

520
00:24:12,696 --> 00:24:16,346
or one form and so it's
going to become relevant

521
00:24:16,566 --> 00:24:18,616
as to how you represent
these numbers on disk,

522
00:24:18,616 --> 00:24:20,316
how you manipulate
these numbers on disk,

523
00:24:20,676 --> 00:24:24,436
but this really only gives us
the ability to express numbers

524
00:24:24,476 --> 00:24:25,596
at this point in time.

525
00:24:25,906 --> 00:24:27,876
We ultimately wanna be
able to express letters

526
00:24:27,876 --> 00:24:29,606
so we can implement
things like Gmail

527
00:24:29,606 --> 00:24:31,646
and actually implement
text on the screen.

528
00:24:31,996 --> 00:24:32,976
So how do we do that?

529
00:24:33,286 --> 00:24:38,216
Well first, a cartoon that you
might now actually get those

530
00:24:38,596 --> 00:24:39,846
otherwise unfamiliar.

531
00:24:40,516 --> 00:24:47,696
[ Pause ]

532
00:24:48,196 --> 00:24:50,466
>> See this happens every
semester that the chuckles,

533
00:24:50,466 --> 00:24:52,546
the grimaces, kind of
roll through the crowd

534
00:24:52,546 --> 00:24:54,586
as people catch on,
but there you go.

535
00:24:54,586 --> 00:24:56,056
Now, you get geek humor.

536
00:24:56,056 --> 00:24:58,446
And for better or for worse
we've done a lot of damage

537
00:24:58,446 --> 00:24:59,716
over the past few
years for this stuff.

538
00:24:59,976 --> 00:25:02,256
When you get more and more
acquainted with geek humor

539
00:25:02,256 --> 00:25:05,506
from the likes of this is
Foxtrot XKCD is popular,

540
00:25:05,506 --> 00:25:08,316
Ph.D. comics, there's a whole
bunch of geekery out there

541
00:25:08,316 --> 00:25:11,146
that will perhaps turn to-- so
that you do exit this course

542
00:25:11,146 --> 00:25:13,986
as a complete side effect
with an understanding

543
00:25:13,986 --> 00:25:16,866
as to why it was funny or at
least awkward that we did,

544
00:25:16,866 --> 00:25:20,016
in fact on Wednesday, have
a CAT pictured in a ceiling.

545
00:25:20,296 --> 00:25:24,496
So, how do you go from just
numbers, zeros and ones

546
00:25:24,496 --> 00:25:26,126
to actually useful information?

547
00:25:26,256 --> 00:25:29,566
Well it turns out a bunch of
years ago people just decided

548
00:25:29,836 --> 00:25:32,606
that if we have the
ability to express numbers

549
00:25:32,796 --> 00:25:35,076
and we actually care
about expressing letters

550
00:25:35,076 --> 00:25:37,566
of the alphabet, well we just
need some kind of mapping

551
00:25:37,566 --> 00:25:38,856
between one and the other,

552
00:25:39,076 --> 00:25:41,786
and so thus was born what's
generally called the ASCII.

553
00:25:42,086 --> 00:25:44,936
This is just an acronym that
you still see and use today,

554
00:25:44,936 --> 00:25:47,206
though there were newer
systems that are more expressive

555
00:25:47,256 --> 00:25:49,906
for other languages,
non-romance languages.

556
00:25:50,156 --> 00:25:51,486
But ASCII is just a code.

557
00:25:51,656 --> 00:25:53,256
This might look a
little cryptic at first,

558
00:25:53,446 --> 00:25:56,276
but it turns there is some
very obvious patterns here,

559
00:25:56,276 --> 00:25:56,936
for instance.

560
00:25:56,936 --> 00:26:01,136
According to this chart, if you
want to represent the letter A

561
00:26:01,136 --> 00:26:03,776
in the so called ASCII system,

562
00:26:03,776 --> 00:26:06,216
what number do you
actually use to do so?

563
00:26:06,716 --> 00:26:07,806
>> Ninety-seven?

564
00:26:07,806 --> 00:26:08,396
>> So, okay.

565
00:26:08,396 --> 00:26:13,086
So lower case A would be 97
and upper case A would be 65.

566
00:26:13,316 --> 00:26:14,646
So this is kind of
a nice chichi.

567
00:26:14,686 --> 00:26:16,306
Now, once you know
each of those,

568
00:26:16,306 --> 00:26:18,586
you can now do 25 more
upper case letters

569
00:26:18,586 --> 00:26:21,406
and 25 more lower case letters
then there are some other stuff

570
00:26:21,406 --> 00:26:23,256
like punctuation and things
you see on the keyboard.

571
00:26:23,256 --> 00:26:25,796
All of those do need their
own encodings, but at the end

572
00:26:25,796 --> 00:26:27,656
of the day, if you already
knew coming into this class

573
00:26:28,056 --> 00:26:30,786
that your computer stores
information, somehow,

574
00:26:31,036 --> 00:26:33,616
using zeros and ones,
well it turns out that

575
00:26:33,616 --> 00:26:37,316
if you have typed the letter A
on your keyboard and thus need

576
00:26:37,316 --> 00:26:39,796
to store that inside
of your computer, well,

577
00:26:39,796 --> 00:26:44,256
how do you actually go from
numbers to that letter and back?

578
00:26:44,886 --> 00:26:47,636
Well again, we can go to
our ones column, twos,

579
00:26:47,726 --> 00:26:51,646
four, eight, 16, damn, 32.

580
00:26:51,646 --> 00:27:00,996
So we can go to 32 and then
we can go to 64, and then 128,

581
00:27:00,996 --> 00:27:04,836
and this gives me 1,
2, 3, 4, 5, 6, 7, 8.

582
00:27:05,276 --> 00:27:06,326
So this is intentional.

583
00:27:06,326 --> 00:27:08,846
I now have eight columns,
though sloppily written there,

584
00:27:09,126 --> 00:27:10,806
eight columns or 8 bits.

585
00:27:11,176 --> 00:27:12,876
So this is what we
know as a byte.

586
00:27:13,166 --> 00:27:15,286
You've probably been surrounded
for years now by bytes

587
00:27:15,286 --> 00:27:17,746
and kilobytes, and megabytes
and terabytes, and the like.

588
00:27:17,956 --> 00:27:20,206
Well, this is just a
way of saying not bits,

589
00:27:20,536 --> 00:27:22,806
but units of eight
bits at a time.

590
00:27:22,806 --> 00:27:23,786
Why is this useful?

591
00:27:23,786 --> 00:27:25,996
Well one bit, really
not all that useful,

592
00:27:25,996 --> 00:27:27,726
two bits still not
all that useful.

593
00:27:27,726 --> 00:27:28,236
Eight bits?

594
00:27:28,356 --> 00:27:30,136
Starting to get useful because,

595
00:27:30,136 --> 00:27:33,286
in fact with eight bits can
you implement things like this?

596
00:27:33,786 --> 00:27:38,586
So with eight bits can you
express 256 possible characters?

597
00:27:38,936 --> 00:27:40,086
That's pretty good 'cause now,

598
00:27:40,086 --> 00:27:42,416
I can pretty much cover my
American keyboard pretty

599
00:27:42,416 --> 00:27:44,376
effectively both upper
case, lower case,

600
00:27:44,376 --> 00:27:45,826
and a whole bunch
of punctuation.

601
00:27:46,046 --> 00:27:49,696
For other languages as in aside,
there are other encoding schemes

602
00:27:49,826 --> 00:27:51,076
when there are more characters.

603
00:27:51,286 --> 00:27:54,236
So what would be the
letter A in binary inside

604
00:27:54,236 --> 00:27:56,546
of a computer's memory
when you hit the letter A

605
00:27:56,546 --> 00:27:58,666
on the keyboard, for capital A?

606
00:27:59,696 --> 00:28:01,506
>> Zero, one, zero, zero,
zero, zero, zero, one.

607
00:28:01,896 --> 00:28:05,186
>> Zero, one, zero, zero,
zero, zero, zero, one.

608
00:28:05,426 --> 00:28:08,636
And so this, in fact, when
you say your computer stores

609
00:28:08,676 --> 00:28:10,176
information as zeros and ones,

610
00:28:10,436 --> 00:28:11,986
you've just typed
the capital letter A,

611
00:28:11,986 --> 00:28:14,046
that's all that's
going on inside.

612
00:28:14,226 --> 00:28:16,076
Now, how is that actually
being implemented?

613
00:28:16,076 --> 00:28:17,576
It actually depends
on the technology.

614
00:28:17,576 --> 00:28:19,686
You probably know your computer
has something called RAM

615
00:28:19,976 --> 00:28:20,926
which is where documents

616
00:28:20,926 --> 00:28:23,516
and programs live just
temporarily while you're running

617
00:28:23,516 --> 00:28:25,056
them 'cause RAM tends
to be pretty fast

618
00:28:25,056 --> 00:28:27,656
and whereas your hard
disk is where programs

619
00:28:27,656 --> 00:28:29,076
and files live permanently.

620
00:28:29,076 --> 00:28:31,796
You can pull the cord and your
files should not actually get

621
00:28:31,796 --> 00:28:33,896
deleted but the hard disks are--

622
00:28:33,896 --> 00:28:35,666
use a different technology
altogether.

623
00:28:35,666 --> 00:28:38,326
But at the end of the day,
and here's why we just went

624
00:28:38,326 --> 00:28:40,736
with binary and not
decimal years ago,

625
00:28:41,236 --> 00:28:44,596
when you have one implement a
computer with a physical device,

626
00:28:45,306 --> 00:28:47,046
it's actually really easy

627
00:28:47,226 --> 00:28:50,636
to represent the
notion of on or off.

628
00:28:50,826 --> 00:28:51,856
Consider a light switch.

629
00:28:51,856 --> 00:28:54,666
You can go on or you
can go off and actually,

630
00:28:54,926 --> 00:28:56,656
how about some gratuitous
use of technology?

631
00:28:56,976 --> 00:28:59,456
For those of you for--
with iPhone 4, by far,

632
00:28:59,456 --> 00:29:03,216
the most useful application
for 99 cents is this flashlight

633
00:29:03,216 --> 00:29:03,946
in the middle of the night.

634
00:29:04,436 --> 00:29:06,196
So this might represent a one.

635
00:29:06,756 --> 00:29:08,256
This now represents a zero.

636
00:29:08,536 --> 00:29:11,996
One, zero, one-- promise it
gets more interesting, zero.

637
00:29:12,426 --> 00:29:14,516
So why is this germane?

638
00:29:14,516 --> 00:29:17,756
Well, if you're implementing
a computer that needs

639
00:29:17,756 --> 00:29:20,066
to represent information now
all you need is something

640
00:29:20,066 --> 00:29:23,826
like electricity which can
be on or off, on or off.

641
00:29:23,826 --> 00:29:25,706
And when you actually wanna
store information on disk,

642
00:29:25,706 --> 00:29:27,936
can you actually
use, as we'll see

643
00:29:27,936 --> 00:29:29,696
in a moment, magnetic particles.

644
00:29:29,756 --> 00:29:33,316
But just to hammer home this
point, if I could, we have time

645
00:29:33,366 --> 00:29:35,876
for one awkward demonstration.

646
00:29:36,646 --> 00:29:40,836
And for that I need a byte
of volunteers, alright?

647
00:29:41,486 --> 00:29:44,056
First eight hands that I
saw we're roughly in here.

648
00:29:44,056 --> 00:29:48,706
So come on up and I'll-- okay,
one, two, three, four, five--

649
00:29:48,706 --> 00:29:52,216
five, six, seven, eight.

650
00:29:52,216 --> 00:29:53,396
But you have to be
comfortable on stage,

651
00:29:53,396 --> 00:29:56,056
that's the catch 'cause
this will be going

652
00:29:56,056 --> 00:29:56,706
out on the Internet.

653
00:29:57,636 --> 00:30:01,036
One, two, three, four, five,
six, seven-- sure, come on up,

654
00:30:01,036 --> 00:30:02,936
and the eighth person
to stand up right here.

655
00:30:03,236 --> 00:30:04,706
>> Come on up.

656
00:30:04,706 --> 00:30:07,636
Okay. So we now have a
byte's worth of volunteers.

657
00:30:07,636 --> 00:30:09,516
Why don't we use this
side of the stage

658
00:30:09,516 --> 00:30:10,786
and I'll make some room for you.

659
00:30:11,016 --> 00:30:15,986
You are the 128th's column,
you're the 64's, 32's, 16's,

660
00:30:17,306 --> 00:30:20,916
8's, and we're gonna have to
fix this slightly in a moment.

661
00:30:22,346 --> 00:30:24,396
Otherwise the math is gonna
get completely screwed up.

662
00:30:24,666 --> 00:30:26,566
So if you guys could
come over here

663
00:30:26,566 --> 00:30:28,106
but still maintain
your alignment

664
00:30:28,106 --> 00:30:30,666
so 128 should be all the
way over here and one,

665
00:30:30,856 --> 00:30:32,736
you should be all
the way over here

666
00:30:32,926 --> 00:30:34,606
and you guys similarly
need to shuffle.

667
00:30:35,356 --> 00:30:37,326
And if you wanna come
up closer to the stage

668
00:30:37,326 --> 00:30:40,626
where everyone can see you?

669
00:30:41,016 --> 00:30:44,526
Alright! We now have a
byte's worth of volunteers.

670
00:30:44,746 --> 00:30:46,966
Now fortunately this time,
it's not speaking role.

671
00:30:46,966 --> 00:30:51,606
You just need to express in
ASCII character, one at a time,

672
00:30:51,816 --> 00:30:54,546
and these brave souls are
simply going to spell something

673
00:30:54,546 --> 00:30:56,786
out for us to hammer home
this point of binary.

674
00:30:57,006 --> 00:30:59,306
Now, each of you is essentially
a place holder or a column.

675
00:30:59,476 --> 00:31:02,186
Why don't we have you, when
the time comes, raise your hand

676
00:31:02,286 --> 00:31:04,196
if you're supposed to
be representing a one?

677
00:31:04,526 --> 00:31:08,236
So on, and or true, is another
way we generally put this.

678
00:31:08,456 --> 00:31:11,166
And if you're our zero just
continue standing there

679
00:31:11,166 --> 00:31:12,206
awkwardly, alright.

680
00:31:12,676 --> 00:31:15,026
So with that said, we
have a three-letter word

681
00:31:15,026 --> 00:31:17,476
so we'll have three rounds
of hands going up or down.

682
00:31:17,696 --> 00:31:19,046
On the back, they
have a little chichi

683
00:31:19,046 --> 00:31:20,366
that tells them what to do.

684
00:31:20,636 --> 00:31:24,266
So folks [laughter], otherwise
this gets really awkward

685
00:31:24,266 --> 00:31:26,346
and really inaccurate
quickly under pressure.

686
00:31:26,346 --> 00:31:29,396
So go ahead, round one.

687
00:31:31,916 --> 00:31:32,616
That's okay.

688
00:31:33,346 --> 00:31:37,386
So what number first
are they presenting?

689
00:31:37,476 --> 00:31:40,366
Sixty-six, right 'cause we've
got one in the 64's column

690
00:31:40,366 --> 00:31:42,016
and a one in the
two's column, okay.

691
00:31:42,176 --> 00:31:43,796
So now, a little chichi up top,

692
00:31:43,796 --> 00:31:44,916
they are representing
the letter?

693
00:31:44,916 --> 00:31:44,983
>> B.

694
00:31:45,716 --> 00:31:49,236
>> Okay B. Alright, so a little
pause, just to let that sink in.

695
00:31:50,256 --> 00:31:53,066
Okay, round two,
execute [laughter].

696
00:32:00,846 --> 00:32:01,946
They really-- it
says on the back

697
00:32:01,946 --> 00:32:03,086
of your seat what
you should be doing.

698
00:32:04,456 --> 00:32:06,366
Alright, what number are
they representing now?

699
00:32:06,866 --> 00:32:08,036
>> Seventy-nine.

700
00:32:08,236 --> 00:32:08,986
>> Seventy-nine.

701
00:32:09,346 --> 00:32:10,986
In fact, if you add up
each of these numbers,

702
00:32:10,986 --> 00:32:13,196
you should get the
decimal number 79

703
00:32:13,196 --> 00:32:14,586
from this binary number here.

704
00:32:14,836 --> 00:32:18,146
Seventy-nine is, of course, the
letter O so we're almost there.

705
00:32:18,176 --> 00:32:19,506
Round three, go.

706
00:32:19,506 --> 00:32:23,856
And this spells?

707
00:32:24,006 --> 00:32:27,506
No it shouldn't spell
boy [laughter].

708
00:32:27,506 --> 00:32:29,446
We have an error somewhere.

709
00:32:29,706 --> 00:32:39,206
Its 87 if I did the math
right myself, 87which is?

710
00:32:39,406 --> 00:32:39,836
>> Bow.

711
00:32:40,026 --> 00:32:41,686
>> Bow, so take your bow.

712
00:32:41,686 --> 00:32:42,416
Thank you very much.

713
00:32:42,916 --> 00:32:47,176
[ Applause ]

714
00:32:47,676 --> 00:32:48,026
>> Alright.

715
00:32:48,086 --> 00:32:50,616
That's the soon-- we came a
little better prepared this time

716
00:32:50,616 --> 00:32:52,676
and we brought CS50
stress balls for everyone

717
00:32:52,676 --> 00:32:54,656
to make this all the
more of an incentive.

718
00:32:54,656 --> 00:32:55,896
Oh, that was sweet.

719
00:32:56,076 --> 00:32:56,726
What a gift.

720
00:32:56,726 --> 00:33:00,446
Oh-- but if you can meet with
our teaching fellows over there,

721
00:33:00,446 --> 00:33:02,496
they're gonna ask
the legal department

722
00:33:02,556 --> 00:33:03,926
to see you next so [laughter].

723
00:33:04,536 --> 00:33:07,006
Alright, I think we're all set.

724
00:33:07,006 --> 00:33:08,456
Is that number eight?

725
00:33:08,456 --> 00:33:09,436
Okay. Oh, you can keep that.

726
00:33:09,766 --> 00:33:10,366
You can keep that.

727
00:33:10,366 --> 00:33:16,756
Okay. So what does-- how
does that actually apply

728
00:33:16,756 --> 00:33:18,376
to real computers, right?

729
00:33:18,376 --> 00:33:19,716
So this is not the
most efficient way

730
00:33:19,716 --> 00:33:20,756
of representing information.

731
00:33:20,906 --> 00:33:22,816
What actually goes on
underneath the hood?

732
00:33:22,976 --> 00:33:23,926
Well, it turns out some

733
00:33:23,926 --> 00:33:26,506
of you might recall this
silly game from years ago.

734
00:33:26,506 --> 00:33:27,136
Wooly Willy?

735
00:33:27,136 --> 00:33:28,616
It turns out these
are really hard

736
00:33:28,616 --> 00:33:30,046
to find in the 21st century.

737
00:33:30,306 --> 00:33:33,326
So these things called Wooly
Willy is just a plastic--

738
00:33:33,676 --> 00:33:36,156
it's this paper mend
with a plastic cover

739
00:33:36,296 --> 00:33:39,526
and there's all these black
magnetic particles here.

740
00:33:39,526 --> 00:33:42,156
And if you use this little
magnet you can draw a mustache

741
00:33:42,156 --> 00:33:45,286
or hair, or eyebrows or whatever
on this particular face.

742
00:33:45,536 --> 00:33:48,856
Now this is mostly irrelevant to
what we're talking about except

743
00:33:48,896 --> 00:33:51,576
for the fact that there are
magnetic particles involved

744
00:33:51,576 --> 00:33:54,456
and they're really small because
it turns out what's inside most

745
00:33:54,456 --> 00:33:56,796
of your laptops and desktop
computers are these things

746
00:33:56,796 --> 00:33:58,036
called hard drives.

747
00:33:58,036 --> 00:34:00,496
And a hard drive, if you
ever open that thing up,

748
00:34:00,756 --> 00:34:04,046
is actually just a big metal
container that contains one

749
00:34:04,046 --> 00:34:07,536
or more platters, so to speak,
these circular pieces of metal

750
00:34:07,826 --> 00:34:09,646
and on top of those
circular pieces

751
00:34:09,646 --> 00:34:14,426
of metal are essentially tiny
little magnetic particles

752
00:34:14,426 --> 00:34:16,886
like this, so small that
you actually can't see them.

753
00:34:16,886 --> 00:34:19,086
But if you touch them
or open the hard drive

754
00:34:19,346 --> 00:34:21,636
and actually poke
around, you will do damage

755
00:34:21,826 --> 00:34:22,966
because you'll mangle these--

756
00:34:22,966 --> 00:34:24,606
the alignment of
these particles.

757
00:34:24,606 --> 00:34:25,666
You'll corrupt your data.

758
00:34:25,666 --> 00:34:28,386
But what's interesting is
when they're actually there,

759
00:34:28,606 --> 00:34:30,686
essentially, if this
is a magnetic particle,

760
00:34:31,066 --> 00:34:34,476
you can actually represent
zeros and ones pretty easily.

761
00:34:34,476 --> 00:34:36,996
You don't need a light switch
or a light bulb or an iPhone app

762
00:34:36,996 --> 00:34:38,446
to turn something on or off.

763
00:34:38,666 --> 00:34:40,406
You can take something
that's magnetic

764
00:34:40,406 --> 00:34:44,366
with a north-south magnetization
and just say arbitrarily,

765
00:34:44,716 --> 00:34:46,076
this will represent one.

766
00:34:46,436 --> 00:34:49,236
But if its south-north,
this will represent zero.

767
00:34:49,476 --> 00:34:53,066
So on your computer's hard
drive are again, a whole bunch

768
00:34:53,066 --> 00:34:56,306
of tiny magnetic particles
aligned this way or this way.

769
00:34:56,306 --> 00:34:59,676
And now that you have patterns
of bits after bits after bits,

770
00:34:59,816 --> 00:35:02,666
and if you chunk them up into
say, eight-unit quantities,

771
00:35:02,906 --> 00:35:06,426
now can you start storing words
and sentences, and paragraphs.

772
00:35:06,426 --> 00:35:09,226
And as we'll see mid semester,
photographs and videos

773
00:35:09,226 --> 00:35:11,256
and other more sophisticated
file formats.

774
00:35:11,486 --> 00:35:13,126
It's one thing to
talk about this.

775
00:35:13,406 --> 00:35:14,526
Let me go ahead and open

776
00:35:14,526 --> 00:35:19,516
up instead a more interesting
graphical carnation--

777
00:35:19,616 --> 00:35:21,466
incarnation of this.

778
00:35:21,466 --> 00:35:22,236
[ Background Music ]

779
00:35:22,236 --> 00:35:26,156
>> The hard drive is
where your PC stores most

780
00:35:26,156 --> 00:35:27,296
of its permanent data.

781
00:35:27,886 --> 00:35:32,016
To do that the data travels from
RAM along with software signals

782
00:35:32,086 --> 00:35:34,526
that tell the hard drive
how to store that data.

783
00:35:35,266 --> 00:35:37,746
The hard drive circuits
translate those signals

784
00:35:37,786 --> 00:35:39,396
into to voltage fluctuations.

785
00:35:40,126 --> 00:35:42,896
These in turn control the
hard drive's moving parts.

786
00:35:43,476 --> 00:35:46,346
Some of the few moving parts
left in the modern computer.

787
00:35:47,116 --> 00:35:48,756
Some of the signals
control a motor

788
00:35:48,756 --> 00:35:50,636
which spins metal-coded
platters.

789
00:35:51,296 --> 00:35:53,926
Your data is actually
stored on these platters.

790
00:35:54,666 --> 00:35:57,706
Other signals move the
read/write heads to read

791
00:35:57,706 --> 00:35:59,196
or write data on the platters.

792
00:35:59,926 --> 00:36:01,746
This machinery is so precise

793
00:36:02,236 --> 00:36:04,766
that a human hair couldn't
even pass between the heads

794
00:36:04,766 --> 00:36:08,916
and spinning platters, yet it
all works in terrific speeds.

795
00:36:09,386 --> 00:36:11,086
>> So if you've ever
done something bad

796
00:36:11,086 --> 00:36:14,146
like drop your laptop
or smack your computer

797
00:36:14,146 --> 00:36:15,926
and you've never been
told not to do this.

798
00:36:15,986 --> 00:36:18,836
It's largely because--
and if you've never been,

799
00:36:18,836 --> 00:36:20,426
don't do this, it's largely

800
00:36:20,426 --> 00:36:23,326
because there are these
very sensitive moving parts

801
00:36:23,326 --> 00:36:25,416
and there are relatively
few moving parts left

802
00:36:25,416 --> 00:36:26,246
in today's computers.

803
00:36:26,246 --> 00:36:29,336
Some of you, in fact, might not
even have these devices anymore

804
00:36:29,336 --> 00:36:30,306
with moving parts.

805
00:36:30,516 --> 00:36:32,866
The fan is perhaps the only
thing left in those of you

806
00:36:32,866 --> 00:36:35,136
who have computers with
what are called SSDs.

807
00:36:35,286 --> 00:36:37,396
But in hard drives where we
actually have these moving

808
00:36:37,396 --> 00:36:40,046
platters, here is in fact,
how we get from electricity

809
00:36:40,046 --> 00:36:42,696
and magnetism to
those zeros and ones.

810
00:36:42,696 --> 00:36:43,046
[ Background Music ]

811
00:36:43,046 --> 00:36:45,836
>> Let's look at what we
just saw in slow motion.

812
00:36:46,106 --> 00:36:48,486
When a brief pulse of
electricity is sent

813
00:36:48,486 --> 00:36:51,466
to the read/write head it
flips on a tiny electromagnet

814
00:36:51,466 --> 00:36:52,696
for a fraction of a second.

815
00:36:53,256 --> 00:36:56,636
The magnet creates a field which
changes the polarity of a tiny,

816
00:36:56,706 --> 00:36:58,776
tiny portion of the
metal particles

817
00:36:58,776 --> 00:37:00,366
which coat each platter's
surface.

818
00:37:01,356 --> 00:37:03,646
A patterned series of
these tiny charged berries

819
00:37:03,646 --> 00:37:05,566
on the disk represents
a single bit of data

820
00:37:05,566 --> 00:37:08,976
in the binary number
system used by computers.

821
00:37:09,066 --> 00:37:11,386
Now, if the current is sent one
way through the read/write head,

822
00:37:11,476 --> 00:37:13,676
the area is polarized
in one direction.

823
00:37:14,046 --> 00:37:16,206
If the current is sent in
the opposite direction,

824
00:37:16,206 --> 00:37:17,896
the polarization is reversed.

825
00:37:18,276 --> 00:37:20,896
How do you get data
off the hard disk?

826
00:37:20,956 --> 00:37:21,946
Just reverse the process.

827
00:37:22,506 --> 00:37:24,966
So as the particles on the
disk that get the current

828
00:37:24,966 --> 00:37:27,456
in the read/write head
moving put together millions

829
00:37:27,456 --> 00:37:30,676
of these magnetized segments
and you've got a file.

830
00:37:31,276 --> 00:37:34,116
Now, the pieces of a single file
may be scanning all the hard

831
00:37:34,516 --> 00:37:37,396
drive's platters, patterns--
the mess of papers on your desk.

832
00:37:38,456 --> 00:37:41,366
So the special active file keeps
track of whatever you missed.

833
00:37:41,366 --> 00:37:44,906
Don't you wish that you
had something like that?

834
00:37:44,906 --> 00:37:44,976
[ Laughter ]

835
00:37:44,976 --> 00:37:45,466
>> Alright.

836
00:37:45,536 --> 00:37:46,916
So take it with a grain of salt.

837
00:37:46,916 --> 00:37:48,436
It was made by a
computer scientist,

838
00:37:48,846 --> 00:37:50,786
but now, it's fun time.

839
00:37:50,846 --> 00:37:53,466
So, these things here that
you have in your hand,

840
00:37:53,596 --> 00:37:55,156
is the results of my
spontaneously going

841
00:37:55,156 --> 00:37:56,196
to staples before class.

842
00:37:56,196 --> 00:37:58,416
I was the only one, believe
it or not, at the store,

843
00:37:58,416 --> 00:37:59,656
who was buying floppies
today [laughter].

844
00:38:00,106 --> 00:38:03,126
What you'll notice
very delicately is

845
00:38:03,126 --> 00:38:06,206
that there's not all that much
to this so called floppy disk.

846
00:38:06,206 --> 00:38:09,036
It's actually a much
cheaper example

847
00:38:09,036 --> 00:38:10,286
than handing out hard drives.

848
00:38:10,286 --> 00:38:12,786
But at the end of the day, if
you move this metal thing aside

849
00:38:12,786 --> 00:38:14,396
with your fingers, don't
squeeze it too hard.

850
00:38:14,676 --> 00:38:16,936
You will see inside
that there's a platter,

851
00:38:16,936 --> 00:38:19,116
but it's literally
a floppy platter,

852
00:38:19,116 --> 00:38:21,936
even though the disks themselves
are fairly firm plastic.

853
00:38:22,206 --> 00:38:25,846
This thing, if you now go ahead
and very gently corrupt a few

854
00:38:25,846 --> 00:38:28,396
of the bytes, you can see
that you can push down on

855
00:38:28,396 --> 00:38:30,156
that thing pretty easily.

856
00:38:30,556 --> 00:38:33,066
Now-- oh, now we've ruined them.

857
00:38:33,256 --> 00:38:35,416
So why don't we go ahead and
take this just a step further?

858
00:38:35,416 --> 00:38:37,416
Go ahead and just
pinch the metal plates

859
00:38:37,416 --> 00:38:38,626
and watch your eyes
'cause these do--

860
00:38:38,626 --> 00:38:39,776
things do have little springs.

861
00:38:40,006 --> 00:38:42,096
Go ahead and just pull
that metal plate off.

862
00:38:42,096 --> 00:38:46,436
And now you truly can see that
there's not much to this disk.

863
00:38:46,436 --> 00:38:50,696
And then this is where you can
really tear into this device.

864
00:38:50,946 --> 00:38:53,166
If you put your fingers
between the plastic opening

865
00:38:53,166 --> 00:38:55,006
at the bottom, and here's
where you gotta watch

866
00:38:55,006 --> 00:39:03,546
out for the spring, pull the
thing apart and that's it.

867
00:39:04,596 --> 00:39:07,376
The irony is-- these things
cost like a dollar each now

868
00:39:07,376 --> 00:39:08,876
because no one needs
them anymore.

869
00:39:08,876 --> 00:39:12,426
It's kind of an Ec 10
thing there, but that's it.

870
00:39:13,016 --> 00:39:18,516
So unfortunately, we had just
ruined roughly 1.4 million bytes

871
00:39:18,986 --> 00:39:20,546
worth of data.

872
00:39:20,546 --> 00:39:23,546
They came formatted or blank
so not such a big deal.

873
00:39:23,756 --> 00:39:26,326
But in fact, if you just run
your fingers again and again

874
00:39:26,326 --> 00:39:30,066
over this floppy disk you are
disorienting those particles

875
00:39:30,066 --> 00:39:32,426
or knocking them off
perhaps altogether,

876
00:39:32,476 --> 00:39:33,346
depending on the medium.

877
00:39:33,346 --> 00:39:34,436
And the rest of this thing,

878
00:39:34,656 --> 00:39:37,216
this is just a cheap
piece of molded plastic.

879
00:39:37,446 --> 00:39:41,816
This little felt thing there is
just to avoid too much friction

880
00:39:41,816 --> 00:39:42,906
from building up
with the plastic

881
00:39:42,906 --> 00:39:45,476
or to keep it slightly separated
from the case, but that's it.

882
00:39:45,896 --> 00:39:48,716
That's all there was to those
things called floppy disks.

883
00:39:48,986 --> 00:39:52,186
But what's amazing is that
the hard drives we have today

884
00:39:52,186 --> 00:39:54,666
in a desktop or in a
laptop, a hard drive

885
00:39:54,666 --> 00:39:56,386
and a laptop is only
about-- yey big!

886
00:39:56,386 --> 00:40:00,336
It's roughly the same size
physically as those floppy disks

887
00:40:00,536 --> 00:40:03,146
and yet how many, how
much, how many bytes fit

888
00:40:03,146 --> 00:40:04,656
on a typical hard drive today?

889
00:40:05,156 --> 00:40:07,216
[ Inaudible Remark ]

890
00:40:07,716 --> 00:40:13,116
>> What's that 500, so 500
megabytes, this is 1.4 megabytes

891
00:40:13,116 --> 00:40:16,376
so a typical hard drive might
have 500-- no, not 500 megabyte,

892
00:40:16,536 --> 00:40:20,146
500 gigabytes which
is 500 billion bytes

893
00:40:20,416 --> 00:40:22,756
and if you have a desktop
computer you might actually have

894
00:40:22,756 --> 00:40:25,786
a 1 terabyte drive
or 1.5 terabyte drive

895
00:40:25,946 --> 00:40:28,366
or 1-- 2.0 terabyte drive.

896
00:40:28,576 --> 00:40:30,876
These are trillions
now of bytes.

897
00:40:31,096 --> 00:40:34,196
And so it wasn't all that many
years ago frankly that you know,

898
00:40:34,196 --> 00:40:37,326
certainly, people in this class
were still using these things.

899
00:40:37,356 --> 00:40:38,896
Thankfully, they've
fallen into disuse.

900
00:40:39,166 --> 00:40:41,876
They're not the most
robust things anymore.

901
00:40:42,086 --> 00:40:44,906
But the technology and the
fundamentals and the ideas,

902
00:40:44,906 --> 00:40:47,866
it's pretty much exactly the
same as we're using today

903
00:40:47,866 --> 00:40:50,136
in some of our modern computers.

904
00:40:50,426 --> 00:40:56,916
So, any questions
on zeroes and ones?

905
00:40:57,336 --> 00:40:59,016
It's a little awkward
asking questions in here

906
00:40:59,016 --> 00:41:03,666
but let's give it a try.

907
00:41:03,666 --> 00:41:03,886
[ Inaudible Question ]

908
00:41:03,886 --> 00:41:05,056
>> Good question,
so what happens

909
00:41:05,056 --> 00:41:07,776
when you format a floppy
disk or format a hard drive?

910
00:41:08,076 --> 00:41:11,806
To be honest, not very much
and this is one of the reasons

911
00:41:11,806 --> 00:41:14,146
that forensic investigators
are actually able to do

912
00:41:14,146 --> 00:41:17,186
such a good job for the police
and FBI or the like because

913
00:41:17,186 --> 00:41:20,256
so many computer programs, Mac
OS and Windows, they'll say,

914
00:41:20,256 --> 00:41:23,266
are you sure you want to do
this, this will erase everything

915
00:41:23,266 --> 00:41:25,676
from your disk, this will erase
everything from your hard drive

916
00:41:25,906 --> 00:41:28,526
if you so called format
it or partition it.

917
00:41:28,526 --> 00:41:30,176
It's another piece of
jargon you often hear.

918
00:41:30,506 --> 00:41:33,926
That generally is just
modifying a few bytes,

919
00:41:33,996 --> 00:41:36,316
a few kilobytes worth
of information.

920
00:41:36,656 --> 00:41:40,606
The end result of which is that
the computer forgets where all

921
00:41:40,606 --> 00:41:42,136
of your files and folders are.

922
00:41:42,386 --> 00:41:44,766
But the bits that actually
compose those files

923
00:41:44,766 --> 00:41:48,296
and folders are in fact
exactly where you left them.

924
00:41:48,616 --> 00:41:51,046
And so this is why in
problem set 5 last year

925
00:41:51,046 --> 00:41:52,516
when I took photographs
on campus

926
00:41:52,516 --> 00:41:55,376
with a compact flash card,
just standard consumer media

927
00:41:55,556 --> 00:41:58,786
and I accidentally formatted
that compact flash card,

928
00:41:58,916 --> 00:42:01,186
I only lost the first
few kilobytes,

929
00:42:01,186 --> 00:42:04,486
the first few thousand bits
on that compact flash card.

930
00:42:04,486 --> 00:42:06,326
But as you probably know,
a JPEG, a photograph,

931
00:42:06,526 --> 00:42:08,996
these are usually multiple
megabytes these days

932
00:42:08,996 --> 00:42:11,416
which means there's a
ridiculous amount of data still

933
00:42:11,416 --> 00:42:12,816
on that digital camera.

934
00:42:13,106 --> 00:42:15,966
And so if you know what patterns
of zeroes and ones to look

935
00:42:15,966 --> 00:42:17,876
for as you will yourselves
this year

936
00:42:18,236 --> 00:42:19,636
in that forensics problem set,

937
00:42:19,846 --> 00:42:21,666
can you recover a
lot of that data?

938
00:42:21,716 --> 00:42:24,966
So in short, formatting a disk
gets it ready for new data

939
00:42:24,966 --> 00:42:27,206
but it tends not to
get rid of the old

940
00:42:27,206 --> 00:42:30,866
and only recently are Apple
and Microsoft finally realizing

941
00:42:30,866 --> 00:42:34,416
that consumers actually want to
securely erase their information

942
00:42:34,416 --> 00:42:36,586
for privacy or financial
or whatever reasons,

943
00:42:36,846 --> 00:42:38,346
but up until now and even know

944
00:42:38,346 --> 00:42:40,126
on some operating
systems it's not

945
00:42:40,126 --> 00:42:42,816
in fact very easy,
other questions.

946
00:42:43,146 --> 00:42:43,276
Yeah.

947
00:42:44,516 --> 00:42:53,946
[ Inaudible Question ]

948
00:42:54,446 --> 00:42:57,636
>> Exactly, so you might
have been thought especially

949
00:42:57,636 --> 00:42:59,806
in years past to
defragment, so to speak,

950
00:42:59,806 --> 00:43:00,986
your hard drive once in a while.

951
00:43:01,226 --> 00:43:03,286
And this is because
as you start writing--

952
00:43:03,286 --> 00:43:04,926
saving files to your hard drive,

953
00:43:05,226 --> 00:43:08,126
what happens is you might save
this file here, then this one,

954
00:43:08,126 --> 00:43:09,086
then this one, then this one,

955
00:43:09,286 --> 00:43:11,276
but very reasonably you
might go back eventually

956
00:43:11,276 --> 00:43:12,176
and delete this one.

957
00:43:12,386 --> 00:43:14,556
Now you have a gap
in your data and

958
00:43:14,556 --> 00:43:16,856
yet you still have some
magnetic particles there

959
00:43:16,856 --> 00:43:18,286
that could be useful.

960
00:43:18,536 --> 00:43:21,536
So thankfully, modern operating
systems allow you to put parts

961
00:43:21,536 --> 00:43:23,426
of files here, parts
of your file here,

962
00:43:23,426 --> 00:43:25,756
parts of your file here,
but the end result is

963
00:43:25,756 --> 00:43:27,856
that your files get fragmented.

964
00:43:28,096 --> 00:43:30,466
The operating system still
finds it but this process

965
00:43:30,466 --> 00:43:33,146
of getting some of your files
bits from here, some from here,

966
00:43:33,146 --> 00:43:35,956
some from here, induces
a potential slow down.

967
00:43:36,106 --> 00:43:38,386
Now defragmentation
as an aside is not

968
00:43:38,386 --> 00:43:39,966
so much necessary these days

969
00:43:39,966 --> 00:43:42,446
because hardware has
gotten so performance.

970
00:43:42,776 --> 00:43:45,976
But it is in fact indicative
of what's actually going

971
00:43:46,166 --> 00:43:48,386
on underneath the hood and
for a forensic investigator

972
00:43:48,386 --> 00:43:50,186
to be honest and
even problem set 5,

973
00:43:50,376 --> 00:43:53,356
it poses a potential
wrinkle if those JPEGs,

974
00:43:53,386 --> 00:43:56,476
those photos we took are not
contiguous back to back to back

975
00:43:56,476 --> 00:43:57,496
but are all over the place.

976
00:43:57,716 --> 00:43:59,996
That's a lot harder of a
problem to solve if you need

977
00:43:59,996 --> 00:44:02,456
to reconstruct the
fragmentation, yeah.

978
00:44:03,516 --> 00:44:08,656
[ Inaudible Question ]

979
00:44:09,156 --> 00:44:12,246
>> Good question, so there is
this little reading head kind

980
00:44:12,246 --> 00:44:14,896
of like in an old phonograph
player that does move back

981
00:44:14,896 --> 00:44:17,496
and forth and in and out
depending on the implementation.

982
00:44:17,766 --> 00:44:20,666
So there is generally a little
computer, a circuit board

983
00:44:20,666 --> 00:44:23,846
in the hard drive itself that
knows how to tell it to go up,

984
00:44:23,846 --> 00:44:25,266
down, left, right and so forth.

985
00:44:25,426 --> 00:44:27,886
And if you've ever heard
the word driver software

986
00:44:28,096 --> 00:44:30,756
that you install in your
computer so that your computer,

987
00:44:30,756 --> 00:44:33,676
your operating system, Windows
or Mac OS, knows how to talk

988
00:44:33,676 --> 00:44:36,726
to that piece of hardware,
it's that driver software

989
00:44:36,726 --> 00:44:39,336
that then empowers the operating
systems to send those commands

990
00:44:39,336 --> 00:44:41,956
as well, up, down, left, right
depending on how the disk--

991
00:44:42,236 --> 00:44:44,016
the data is laid
out on the disk.

992
00:44:44,286 --> 00:44:46,206
Let me go over here
for another question.

993
00:44:46,206 --> 00:44:46,371
[ Inaudible Question ]

994
00:44:46,371 --> 00:44:46,536
[ Laughter ]

995
00:44:46,536 --> 00:44:53,266
>> Okay, what are the advantages
of SSDs and how do they work.

996
00:44:53,266 --> 00:44:54,386
Let me answer this briefly now,

997
00:44:54,386 --> 00:44:56,596
I'm happy to chat more
during break time.

998
00:44:56,596 --> 00:44:57,906
SSDs are where things are going.

999
00:44:58,056 --> 00:45:01,216
SSDs is like using
compact flash or SD cards.

1000
00:45:01,246 --> 00:45:03,206
The things we've had in
cameras for several years

1001
00:45:03,366 --> 00:45:06,486
by putting gigabytes, soon
hopefully terabytes of storage

1002
00:45:06,646 --> 00:45:08,506
on a device that
has no moving parts.

1003
00:45:08,776 --> 00:45:11,886
Generally, the feature is
that they are much faster

1004
00:45:12,196 --> 00:45:13,926
because they have
no moving parts.

1005
00:45:14,036 --> 00:45:17,526
Even though a hard drive
spins at 7200 times per minute

1006
00:45:17,526 --> 00:45:20,086
or even faster, that's
relatively slower

1007
00:45:20,086 --> 00:45:20,886
than the speed of light.

1008
00:45:21,086 --> 00:45:22,626
And so when you actually
have a device

1009
00:45:22,866 --> 00:45:25,546
that is purely electronic,
and even that doesn't--

1010
00:45:25,546 --> 00:45:27,496
the bits don't flow at the
speed of light in reality.

1011
00:45:27,716 --> 00:45:29,376
It's a lot faster of a device.

1012
00:45:29,456 --> 00:45:31,936
But these days you pay for
it which is why you get SSDs,

1013
00:45:31,986 --> 00:45:34,446
you're paying several
hundred dollars more.

1014
00:45:34,676 --> 00:45:37,566
So let's transition from
these lower level details.

1015
00:45:37,566 --> 00:45:39,546
I'm happy to take questions
during break and after

1016
00:45:39,796 --> 00:45:44,346
to actually making this hardware
do something in software.

1017
00:45:44,346 --> 00:45:47,046
And for this I need
one volunteer.

1018
00:45:47,916 --> 00:45:52,056
Let's go and okay, right here.

1019
00:45:52,276 --> 00:45:53,096
Oh, but let's see.

1020
00:45:53,096 --> 00:45:55,356
Okay, I need a second
volunteer for reasons

1021
00:45:55,356 --> 00:45:56,256
that will be clear in a second.

1022
00:45:56,256 --> 00:46:00,126
Come on down.

1023
00:46:00,336 --> 00:46:01,466
Your role has just been changed

1024
00:46:01,466 --> 00:46:02,956
because you're not
wearing any socks today.

1025
00:46:04,036 --> 00:46:08,816
So come on over here, we're
gonna have-- what's your name?

1026
00:46:09,576 --> 00:46:09,876
>> Sam.

1027
00:46:10,026 --> 00:46:10,646
>> Sam and?

1028
00:46:10,956 --> 00:46:11,166
>> Philip.

1029
00:46:11,456 --> 00:46:12,186
>> Sam and Philip.

1030
00:46:12,186 --> 00:46:14,776
Alright, I'm David and
let's have Philip sit down.

1031
00:46:15,306 --> 00:46:17,906
You'll play role sort of a
shoe salesman, shall we say?

1032
00:46:18,096 --> 00:46:18,266
>> Okay.

1033
00:46:18,416 --> 00:46:19,496
>> Alright, and don't
worry I have a script

1034
00:46:19,496 --> 00:46:20,576
for you but not for Philip.

1035
00:46:21,626 --> 00:46:24,726
So here is a computer program.

1036
00:46:24,726 --> 00:46:26,926
This is not written in
a particular language.

1037
00:46:26,966 --> 00:46:30,216
This is what's written in what
we'll generally call pseudocode.

1038
00:46:30,356 --> 00:46:31,296
Pseudocode is kind

1039
00:46:31,296 --> 00:46:33,806
of an English-like syntax
that's just a useful way

1040
00:46:33,806 --> 00:46:37,146
of expressing yourself fairly
succinctly, fairly computer-like

1041
00:46:37,306 --> 00:46:39,286
but without having to
worry about stupid details

1042
00:46:39,286 --> 00:46:41,376
like whether it's
Java or C or whatever.

1043
00:46:41,596 --> 00:46:43,786
You just kind of say what you
mean but you say it succinctly.

1044
00:46:43,786 --> 00:46:44,646
But what's interesting

1045
00:46:44,646 --> 00:46:47,476
about this particular example
is one, it's imperfect.

1046
00:46:47,776 --> 00:46:49,576
I didn't quite get
this program right.

1047
00:46:49,896 --> 00:46:53,536
Two, it also highlights
a number of constructs

1048
00:46:53,536 --> 00:46:55,846
that we'll see today and we'll
see recurring throughout this

1049
00:46:55,846 --> 00:46:58,946
world of programming, things
like loops and conditions

1050
00:46:58,946 --> 00:46:59,976
and variables and the like.

1051
00:47:00,216 --> 00:47:04,306
So what we have Philip here
for is to actually be the one

1052
00:47:04,476 --> 00:47:06,876
who if you could start taking
off your shoes and socks,

1053
00:47:07,686 --> 00:47:10,896
I didn't just claim this in the
beginning but we very quickly

1054
00:47:10,896 --> 00:47:13,926
in this course start to
hemorrhage volunteers overtime.

1055
00:47:14,456 --> 00:47:14,626
Alright.

1056
00:47:14,751 --> 00:47:16,751
[ Laughter ]

1057
00:47:16,876 --> 00:47:21,486
>> You don't have to--
you don't have to touch.

1058
00:47:21,486 --> 00:47:21,816
It's okay.

1059
00:47:22,226 --> 00:47:24,926
So this first line of
code, I will get it started

1060
00:47:24,926 --> 00:47:26,436
but then I'm gonna
have you take over

1061
00:47:26,436 --> 00:47:28,946
and actually instruct
Philip what to do here.

1062
00:47:29,256 --> 00:47:30,476
So this first line of code,

1063
00:47:30,476 --> 00:47:31,826
and some of this again
we'll be rehashed

1064
00:47:31,826 --> 00:47:33,526
but at least you can
delight in the awkwardness

1065
00:47:33,526 --> 00:47:34,596
that is this demonstration.

1066
00:47:34,596 --> 00:47:36,336
So line 1, I've numbered
the lines just

1067
00:47:36,336 --> 00:47:37,236
for the sake of discussion.

1068
00:47:37,526 --> 00:47:39,436
Line 1 is creating what
we'll call a variable.

1069
00:47:39,436 --> 00:47:41,566
Now, unlike high school
math or in algebra

1070
00:47:41,566 --> 00:47:44,106
where you call things X and
Y and Z, in programming,

1071
00:47:44,106 --> 00:47:46,936
in computer science, you're
actually dealing with humans

1072
00:47:46,936 --> 00:47:50,156
where it's useful to have
a variable name that's more

1073
00:47:50,156 --> 00:47:53,666
descriptive than X and Y and Z.
So here is an example of socks

1074
00:47:53,666 --> 00:47:55,546
on feet being the
number of socks

1075
00:47:55,546 --> 00:47:58,296
on Philip's feet
which is in fact 0.

1076
00:47:58,456 --> 00:48:01,286
On line 2, we're gonna
induce what's called a loop,

1077
00:48:01,356 --> 00:48:03,526
so do you wanna go ahead and
Philip henceforth you get

1078
00:48:03,526 --> 00:48:04,996
to stare, stare at
the awkwardness.

1079
00:48:05,516 --> 00:48:12,606
Go ahead and recite for
us line 2 of this program.

1080
00:48:12,606 --> 00:48:13,026
[ Inaudible Remark ]

1081
00:48:13,026 --> 00:48:15,696
>> Okay, so while not
equal to, so bang equals,

1082
00:48:15,696 --> 00:48:17,856
exclamation point equals
is computer science syntax

1083
00:48:18,016 --> 00:48:21,676
for saying not equal
to, while not equal to 2

1084
00:48:21,676 --> 00:48:23,286
which it is not,
it is equal to 0.

1085
00:48:23,286 --> 00:48:29,226
What should he do next?

1086
00:48:29,226 --> 00:48:30,926
[ Inaudible Remark ]

1087
00:48:30,926 --> 00:48:31,336
>> Okay.

1088
00:48:31,336 --> 00:48:32,896
[ Inaudible Remark ]

1089
00:48:32,896 --> 00:48:34,756
>> Excellent, look
for sock, alright.

1090
00:48:34,916 --> 00:48:36,436
Step 5, now this is an example

1091
00:48:36,436 --> 00:48:39,026
of what we'll henceforth
call a condition or a branch

1092
00:48:39,226 --> 00:48:44,736
because it's sort of
a fork in the road.

1093
00:48:44,736 --> 00:48:45,376
[ Inaudible Remark ]

1094
00:48:45,376 --> 00:48:49,926
>> Okay, if you find the
sock then put on sock.

1095
00:48:51,736 --> 00:48:53,796
Intentionally, I bought
the worst socks I could.

1096
00:48:53,856 --> 00:48:54,966
>> No, I like it.

1097
00:48:54,966 --> 00:48:55,236
[ Laughter ]

1098
00:48:55,236 --> 00:49:03,156
>> I get this.

1099
00:49:03,156 --> 00:49:03,366
[ Laughter ]

1100
00:49:03,366 --> 00:49:04,626
>> Available at the
Harvard Coop.

1101
00:49:05,906 --> 00:49:07,376
Alright, next line.

1102
00:49:07,686 --> 00:49:09,956
>> Socks on feet plus plus.

1103
00:49:09,956 --> 00:49:11,296
>> So this plus plus is sort

1104
00:49:11,296 --> 00:49:13,146
of shorthand notation
for incrementation.

1105
00:49:13,146 --> 00:49:16,516
So just add 1 to this variable's
value, so now socks on feet is

1106
00:49:16,516 --> 00:49:19,716
of course 1 and so next.

1107
00:49:20,126 --> 00:49:23,166
>> Look for matching sock.

1108
00:49:23,646 --> 00:49:26,836
>> Problem, so where
are we at line 9?

1109
00:49:26,836 --> 00:49:30,396
>> If you find a matching sock
which you didn't so then else.

1110
00:49:30,486 --> 00:49:33,226
>> Okay, so notice again
the control flow and this--

1111
00:49:33,406 --> 00:49:35,886
the choice of indentation
here is deliberate throughout

1112
00:49:35,886 --> 00:49:38,656
programming whether it's
in C or PHP or JavaScript

1113
00:49:38,656 --> 00:49:39,596
in this particular course.

1114
00:49:39,746 --> 00:49:41,806
The indentation gives
you a visual cue as to

1115
00:49:41,806 --> 00:49:43,946
which lines are related
to each other.

1116
00:49:43,946 --> 00:49:46,226
So the fact that
line 9 did not match,

1117
00:49:46,556 --> 00:49:48,786
if you find a matching
sock which we didn't,

1118
00:49:48,896 --> 00:49:51,856
that means we immediately jump
to the else which is line 13

1119
00:49:52,036 --> 00:49:53,476
and so Philip now
has to do what?

1120
00:49:54,116 --> 00:49:56,276
>> Remove first sock from foot.

1121
00:49:56,276 --> 00:49:57,396
>> Oh man!

1122
00:49:58,276 --> 00:50:00,656
>> Socks on feet, minus minus.

1123
00:50:00,656 --> 00:50:02,876
>> We decrement so to
speak the variable and then

1124
00:50:02,876 --> 00:50:05,906
where will we go next in
terms of the flow control,

1125
00:50:05,906 --> 00:50:06,976
the flow of this program.

1126
00:50:07,526 --> 00:50:08,866
>> Back to the while.

1127
00:50:09,116 --> 00:50:09,866
>> Right, so the fact

1128
00:50:09,866 --> 00:50:12,326
that everything is indented
means while we're doing,

1129
00:50:12,326 --> 00:50:14,906
while the variable is not
equal to 2, keep doing this.

1130
00:50:15,176 --> 00:50:16,716
And so we're going to repeat.

1131
00:50:16,716 --> 00:50:18,546
Now, rather than go through
this again and again,

1132
00:50:18,546 --> 00:50:19,886
because frankly you
can probably see

1133
00:50:19,886 --> 00:50:23,176
where this goes before long
we're gonna hit some problems.

1134
00:50:23,176 --> 00:50:24,526
What's one of the
problems you see

1135
00:50:25,436 --> 00:50:26,576
or that we tripped over already?

1136
00:50:26,576 --> 00:50:28,376
[ Inaudible Remark ]

1137
00:50:28,376 --> 00:50:33,086
>> If there is no matching
socks then what's gonna happen

1138
00:50:33,086 --> 00:50:33,776
with Philip here?

1139
00:50:34,206 --> 00:50:37,436
We're gonna have a so called
a loop, an infinite loop

1140
00:50:37,436 --> 00:50:39,396
and you actually experience
this in the real world.

1141
00:50:39,396 --> 00:50:40,856
If you've ever been
using some program,

1142
00:50:41,066 --> 00:50:43,086
you did something completely
innocuous, pull something

1143
00:50:43,086 --> 00:50:44,406
down from a menu,
click a button,

1144
00:50:44,596 --> 00:50:46,496
you didn't do anything
wrong and yet all

1145
00:50:46,496 --> 00:50:48,136
of a sudden the thing
just starts hanging there

1146
00:50:48,136 --> 00:50:49,916
or you get a little
spinning beach ball

1147
00:50:49,916 --> 00:50:51,276
or the hour glass
or whatever it is.

1148
00:50:51,276 --> 00:50:54,066
Something completely not
your fault, it's not.

1149
00:50:54,226 --> 00:50:56,486
It's because the programmer
or programmers that wrote

1150
00:50:56,486 --> 00:50:58,816
that software might have
been pretty specific

1151
00:50:59,016 --> 00:51:00,356
but not specific enough.

1152
00:51:00,356 --> 00:51:02,576
Computers smart though
they may be

1153
00:51:02,576 --> 00:51:04,876
and intimidating though
they may be, they can not

1154
00:51:04,986 --> 00:51:06,726
yet think for themselves.

1155
00:51:06,726 --> 00:51:08,476
They can't just know
what you mean.

1156
00:51:08,626 --> 00:51:10,746
And so similarly as
Gmail's priority inbox,

1157
00:51:10,746 --> 00:51:13,456
it's gonna be pretty flawed for
a lot of us at least initially

1158
00:51:13,616 --> 00:51:17,106
because it just doesn't know
the answers a priori correctly

1159
00:51:17,106 --> 00:51:19,966
unless we teach it or
program it to be better.

1160
00:51:19,966 --> 00:51:23,536
There is also another flaw in
here or multiple flaws arguably.

1161
00:51:23,536 --> 00:51:25,206
But something else
catch your eye?

1162
00:51:25,446 --> 00:51:28,256
This is a very imperfect
program.

1163
00:51:28,256 --> 00:51:29,996
[ Inaudible Remark ]

1164
00:51:29,996 --> 00:51:35,026
>> So, in fact we may not
find a sock at all and

1165
00:51:35,026 --> 00:51:37,526
yet we don't seem to
handle that at all unless

1166
00:51:37,836 --> 00:51:40,136
if we don't find any
socks, what do we do?

1167
00:51:41,116 --> 00:51:42,386
We replenish the sock drawer.

1168
00:51:42,386 --> 00:51:44,566
So if we tease this apart
and this is actually nice,

1169
00:51:44,566 --> 00:51:45,396
every semester I get beat

1170
00:51:45,396 --> 00:51:46,856
up for just how bad
this thing actually is

1171
00:51:46,856 --> 00:51:48,136
if you start really
picking it apart.

1172
00:51:48,436 --> 00:51:50,736
You find that there's a lot of
flaws but this is intentional

1173
00:51:50,946 --> 00:51:54,156
and this is one of the sort of
arts that you learn early on is

1174
00:51:54,156 --> 00:51:56,476
that with computers you
really do need to be careful

1175
00:51:56,476 --> 00:51:58,676
and you need to think about
this so called corner cases

1176
00:51:58,856 --> 00:52:01,596
and even you might use popular
websites or programs that have

1177
00:52:01,806 --> 00:52:05,136
so called bugs or mistakes
and that is simply the results

1178
00:52:05,136 --> 00:52:09,266
of the programmer not
having anticipated some

1179
00:52:09,266 --> 00:52:10,286
particular problem.

1180
00:52:10,286 --> 00:52:11,926
So we have a parting gift.

1181
00:52:11,926 --> 00:52:15,816
Why don't we give Philip
both the matching pair

1182
00:52:16,016 --> 00:52:17,656
and give you each a pair of
socks since he didn't get

1183
00:52:17,656 --> 00:52:20,106
to wear this one, they are brand
new, in fact, and a big round

1184
00:52:20,106 --> 00:52:21,946
of applause for what I'm
sure is a little difficult.

1185
00:52:22,516 --> 00:52:28,476
[ Applause ]

1186
00:52:28,976 --> 00:52:29,676
>> Take your time.

1187
00:52:30,166 --> 00:52:34,096
So, let's actually now
take a 2-minute break,

1188
00:52:34,096 --> 00:52:35,436
we're gonna change tapes
and we'll come back

1189
00:52:35,436 --> 00:52:37,866
and we'll actually write
programs with a real computer

1190
00:52:37,866 --> 00:52:40,376
and some real animated
characters.

1191
00:52:40,376 --> 00:52:41,796
More in just a couple
of minutes.

1192
00:52:41,796 --> 00:52:42,346
[ Silence ]

1193
00:52:42,346 --> 00:52:48,086
>> Alright, we are back

1194
00:52:48,546 --> 00:52:52,866
so we just wrote a
programming pseudocode or walk

1195
00:52:52,866 --> 00:52:54,136
through a programming
pseudocode.

1196
00:52:54,136 --> 00:52:55,566
Let's take a quick
glimpse of what it's

1197
00:52:55,566 --> 00:52:58,866
like to write a real program
albeit a very trivial one.

1198
00:52:58,866 --> 00:53:02,476
Starting next week you'll
actually be using a cluster

1199
00:53:02,476 --> 00:53:06,926
of systems that CS50 itself
runs called Cloud.CS50.net.

1200
00:53:07,166 --> 00:53:09,026
This is a bunch of computers
running an operating system

1201
00:53:09,026 --> 00:53:11,836
called Linux which has a
variety of interfaces too

1202
00:53:11,836 --> 00:53:12,956
and more on that next week.

1203
00:53:13,156 --> 00:53:14,316
But it turns out that on Macs

1204
00:53:14,426 --> 00:53:16,656
and if you have the right
software on PCs, you can kind

1205
00:53:16,656 --> 00:53:19,476
of get a teaser of what this
environment tends to look like

1206
00:53:19,676 --> 00:53:20,836
and be forewarned, what you're

1207
00:53:20,836 --> 00:53:23,576
about to see is intentionally
very underwhelming.

1208
00:53:23,576 --> 00:53:25,966
I am going to run a
program called Nano

1209
00:53:26,296 --> 00:53:30,276
and type nano hello.c and I'm
gonna type the following very

1210
00:53:30,276 --> 00:53:33,486
quickly without much
explanation because we'll dive

1211
00:53:33,486 --> 00:53:34,796
into this more next week.

1212
00:53:35,236 --> 00:53:38,036
But what I'm doing
here is programming.

1213
00:53:38,976 --> 00:53:44,626
Sorry, I didn't mean to make
it sound so condescending.

1214
00:53:44,626 --> 00:53:44,736
[ Laughter ]

1215
00:53:44,736 --> 00:53:47,596
>> So this is just a dumb
text editor called Nano,

1216
00:53:47,596 --> 00:53:49,426
dumb in that it's very
simple which is nice

1217
00:53:49,466 --> 00:53:51,206
because it eliminates a
lot of the complexities

1218
00:53:51,206 --> 00:53:52,086
of learning a new tool

1219
00:53:52,086 --> 00:53:54,326
when really all you wanna do
is solve an actual problem.

1220
00:53:54,516 --> 00:53:57,556
So I'm gonna go ahead and
save this file, it's hello.c,

1221
00:53:57,796 --> 00:54:00,516
dot C hints at the fact that
this program I just whipped

1222
00:54:00,516 --> 00:54:03,436
up is written in a
language called C. Now,

1223
00:54:03,436 --> 00:54:06,016
this is underwhelming with what
you can do in just a second

1224
00:54:06,016 --> 00:54:07,286
in this particular language.

1225
00:54:07,506 --> 00:54:09,826
But again as per
Wednesday's lecture by the end

1226
00:54:09,826 --> 00:54:11,236
of the semester,
you'll be making things

1227
00:54:11,236 --> 00:54:13,286
like isawyouharvard.com

1228
00:54:13,286 --> 00:54:15,156
or any websites really
that you can imagine.

1229
00:54:15,156 --> 00:54:18,856
Can you probably produce
after just several weeks

1230
00:54:18,856 --> 00:54:20,626
of fundamentals like these?

1231
00:54:20,626 --> 00:54:23,306
So I'm saving this file,
notice there is a few keywords

1232
00:54:23,306 --> 00:54:25,496
that we'll come back to next
week like main which is kind

1233
00:54:25,496 --> 00:54:26,986
of the main part of my program.

1234
00:54:27,156 --> 00:54:30,486
There is this include which
allows me to include code,

1235
00:54:30,486 --> 00:54:32,846
source code so to speak
that other people wrote

1236
00:54:33,026 --> 00:54:34,686
so I don't have to
figure out for myself how

1237
00:54:34,686 --> 00:54:35,946
to print things to the screen.

1238
00:54:36,146 --> 00:54:37,746
Other people are
telling me how to--

1239
00:54:38,126 --> 00:54:40,786
other people had solved that
problem for me, and this thing

1240
00:54:40,786 --> 00:54:42,746
between quotes, we'll
start calling a string.

1241
00:54:42,966 --> 00:54:45,326
It's just a sentence or words
or whatever that I wanna print.

1242
00:54:45,326 --> 00:54:47,876
And then there are some stupid
syntax like curly braces

1243
00:54:47,876 --> 00:54:50,316
and semicolons that
initially are kind of annoying

1244
00:54:50,316 --> 00:54:52,156
because you have to remember
where all this junk goes.

1245
00:54:52,156 --> 00:54:55,516
But at the end of the day, it's
really going to be identical

1246
00:54:55,516 --> 00:54:57,256
to what we see pictorially
with Scratch.

1247
00:54:57,486 --> 00:54:59,066
I'm now gonna run a
program called GCC

1248
00:54:59,116 --> 00:55:01,126
which is what's called
the compiler.

1249
00:55:01,406 --> 00:55:03,566
Compiler, we'll also
revisit next week takes--

1250
00:55:03,826 --> 00:55:08,976
human text, ASCII text as input
and output zeroes and ones

1251
00:55:09,246 --> 00:55:13,686
that represent that program
in a manner that the CPU,

1252
00:55:13,686 --> 00:55:17,726
the Intel Inside, actually
understands which is--

1253
00:55:17,726 --> 00:55:21,616
interesting, dammit, not
installed on this computer.

1254
00:55:21,996 --> 00:55:26,006
So what we would see, I'm
going to completely fake this,

1255
00:55:27,266 --> 00:55:30,586
is a program that
operates like that.

1256
00:55:30,586 --> 00:55:32,886
So I just realized this
is a new MacBook Pro

1257
00:55:32,886 --> 00:55:35,946
that I didn't actually download
the compiler onto it apparently.

1258
00:55:36,296 --> 00:55:38,526
So good, that will give us
something to do next week.

1259
00:55:38,706 --> 00:55:41,416
But what you would have seen
is the program just doing this,

1260
00:55:41,526 --> 00:55:42,776
printing out to the screen.

1261
00:55:42,776 --> 00:55:45,036
But unfortunately, this is
incredibly underwhelming

1262
00:55:45,036 --> 00:55:46,596
certainly in the first
week of a course.

1263
00:55:46,856 --> 00:55:49,506
It's much more interesting if
you can actually build things

1264
00:55:49,736 --> 00:55:52,576
that are interactive, that
are animated, that play music

1265
00:55:52,576 --> 00:55:55,316
and so forth and not just
produce things on day 1

1266
00:55:55,526 --> 00:55:57,376
that outputs zeroes
and ones like this.

1267
00:55:57,376 --> 00:55:59,776
So those are in fact the
patterns of zeroes and ones,

1268
00:55:59,776 --> 00:56:02,746
the bytes that would have been
outputted had I remembered

1269
00:56:02,966 --> 00:56:06,616
to download the compiler
to this computer and run it

1270
00:56:07,096 --> 00:56:09,546
on that little hello.c file.

1271
00:56:09,546 --> 00:56:11,256
But we'll do that again
correctly next time.

1272
00:56:11,496 --> 00:56:13,926
But it turns out that with
this language we glimpsed

1273
00:56:13,926 --> 00:56:16,556
on Wednesday, you can do the
step so much more easily and

1274
00:56:16,556 --> 00:56:19,236
yet the ideas are identical
to what we're gonna explore

1275
00:56:19,236 --> 00:56:21,066
at a lower level in
the weeks to come.

1276
00:56:21,296 --> 00:56:23,966
So this is the simplest
program you can probably write

1277
00:56:23,966 --> 00:56:25,706
in this language called Scratch.

1278
00:56:25,956 --> 00:56:28,326
Scratch is a program that
you can download for free

1279
00:56:28,556 --> 00:56:32,146
and you will see in problem
set 0 which will be posted

1280
00:56:32,146 --> 00:56:35,476
on CS50.net tonight under
problem sets in both standard

1281
00:56:35,476 --> 00:56:38,646
and hacker editions, can
you do the-- can you--

1282
00:56:38,646 --> 00:56:40,146
it will walk you
through the process

1283
00:56:40,146 --> 00:56:41,256
of getting this software.

1284
00:56:41,526 --> 00:56:43,216
This software now
looks like this.

1285
00:56:43,646 --> 00:56:46,136
On the top right hand side
is the so called stage.

1286
00:56:46,136 --> 00:56:47,236
This is where as we saw

1287
00:56:47,236 --> 00:56:50,106
on Wednesday are sprites
or our characters.

1288
00:56:50,106 --> 00:56:53,536
Our little characters
are going to play or move

1289
00:56:53,536 --> 00:56:56,016
as we instruct them to do.

1290
00:56:56,236 --> 00:56:59,416
On the left hand side is
my palette of puzzle pieces

1291
00:56:59,416 --> 00:57:01,036
or building blocks and
those are the things.

1292
00:57:01,036 --> 00:57:02,966
In a moment, I'm gonna
drag and drop in order

1293
00:57:02,966 --> 00:57:04,786
to induce this right
to do something.

1294
00:57:05,066 --> 00:57:07,316
And in the middle of this
blank slate, in the middle,

1295
00:57:07,616 --> 00:57:09,556
this is where my
program is going to go.

1296
00:57:09,786 --> 00:57:11,186
And so as we saw on Wednesday,

1297
00:57:11,186 --> 00:57:13,126
if I wanna just get
this thing started,

1298
00:57:13,406 --> 00:57:16,706
notice that there is this thing
here when green flag clicked,

1299
00:57:16,996 --> 00:57:20,236
that scratches version
of that keyword "main".

1300
00:57:20,496 --> 00:57:22,536
So main is equivalent
to this green flag.

1301
00:57:22,776 --> 00:57:23,616
What can I now do?

1302
00:57:23,616 --> 00:57:26,196
We'll shape like a puzzle
piece that will allow me to go

1303
00:57:26,196 --> 00:57:28,216
to the looks palette at top left

1304
00:57:28,216 --> 00:57:29,996
and it's pretty
self-explanatory.

1305
00:57:29,996 --> 00:57:33,006
The categories, the colors,
very quickly start to make sense

1306
00:57:33,006 --> 00:57:34,216
when you just start
poking around.

1307
00:57:34,216 --> 00:57:37,316
I'm gonna drag this
thing here, zoom in,

1308
00:57:37,416 --> 00:57:39,116
and voila, what do I now see?

1309
00:57:39,376 --> 00:57:41,566
Well, I now have two
blocks interlocked

1310
00:57:41,566 --> 00:57:43,986
and you can pretty much
guess what this is gonna do.

1311
00:57:43,986 --> 00:57:48,436
It's going to say hello
and I can change this here.

1312
00:57:48,706 --> 00:57:50,646
I can change the number of
seconds that it does it.

1313
00:57:50,646 --> 00:57:52,766
And if I want to detach
it, I just click and drag,

1314
00:57:52,766 --> 00:57:54,906
and now I can go back and
put different puzzle pieces

1315
00:57:54,906 --> 00:57:56,186
there altogether.

1316
00:57:56,416 --> 00:57:58,176
But it turns out
though simple, though,

1317
00:57:58,176 --> 00:58:01,356
some of these puzzle
pieces actually are,

1318
00:58:01,596 --> 00:58:05,376
we can actually start to do very
interesting things very quickly.

1319
00:58:05,616 --> 00:58:06,906
So, some jargon first.

1320
00:58:07,226 --> 00:58:09,116
Computers generally
have things called--

1321
00:58:09,116 --> 00:58:12,126
computer languages generally
have things called "statements".

1322
00:58:12,126 --> 00:58:13,476
A statement is an action.

1323
00:58:13,476 --> 00:58:14,396
It's an instruction.

1324
00:58:14,636 --> 00:58:18,026
It's a capability that just
tells the computer what to do.

1325
00:58:18,296 --> 00:58:21,776
Much as you might instruct as a
human, a robot, to do something

1326
00:58:21,776 --> 00:58:23,396
to the extent that's
currently possible.

1327
00:58:23,646 --> 00:58:25,226
So say, "Oh, hi world."

1328
00:58:25,426 --> 00:58:26,856
This would be called
a statement.

1329
00:58:26,856 --> 00:58:28,896
Another one might
be "wait one second"

1330
00:58:29,146 --> 00:58:31,746
which is another building block
or puzzle piece and scratch.

1331
00:58:31,746 --> 00:58:34,316
And another one might
be "play sound meow."

1332
00:58:34,506 --> 00:58:36,556
So now you can start to
see the things can get

1333
00:58:36,556 --> 00:58:37,946
interesting quickly.

1334
00:58:37,946 --> 00:58:39,256
Well, let's go ahead
and do that.

1335
00:58:39,426 --> 00:58:40,906
Let me go ahead and open up some

1336
00:58:40,906 --> 00:58:42,826
of the examples that
I came with.

1337
00:58:42,916 --> 00:58:44,906
All of these are available
on the courses' website

1338
00:58:45,096 --> 00:58:46,446
so you can play afterward.

1339
00:58:46,576 --> 00:58:49,436
And I'm gonna go ahead and
open a file called hai2.sb

1340
00:58:50,356 --> 00:58:52,366
which is the file
extension for this language.

1341
00:58:52,656 --> 00:58:55,056
Now, notice in the middle, I
wrote this at home already,

1342
00:58:55,326 --> 00:58:57,916
you can pretty much
read this stuff top

1343
00:58:57,916 --> 00:58:59,306
to bottom to see what it does.

1344
00:58:59,486 --> 00:59:01,496
When green flag clicked,
I'm gonna say, "Oh,

1345
00:59:01,496 --> 00:59:05,726
hi world for a second", then
wait, say it again and wait,

1346
00:59:05,886 --> 00:59:08,136
say it again, and
then that's it.

1347
00:59:08,136 --> 00:59:09,986
So let's go ahead and
click the green flag.

1348
00:59:09,986 --> 00:59:12,926
And in fact, very overwhelming.

1349
00:59:13,366 --> 00:59:16,556
So let's actually start to
make this more interesting.

1350
00:59:16,556 --> 00:59:20,936
Let me go ahead and instead
open up what I called high3.

1351
00:59:21,806 --> 00:59:23,666
I'm gonna go ahead and say don't
say it 'cause I didn't make

1352
00:59:23,666 --> 00:59:24,146
any changes.

1353
00:59:24,146 --> 00:59:24,836
I'll play this--

1354
00:59:25,516 --> 00:59:30,576
[ Sound Effects ]

1355
00:59:31,076 --> 00:59:31,956
>> Okay, getting cute.

1356
00:59:31,956 --> 00:59:34,466
You do that on loop too long,
it gets really annoying quickly.

1357
00:59:35,666 --> 00:59:37,576
[Laughter] But it's just
a different statement.

1358
00:59:37,576 --> 00:59:40,076
And so when you start to poke
around the puzzle pieces,

1359
00:59:40,076 --> 00:59:42,256
you see that it can say
things, we'll see movement

1360
00:59:42,256 --> 00:59:44,666
in just a moment and sound
and so forth, but at the end

1361
00:59:44,666 --> 00:59:47,466
of the day, this is just like
the program we used a moment ago

1362
00:59:47,466 --> 00:59:50,346
for the sock exercise where
we just hold Philip a step

1363
00:59:50,346 --> 00:59:53,286
after step after step, do this,
do this, do that, and yet,

1364
00:59:53,376 --> 00:59:55,906
here too, we already
see an opportunity

1365
00:59:56,066 --> 00:59:57,476
for that thing we
called the loop.

1366
00:59:57,476 --> 01:00:00,596
I am meowing three times
and you can very quick--

1367
01:00:00,596 --> 01:00:02,976
easily see perhaps that this
very quickly could get messy.

1368
01:00:03,216 --> 01:00:05,626
>> I could do a whole lot
of copy-paste essentially.

1369
01:00:05,836 --> 01:00:08,216
If I want the cats meow six
times, I just make a copy

1370
01:00:08,216 --> 01:00:09,896
of this which you can
do by right-clicking

1371
01:00:09,896 --> 01:00:11,206
or control-clicking
as you'll see,

1372
01:00:11,416 --> 01:00:14,226
and then I just keep doing this
and this, and this, and this,

1373
01:00:14,226 --> 01:00:15,726
but now, what's the problem?

1374
01:00:15,726 --> 01:00:18,346
Well, one, it just starts
to look ridiculous, frankly.

1375
01:00:18,346 --> 01:00:21,226
You should acquire overtime
this instinctive aversion

1376
01:00:21,226 --> 01:00:25,216
to just code or programs that
just look ugly because things

1377
01:00:25,216 --> 01:00:28,146
that look ugly generally
are not easy to maintain.

1378
01:00:28,286 --> 01:00:30,306
For instance, if I
wanted to change the sound

1379
01:00:30,306 --> 01:00:32,846
that the cat is making, my
God, now I have to change it

1380
01:00:32,846 --> 01:00:34,956
on three places but
like 30 places

1381
01:00:35,006 --> 01:00:36,576
if all I've done
is copy and paste.

1382
01:00:36,876 --> 01:00:38,436
So hopefully, there's
an opportunity

1383
01:00:38,436 --> 01:00:40,546
for actually looping
and, ultimately,

1384
01:00:40,546 --> 01:00:42,176
than doing things conditionally.

1385
01:00:42,436 --> 01:00:44,506
So if you wanna do
something conditionally,

1386
01:00:44,726 --> 01:00:46,346
you generally needs
to ask the question,

1387
01:00:46,556 --> 01:00:48,486
is the following true or false.

1388
01:00:48,486 --> 01:00:51,246
In Philip's case, we ask the
question, does he have zero

1389
01:00:51,396 --> 01:00:54,176
or none zero socks on his feet?

1390
01:00:54,176 --> 01:00:57,506
Or rather, does he have two
or not two socks on his feet?

1391
01:00:57,706 --> 01:01:00,066
That's what's called a
Boolean expression named

1392
01:01:00,066 --> 01:01:01,896
after literally a
man named Boole.

1393
01:01:01,896 --> 01:01:05,186
And this puzzle piece
here colored in blue

1394
01:01:05,186 --> 01:01:08,156
because it's a Boolean
expression is just something

1395
01:01:08,156 --> 01:01:10,666
that-- whose answer
is either a yes or no,

1396
01:01:10,746 --> 01:01:13,346
true or false, one or zero.

1397
01:01:13,556 --> 01:01:16,646
So notice already the parallels
between the lower level details

1398
01:01:16,646 --> 01:01:18,676
and now these higher level
details, so to speak,

1399
01:01:18,886 --> 01:01:19,886
of actually programming.

1400
01:01:20,116 --> 01:01:22,846
This Boolean expression
allows you to say to scratch

1401
01:01:23,126 --> 01:01:25,086
if you are touching
the mouse pointer,

1402
01:01:25,086 --> 01:01:27,126
the little arrow, do something.

1403
01:01:27,486 --> 01:01:28,396
How do we do something?

1404
01:01:28,396 --> 01:01:29,176
We'll see in a moment.

1405
01:01:29,176 --> 01:01:30,696
What else can you
detect as a yes

1406
01:01:30,696 --> 01:01:32,576
or no answer is the mouse down?

1407
01:01:32,896 --> 01:01:33,906
Yes or no.

1408
01:01:33,906 --> 01:01:37,026
That can give you a true or
false, a one or a zero answer.

1409
01:01:37,276 --> 01:01:37,866
What about this?

1410
01:01:38,056 --> 01:01:41,476
Is the number on the left less
than the number on the right?

1411
01:01:41,476 --> 01:01:43,636
That might be a question
you might wanna answer

1412
01:01:43,856 --> 01:01:48,616
or you might wanna say, is the
thing on the left and the thing

1413
01:01:48,616 --> 01:01:50,646
on the right both true?

1414
01:01:50,766 --> 01:01:52,366
Do I have two yes answers

1415
01:01:52,566 --> 01:01:54,886
if you wanna check two
conditions at once?

1416
01:01:55,226 --> 01:01:58,126
You know, this might seem to be
a bit of a new context for some

1417
01:01:58,126 --> 01:02:00,806
of you but consider
a typical website.

1418
01:02:00,806 --> 01:02:03,146
If you've been shopping for
courses on Harvard courses,

1419
01:02:03,276 --> 01:02:05,396
the website we mentioned
on Wednesday, well,

1420
01:02:05,396 --> 01:02:07,226
you'll notice you'll
recall perhaps

1421
01:02:07,346 --> 01:02:10,006
that there's all these
checkboxes, all these checkboxes

1422
01:02:10,006 --> 01:02:12,236
that allow you to, say, you
want a course in the fall

1423
01:02:12,236 --> 01:02:14,926
or the spring, Monday, Tuesday,
Wednesday, Thursday, well, now,

1424
01:02:14,926 --> 01:02:17,816
think about even with today's
basics already in mind,

1425
01:02:18,116 --> 01:02:20,296
that this is just
kind of the same idea

1426
01:02:20,486 --> 01:02:22,746
where if you wanna look for
a course that's on the fall

1427
01:02:22,746 --> 01:02:26,476
and on Monday, Tuesday,
that's like saying I want fall

1428
01:02:26,476 --> 01:02:31,756
and Monday, and Tuesday, so each
of these checkboxes is itself,

1429
01:02:31,756 --> 01:02:33,626
really, a Boolean expression.

1430
01:02:33,626 --> 01:02:35,106
And so you see that
stuff all over the place

1431
01:02:35,106 --> 01:02:36,376
on Facebook, Google,
and the like.

1432
01:02:36,586 --> 01:02:38,046
These are not foreign concepts.

1433
01:02:38,046 --> 01:02:39,966
It's just we're now
slapping some new,

1434
01:02:39,966 --> 01:02:41,196
perhaps jargon on them.

1435
01:02:41,386 --> 01:02:44,246
Now that you have the ability
to ask yes or no questions,

1436
01:02:44,426 --> 01:02:46,506
you wanna be able
to branch or fork

1437
01:02:46,506 --> 01:02:48,516
in the road based
on those answers.

1438
01:02:48,516 --> 01:02:50,836
We saw this with the sock
algorithm a moment ago.

1439
01:02:51,016 --> 01:02:54,106
In scratch, a condition looks
a little something like this.

1440
01:02:54,156 --> 01:02:55,966
It's a puzzle piece
that looks kind of small

1441
01:02:56,206 --> 01:02:57,796
but notice there is
this little funky shape

1442
01:02:57,796 --> 01:02:59,116
up top that's the same shape

1443
01:02:59,116 --> 01:03:01,986
as the blue blocks we've just
looked at 'cause you can drag

1444
01:03:01,986 --> 01:03:03,716
and drop that condition
on top of that,

1445
01:03:03,906 --> 01:03:05,926
and even though there's not
much space in the middle here,

1446
01:03:05,926 --> 01:03:08,646
it turns out what's nice
about scratches, it will grow

1447
01:03:08,646 --> 01:03:10,626
to fill more and
more puzzle pieces.

1448
01:03:10,626 --> 01:03:12,976
If you just drag them and drag
them, and drag them in there,

1449
01:03:13,066 --> 01:03:16,736
it will keep growing to fill--
to fit any such statements.

1450
01:03:16,816 --> 01:03:20,326
And you don't just have to have
an if or nothing construct.

1451
01:03:20,486 --> 01:03:22,616
You can say if this
is true, then go

1452
01:03:22,616 --> 01:03:25,446
and do the following pieces
of code or puzzle pieces

1453
01:03:25,446 --> 01:03:27,976
that are inside this part
of the block, else go ahead

1454
01:03:27,976 --> 01:03:29,866
and do these that are down here.

1455
01:03:29,866 --> 01:03:31,216
And we saw this with
Philip's example.

1456
01:03:31,216 --> 01:03:33,166
But sometimes there are
three roads you can take

1457
01:03:33,436 --> 01:03:35,856
and so even though it starts
to look a little messy,

1458
01:03:35,856 --> 01:03:38,826
if you do this too much, you
can certainly branch this way

1459
01:03:38,826 --> 01:03:40,506
or this way, or this way

1460
01:03:40,506 --> 01:03:43,316
and these are what we
generally call conditions.

1461
01:03:43,316 --> 01:03:47,736
Well, let's actually
put these to use.

1462
01:03:48,226 --> 01:03:50,396
Here's high4.sb.

1463
01:03:50,396 --> 01:03:50,463
[ Sound Effects ]

1464
01:03:50,463 --> 01:03:52,686
>> Kind of a stupid
program, why?

1465
01:03:55,506 --> 01:03:55,806
[Laughter] Right.

1466
01:03:55,896 --> 01:03:59,256
So this is a great way of like
fulfilling the requirements

1467
01:03:59,256 --> 01:04:00,946
of the problem step but
not quite the spirit

1468
01:04:00,946 --> 01:04:02,176
of using a condition.

1469
01:04:02,866 --> 01:04:04,886
So this is not all
that interesting,

1470
01:04:04,916 --> 01:04:08,096
but let's actually take a
look at a 5th version of high

1471
01:04:08,386 --> 01:04:09,856
that actually does
something interesting.

1472
01:04:09,856 --> 01:04:12,316
Oh, it turns out there's a
puzzle piece that allows you

1473
01:04:12,386 --> 01:04:14,896
to pick a random or as we'll
start calling it soon a

1474
01:04:14,896 --> 01:04:18,676
pseudorandom number so that
scratch can effectively roll a

1475
01:04:18,676 --> 01:04:22,406
dice or flip a coin, or give
you a bit of non-determinism.

1476
01:04:22,406 --> 01:04:23,656
It's the fancy way
of saying this.

1477
01:04:23,896 --> 01:04:25,786
And this is useful because
if you're actually writing a

1478
01:04:25,786 --> 01:04:28,056
program that's interactive
or that you want

1479
01:04:28,056 --> 01:04:31,446
to change overtime much like
you would a game, hopefully,

1480
01:04:31,606 --> 01:04:34,556
the game is not going to have
the bad guys, for instance,

1481
01:04:34,556 --> 01:04:37,576
always standing in the
same place or the rocks

1482
01:04:37,576 --> 01:04:39,066
or whatever it is
falling from the sky

1483
01:04:39,066 --> 01:04:41,616
in the same place 'cause it
would very quickly become a very

1484
01:04:41,756 --> 01:04:42,426
tedious game.

1485
01:04:42,426 --> 01:04:45,406
Almost any game you play
has some kind of randomness

1486
01:04:45,406 --> 01:04:47,506
or pseudorandomness which means

1487
01:04:47,506 --> 01:04:50,826
that things change overtime
somewhat unpredictably.

1488
01:04:50,826 --> 01:04:52,676
So now, if I click
the green flag,

1489
01:04:53,276 --> 01:04:56,986
the program seems
broken or-- I don't know.

1490
01:04:56,986 --> 01:04:58,656
It's just the number
that was picked between 1

1491
01:04:58,656 --> 01:05:03,116
and 10 that's probably bigger
than 5 or 6, not smaller.

1492
01:05:03,116 --> 01:05:05,236
Let's try it again.

1493
01:05:05,236 --> 01:05:06,556
Let's try it again.

1494
01:05:06,556 --> 01:05:07,516
[ Sound Effects ]

1495
01:05:07,516 --> 01:05:08,086
>> There we go.

1496
01:05:08,086 --> 01:05:12,416
Now if I do this, an infinite
[sound effects] number of times,

1497
01:05:12,416 --> 01:05:15,096
we should get roughly 50/50,
but now we have the ability

1498
01:05:15,096 --> 01:05:16,796
to do things only
once in awhile.

1499
01:05:16,796 --> 01:05:18,176
So now, loops.

1500
01:05:18,416 --> 01:05:21,576
You wanna do something
again and again.

1501
01:05:21,576 --> 01:05:24,306
You can kind of see where
the syntax is going,

1502
01:05:24,566 --> 01:05:27,276
looks like a loop, but you
can kind of put blocks inside

1503
01:05:27,276 --> 01:05:29,016
of you, and the little arrow
suggests that once you get

1504
01:05:29,016 --> 01:05:31,026
to the bottom just
like our socks example,

1505
01:05:31,276 --> 01:05:32,666
you're gonna do the
following again.

1506
01:05:32,906 --> 01:05:33,976
What else might it look like?

1507
01:05:33,976 --> 01:05:34,916
You might be able to say

1508
01:05:34,916 --> 01:05:36,946
and we'll see this
in C and PHP as well.

1509
01:05:37,166 --> 01:05:40,056
Do the following a bunch of
times, but a finite number

1510
01:05:40,106 --> 01:05:41,476
of times which might be useful.

1511
01:05:41,476 --> 01:05:42,946
So here we have a repeat block.

1512
01:05:43,196 --> 01:05:44,626
Let's actually put
this into play

1513
01:05:44,626 --> 01:05:46,556
with the little sprite
known as scratch.

1514
01:05:46,996 --> 01:05:47,926
So this is high6.

1515
01:05:47,926 --> 01:05:50,766
And now, as promised,
and all even full screen,

1516
01:05:50,766 --> 01:05:51,456
it's really annoying.

1517
01:05:52,516 --> 01:06:00,906
[ Sound Effects ]

1518
01:06:01,406 --> 01:06:02,456
>> If we weren't so
pressed for time,

1519
01:06:02,456 --> 01:06:04,036
I kinda wanna see how
long we could get away

1520
01:06:04,036 --> 01:06:06,876
with this before it gets
awkward, but [laughter] that,

1521
01:06:06,876 --> 01:06:10,256
in fact, is an infinite loop
that's hopefully deliberate.

1522
01:06:10,256 --> 01:06:12,516
Otherwise, it's a
pretty bad mistake,

1523
01:06:12,776 --> 01:06:15,296
but sometimes you do want an
infinite loop, for instance,

1524
01:06:15,296 --> 01:06:18,236
Microsoft Word if you
use it or pages or the

1525
01:06:18,236 --> 01:06:19,886
like on your Mac or PC.

1526
01:06:20,116 --> 01:06:23,116
These days, those things
constantly spell check your

1527
01:06:23,116 --> 01:06:23,976
program for you.

1528
01:06:23,976 --> 01:06:26,746
So in a sense, that's a good
infinite loop that is as long

1529
01:06:26,746 --> 01:06:27,696
as you're using the program,

1530
01:06:27,836 --> 01:06:30,056
it's constantly checking
your grammar and spelling,

1531
01:06:30,056 --> 01:06:30,986
and underlining things,

1532
01:06:31,226 --> 01:06:33,286
so infinite loops
aren't necessarily bad

1533
01:06:33,566 --> 01:06:36,006
if the objective is
actually being met.

1534
01:06:36,256 --> 01:06:38,306
Well, let's take a look
at this high7 variant

1535
01:06:38,686 --> 01:06:41,906
that also use loops,
that also uses a loop.

1536
01:06:42,286 --> 01:06:44,216
So this one is a little
cute, I like to think.

1537
01:06:45,026 --> 01:06:45,636
So notice this.

1538
01:06:45,686 --> 01:06:46,796
My mouse cursor is over here

1539
01:06:46,796 --> 01:06:49,086
on the top right,
now, I touch the cat.

1540
01:06:49,086 --> 01:06:50,026
[ Sound Effects ]

1541
01:06:50,026 --> 01:06:50,506
>> Move away.

1542
01:06:50,506 --> 01:06:51,866
[ Sound Effects ]

1543
01:06:51,866 --> 01:06:55,336
>> So this is my--
petting a cat, example.

1544
01:06:55,336 --> 01:06:56,376
Well, how does this work?

1545
01:06:56,786 --> 01:06:58,936
We'll notice, it's just
a few puzzle pieces.

1546
01:06:58,936 --> 01:07:01,296
I'm forever watching
the situation.

1547
01:07:01,436 --> 01:07:04,026
And now, if I'm touching
mouse pointer,

1548
01:07:04,026 --> 01:07:06,856
we saw that Boolean expression
before, play sound meow,

1549
01:07:06,906 --> 01:07:09,056
wait a couple of seconds,
and then just keep checking.

1550
01:07:09,056 --> 01:07:11,416
So if I left my cursor there
like you're petting the cat,

1551
01:07:11,606 --> 01:07:13,916
it would just keep meowing
and meowing every two seconds,

1552
01:07:13,976 --> 01:07:17,126
but if I move it away, that
Boolean expression becomes false

1553
01:07:17,126 --> 01:07:19,226
or zero and so we
don't do it again.

1554
01:07:19,226 --> 01:07:21,936
And just to be clear, you can
disassemble these programs,

1555
01:07:21,936 --> 01:07:23,296
you can move this
thing over here.

1556
01:07:23,466 --> 01:07:25,496
It's all about dragging
and dropping which is,

1557
01:07:25,496 --> 01:07:26,506
to say, is very intuitive.

1558
01:07:26,506 --> 01:07:28,656
If you think of the idea,
you think of the basic logic,

1559
01:07:28,926 --> 01:07:31,256
and then you go ahead and
drag and drop accordingly.

1560
01:07:31,256 --> 01:07:33,086
Let's look at this
8th variance of this,

1561
01:07:33,086 --> 01:07:34,426
otherwise, simple example.

1562
01:07:34,766 --> 01:07:37,326
Well, this one was me
having a little fun here.

1563
01:07:37,326 --> 01:07:38,156
So play--

1564
01:07:38,156 --> 01:07:39,336
[ Sound Effects ]

1565
01:07:39,336 --> 01:07:40,586
>> Cat meows ad nauseam.

1566
01:07:41,496 --> 01:07:45,266
But touch the cat and--
[sound effects] [laughter].

1567
01:07:46,296 --> 01:07:47,876
So don't touch this
particular cat.

1568
01:07:48,146 --> 01:07:49,696
So how is this working?

1569
01:07:49,856 --> 01:07:52,736
Well, again, I'm not spending
too much time on the specifics

1570
01:07:52,736 --> 01:07:54,226
because again, it
should hopefully start--

1571
01:07:54,226 --> 01:07:56,006
it was, oh, that's
kind of intuitive.

1572
01:07:56,006 --> 01:08:00,076
If something is true do this,
else something is true, do that.

1573
01:08:00,426 --> 01:08:02,526
Well, what are some of the other
constructs we're gonna have

1574
01:08:02,626 --> 01:08:04,366
access to starting
with scratch and see?

1575
01:08:04,526 --> 01:08:05,546
Well, certainly, variables.

1576
01:08:05,666 --> 01:08:08,066
If you want to maintain
states, that is, to say,

1577
01:08:08,066 --> 01:08:10,946
remember some value, well, you
can use what's called a variable

1578
01:08:10,946 --> 01:08:13,276
and here's what it looks like
in this language called scratch.

1579
01:08:13,526 --> 01:08:15,956
Now, who cares about variables
Xs and Ys and all these?

1580
01:08:15,956 --> 01:08:17,476
Well, what about a game?

1581
01:08:17,516 --> 01:08:19,426
Anything where you're actually
keeping track of something

1582
01:08:19,426 --> 01:08:21,546
like a score that's
having a variable

1583
01:08:21,546 --> 01:08:24,786
that you increment plus, plus,
or decrement, minus, minus,

1584
01:08:25,086 --> 01:08:27,306
using the appropriate
puzzle piece could certainly

1585
01:08:27,306 --> 01:08:27,996
be compelling.

1586
01:08:28,226 --> 01:08:29,776
Suppose you want a
character to move

1587
01:08:29,776 --> 01:08:31,796
around the screen
and collect things.

1588
01:08:31,876 --> 01:08:34,306
So it doesn't make sense to just
use a number 'cause you want

1589
01:08:34,306 --> 01:08:35,606
this character to
collect something

1590
01:08:35,606 --> 01:08:37,196
from here or here, or there.

1591
01:08:37,386 --> 01:08:40,326
Well, you can use what's called
an array which is kind of a list

1592
01:08:40,586 --> 01:08:43,236
or a little satchel of that
you might physically carry

1593
01:08:43,366 --> 01:08:45,646
that you can put things
in again and again,

1594
01:08:45,856 --> 01:08:48,006
and with an array can
you actually keep track

1595
01:08:48,006 --> 01:08:51,596
of not just numbers but any
kinds of things, other sprites

1596
01:08:51,596 --> 01:08:54,756
or other pictures, or perhaps
a little something like this.

1597
01:08:54,806 --> 01:08:57,726
Let me go ahead and pull up
this little example here.

1598
01:08:57,726 --> 01:09:00,566
It looks like we're dwelling
on only the most simple.

1599
01:09:01,216 --> 01:09:02,896
Go ahead and click
the green flag.

1600
01:09:03,846 --> 01:09:06,776
You'll see this was written
by a former student of ours.

1601
01:09:07,706 --> 01:09:09,956
Notice the top left is
a picture of inventory.

1602
01:09:09,956 --> 01:09:11,666
That's just a graphical
incarnation

1603
01:09:11,666 --> 01:09:15,376
of this thing called an array
and notice, the games can,

1604
01:09:15,376 --> 01:09:17,076
in fact, interact
with a human much

1605
01:09:17,076 --> 01:09:18,706
like Wednesday, I'm
leaving my house.

1606
01:09:19,716 --> 01:09:20,706
Oh, here are some cherries.

1607
01:09:20,706 --> 01:09:22,776
Oh, look. In my array
at top left,

1608
01:09:22,776 --> 01:09:23,976
they have now cherries in it.

1609
01:09:24,066 --> 01:09:26,286
Let's see what this
is, probably an orange.

1610
01:09:26,956 --> 01:09:28,016
Let me go over here.

1611
01:09:28,016 --> 01:09:29,966
And now think about
how you do this.

1612
01:09:29,966 --> 01:09:32,446
If right arrow key
down, move right.

1613
01:09:32,486 --> 01:09:34,286
If right arrow key
down, move right.

1614
01:09:34,546 --> 01:09:39,326
If touching sign, display
this little cartoon bubble.

1615
01:09:39,326 --> 01:09:42,656
So let me go get the pineapple,
let me go get the bananas.

1616
01:09:42,716 --> 01:09:44,096
Let me go get the apple.

1617
01:09:44,456 --> 01:09:46,236
I think I-- where do I have
to go, the fruit place?

1618
01:09:46,336 --> 01:09:51,116
Oh, there's a man at the
store and now we have a loop

1619
01:09:51,566 --> 01:09:54,486
where he is just looking through
my inventory one at a time

1620
01:09:54,816 --> 01:09:59,906
and reciting what it is
I actually picked up.

1621
01:10:00,046 --> 01:10:02,606
>> Well, what more can we do
with some of these fundamentals?

1622
01:10:02,606 --> 01:10:05,526
Well, with arrays and the like,
after arrays and the like,

1623
01:10:05,526 --> 01:10:06,976
we have these things
called threads.

1624
01:10:07,316 --> 01:10:09,846
So thankfully, you can do
multiple things at once

1625
01:10:09,846 --> 01:10:12,266
in a program because of
what we'll call threads.

1626
01:10:12,576 --> 01:10:14,996
Here is an example in just
a moment of a program,

1627
01:10:15,076 --> 01:10:18,646
a scratch project, so to speak,
with two sprites; one bird

1628
01:10:18,646 --> 01:10:20,706
and one cat, and
what this means is

1629
01:10:20,706 --> 01:10:23,006
that even though the interface
is getting a little complicated,

1630
01:10:23,006 --> 01:10:25,156
realize the student did
this with baby steps,

1631
01:10:25,156 --> 01:10:27,026
implements in a little bit of
this, a little bit of this,

1632
01:10:27,026 --> 01:10:30,026
and then over time, it grows to
be a pretty impressive program,

1633
01:10:30,256 --> 01:10:32,046
but it starts off as
just a clean slate.

1634
01:10:32,286 --> 01:10:33,746
Notice at the bottom right here,

1635
01:10:33,876 --> 01:10:35,356
we have all these
different icons.

1636
01:10:35,496 --> 01:10:36,916
I can click on this guy

1637
01:10:37,126 --> 01:10:39,536
to see what sprites
are controlling him.

1638
01:10:39,776 --> 01:10:40,976
Here is where the guy yawned.

1639
01:10:41,256 --> 01:10:42,476
It's because of that script.

1640
01:10:42,646 --> 01:10:44,756
I can click on the cherries
and see what they're doing,

1641
01:10:45,056 --> 01:10:46,086
you know, not all that much.

1642
01:10:46,206 --> 01:10:47,856
They're just sitting there,
but they are listening

1643
01:10:47,856 --> 01:10:49,206
for something we'll
see in a moment.

1644
01:10:49,476 --> 01:10:54,286
But let me go ahead and open
up, let's say, this cat example,

1645
01:10:54,416 --> 01:10:58,146
move two-- you can even have
different sprites interact

1646
01:10:58,146 --> 01:11:00,116
with each other much like this.

1647
01:11:01,766 --> 01:11:04,286
Now, all the cat is doing is
that it's an infinite loop

1648
01:11:04,496 --> 01:11:08,326
and it's checking if
I am not facing bird,

1649
01:11:08,326 --> 01:11:10,946
face bird, and it's [noise]--

1650
01:11:10,946 --> 01:11:11,466
[ Laughter ]

1651
01:11:11,466 --> 01:11:12,896
>> It's constantly
chasing the bird.

1652
01:11:12,896 --> 01:11:14,996
And so now, you have
two threads so to speak,

1653
01:11:14,996 --> 01:11:16,516
two miniature programs,

1654
01:11:16,516 --> 01:11:19,726
two sprites all interacting
with one another.

1655
01:11:19,726 --> 01:11:21,856
You can do other things too, and
I know I'm flying through these

1656
01:11:21,856 --> 01:11:25,776
but only because it's more fun
to play hands-on on your own--

1657
01:11:25,776 --> 01:11:27,556
oh, actually, this
guy is kinda funny.

1658
01:11:27,556 --> 01:11:29,766
Let's play this.

1659
01:11:29,766 --> 01:11:30,772
[ Noise ]

1660
01:11:30,772 --> 01:11:31,778
[ Laughter ]

1661
01:11:31,778 --> 01:11:32,784
[ Noise ]

1662
01:11:32,786 --> 01:11:34,796
>> This is all he does.

1663
01:11:34,796 --> 01:11:35,166
[ Laughter ]

1664
01:11:35,166 --> 01:11:35,536
[ Noise ]

1665
01:11:35,536 --> 01:11:38,596
>> But again, it hints
with these fundamentals.

1666
01:11:38,596 --> 01:11:41,516
You've got a loop, you've got a
condition and a boolean to check

1667
01:11:41,516 --> 01:11:43,776
if he's touching the edge, and
some statements to tell him

1668
01:11:43,776 --> 01:11:47,576
to shout or to actually
turn around.

1669
01:11:47,576 --> 01:11:49,446
So what more is there?

1670
01:11:49,516 --> 01:11:52,696
There are also these things
called events in programming,

1671
01:11:52,696 --> 01:11:55,536
and specifically, scratch,
whereby you can have two sprites

1672
01:11:55,536 --> 01:11:56,966
such as this boy and this girl,

1673
01:11:57,196 --> 01:12:00,436
and when you actually hit the
spacebar with the like-- oops--

1674
01:12:00,706 --> 01:12:03,166
if you actually hit the
spacebar or the like,

1675
01:12:04,196 --> 01:12:06,106
one sprite can do something,

1676
01:12:06,776 --> 01:12:08,866
and then the other sprite
can do something in response

1677
01:12:08,866 --> 01:12:11,376
because in addition to
When Green Flag Clicked,

1678
01:12:11,686 --> 01:12:14,206
there's also this kind of
starting point, this alternative

1679
01:12:14,206 --> 01:12:17,046
to main that says
When I Receive Event.

1680
01:12:17,426 --> 01:12:20,226
In other words, you can pass
sort of secret messages or notes

1681
01:12:20,226 --> 01:12:24,006
between sprites by giving them a
name, like in this case, events,

1682
01:12:24,006 --> 01:12:26,016
and then having one
talk to the other.

1683
01:12:26,016 --> 01:12:27,296
So this is a simple incarnation

1684
01:12:27,296 --> 01:12:29,676
of that game you might
know as Marco Polo.

1685
01:12:29,896 --> 01:12:32,126
Now, in Problem Set Zero,
well, you get to play

1686
01:12:32,126 --> 01:12:35,056
with exactly these things and
in the hacker edition if you

1687
01:12:35,056 --> 01:12:37,386
so choose, can you play
with one of these things.

1688
01:12:37,446 --> 01:12:39,856
I taught I'd demo just a couple
little snippets with this.

1689
01:12:40,006 --> 01:12:41,586
So to be clear, we
expect most students

1690
01:12:41,586 --> 01:12:44,446
to very comfortably pursue the
standard edition of the P set,

1691
01:12:44,696 --> 01:12:47,436
but for this particular P set,
the hacker edition gives you--

1692
01:12:47,436 --> 01:12:50,086
and we have about 150 or so
of these out on the hallway,

1693
01:12:50,086 --> 01:12:51,836
and we'll have them at office
hours throughout the coming

1694
01:12:51,836 --> 01:12:54,576
week-- this is a little logic
board with lots of sensors;

1695
01:12:54,576 --> 01:12:56,396
the sound sensor,
a light sensor,

1696
01:12:56,476 --> 01:12:58,106
a little joystick and the like.

1697
01:12:58,376 --> 01:13:00,706
And now, that you have not
just the keyboard, but a device

1698
01:13:00,706 --> 01:13:04,236
like this connected via USB
can you do something like this,

1699
01:13:04,596 --> 01:13:06,066
like a former student did.

1700
01:13:06,406 --> 01:13:07,656
This is a little game--

1701
01:13:08,176 --> 01:13:10,046
and again, don't be
overwhelmed by the complexity.

1702
01:13:10,046 --> 01:13:13,106
It's all about building
up bytes after bytes--

1703
01:13:14,516 --> 01:13:18,516
[ Music ]

1704
01:13:19,016 --> 01:13:19,083
[ Background Music ]

1705
01:13:19,083 --> 01:13:20,896
>> So I'm now moving the
joystick, and the goal

1706
01:13:20,896 --> 01:13:25,046
of this game is not to hit--
oh, to get the bananas,

1707
01:13:25,926 --> 01:13:28,836
but now I've been stunned,
there's the man that fell

1708
01:13:28,836 --> 01:13:29,726
from the ceiling on Wednesday,

1709
01:13:29,726 --> 01:13:32,816
and it's responding
to the joystick.

1710
01:13:32,966 --> 01:13:35,106
You can do other things too
though, when you have sound.

1711
01:13:35,206 --> 01:13:38,876
For instance, let me go
ahead and open up masquerade,

1712
01:13:38,876 --> 01:13:41,236
and I'll need Barry to help
here, if you don't mind.

1713
01:13:41,486 --> 01:13:42,396
Let me hit play.

1714
01:13:42,876 --> 01:13:44,916
So here, we have-- come on.

1715
01:13:46,496 --> 01:13:49,466
Wait, don't do that yet.

1716
01:13:49,546 --> 01:13:55,476
Come on. I need more lights.

1717
01:13:55,526 --> 01:13:55,936
Wait a minute.

1718
01:13:55,936 --> 01:13:56,886
[ Laughter ]

1719
01:13:56,886 --> 01:13:58,366
>> We can solve this.

1720
01:13:59,156 --> 01:14:04,536
It's a little dark
in here as well.

1721
01:14:04,536 --> 01:14:04,603
[ Background Music ]

1722
01:14:04,603 --> 01:14:06,816
>> Okay. It's very nice and
beautiful, daylight outside,

1723
01:14:06,816 --> 01:14:07,976
and Barry, could
you kill the lights?

1724
01:14:07,976 --> 01:14:08,043
[Noise ]

1725
01:14:08,043 --> 01:14:15,056
So can you actually
detect sound in this way?

1726
01:14:16,166 --> 01:14:18,436
Let me stop playing this
since it will just go

1727
01:14:18,436 --> 01:14:19,596
on ad nauseam as well.

1728
01:14:20,106 --> 01:14:22,926
So what does this
actually mean for you?

1729
01:14:22,996 --> 01:14:25,936
So with problems at zero, which
will be posted in two forms;

1730
01:14:26,026 --> 01:14:29,506
PDF for standard and PDF for
hacker on cs50.net tonight,

1731
01:14:29,706 --> 01:14:30,586
you will have the choice

1732
01:14:30,586 --> 01:14:34,836
of implementing pretty much any
scratch projects you can think

1733
01:14:34,836 --> 01:14:36,786
of, and which subject
to very few constraints.

1734
01:14:36,886 --> 01:14:38,826
At the end of the day, the goal
is to make something that's

1735
01:14:38,826 --> 01:14:43,756
of interest to you, fun for you,
and you'll see at cs50.net/ohs

1736
01:14:43,756 --> 01:14:46,496
for office hours, what
the schedule is therefore.

1737
01:14:46,496 --> 01:14:48,426
And let me conclude with this,

1738
01:14:48,426 --> 01:14:53,076
you have an amazing
10 weeks ahead of you.

1739
01:14:53,466 --> 01:14:56,516
And so, we will see you

1740
01:14:56,996 --> 01:15:00,926
if I time this correctly
on Wednesday.

1741
01:15:02,516 --> 01:15:09,516
[ Music ]

1742
01:15:10,016 --> 01:15:12,766
[ Silence ]