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[ Silence ]

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>> This is CS50, Week
1, thought we'd begin

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with a cookie love story.

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[ Music ]

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>> So this was a Scratch
project submitted by one

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of last year's students.

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And what's nice is because
not only is it rather cute,

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it also draws in all
of these fundamentals

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that we started talking
about last week.

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And if you haven't
yourself dived

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into your own problem set zero,
whose PDFs are available online,

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realize that if something like
this, given that it's a minute

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or so long, kind of looks you
know, despite the cutesiness

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of it, kind of overwhelming
in that you don't really know

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where to begin, how
could you possibly sketch

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out this whole story line.

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Well, realize one of the themes
in this course and really one

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of the themes in programming
and solving any problem

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with a machine is
going to be really

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to just take small bites one
at a time out of these problems

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until the end result
ultimately is

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that you've actually bitten
off a fairly large fairly

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interesting problem
and in the aggregate,

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you've actually implemented
something pretty sophisticated.

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But if you set out trying to
implement this whole story line

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at once, if you set out trying

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to implement a cookie love
story, it's just not gonna fly.

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You're gonna run into
innumerable bugs most likely,

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by bugs we mean mistakes,

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behaviors that you didn't quite
intend and yet they seem to be--

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and yet your program seems to
be misbehaving in some sense.

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And so with this example
here, I thought I'd propose

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that we consider
exactly how you'd go

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about tackling something
like this.

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Let me restart it but in the
smaller scope of this window.

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And there's a few
things going on here,

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so clearly there is multiple
sprite, multiple characters,

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and recall from last week that
each sprite has its own set

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of strips or programs.

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The scripts are in the
middle, the sprites are

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on the right hand side, so
when I have the stage selected

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at the moment, I'm seeing the
script that are associated

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with sort of the whole program
itself, the so called stage.

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But I could home in
on sprite 1 there

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and just see this
particular sprite script.

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So now let me actually
reverse this

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and go back to the beginning.

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So it's a lot just
happened here.

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But how do you go

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about implementing
something like this.

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Well, honestly start easy.

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The first thing you
probably want to accomplish

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or at least one thing that
seems easy is find the graphic

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of the cookie sheet that you
happen to want for this project.

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Lay it down and make sure it's
in the right position, right?

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Check. So, we're
making progress already.

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It doesn't do anything

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but at least that's one
bite out of this problem.

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Now, focus on just
one of these sprites.

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For instance the cookie,
the gingerbread man,

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the gingerbread woman
or whichever one

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of the cookies you want to
introduce first and focus just

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on getting that cookie to dance.

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And what do we mean by dance?

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Figure out how you can make
it move a little to the left,

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a little to the right, up
and down and then repeat

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in some kind of looping
structure.

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And then after a few
minutes or maybe an hour,

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you hopefully have at
least one dancing cookie.

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Maybe there is no music, so
it's just kind of dancing

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in the middle of a cookie
sheet all by himself.

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But now at least
you have a template

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that you can base cookies
number 2 and 3 and 4 and 5.

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And if you haven't
realized already,

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do realize per the tutorials
that we've linked to online,

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you can do things like
duplicating sprites,

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you don't have to
re-implement the whole thing.

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And so you can really--

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the process starts to pick
up traction pretty quickly.

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Now on the next screen,
once this thing resumes,

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realize that-- come on.

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[ Background Music ]

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>> Once we changes phases,

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realize that a whole
lot more was happening

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on the next screen.

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In just a moment the hearts are
gonna start flowing from bottom

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to top and the cookies are gonna
start moving left and right

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and they seem to be somewhat
synchronized with the music,

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perhaps based on trial
and error by the student

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who implemented this one.

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Well, how do you get this hearts
to move up and own and seemingly

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in these random patterns?

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Well, recall that very basic
building block we introduced

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last week which allowed you to
grab a random number between 1

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and 10 or 1 or a hundred.

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And if you haven't realized
already, the whole point

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of this stage being a rectangle
is that ask him to buy the X

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and Y coordinates there, it's
essentially a grid of dots,

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of so-called pixels that
go up, down, left, right,

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and each of them is
a specific address,

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X comma Y. So if you wanna
pick a random location for one

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of those hearts so it starts
from a different place,

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we'll pick a random X of random
number between 1 and 200 or 300,

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or however wide the stage is,
and then use the move block

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from the appropriate palette
to put that's right there,

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and then start its movement.

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In other words, when you
bite off your own project

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or when you start off by looking
at someone else's projects,

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either that we looked
at last week or some

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of the innumerable links
we included in the specs,

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focus on just one piece
of it, get that working,

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and then move on to the next.

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And this really is going to be
a recurring theme throughout

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the course.

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You can very quickly drive
yourself nuts by sitting down,

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thinking, oh, I can write
this program, no sweat,

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and then you write it and then
there's a tiny little mistake

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here, but then there's a tiny
little mistake here and here,

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and all of these stupid
little things cascade

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until you try running
your program,

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and the thing just doesn't work.

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And then it's the situation

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where you just don't
know where to begin.

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So honestly, one of the best
strategies in this course

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and in programming in general
is-- generally speaking design,

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but it's these little
tricks of the trade

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that will honestly make things
much more fun, much more easier,

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and really allow you to focus on
the juicy parts of the problem

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and not little tiny bugs

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that you may have started
tripping over already.

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So hopefully, approach
if you haven't already,

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problem set zero
with that mindset.

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Take quite literally baby steps.

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And from each of these
individual parts, well,

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you very likely come out
with a very impressive whole.

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So, with that said, we like
to Google ourselves sometimes

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and so we discovered that FlyBy
last night mentioned CS50 among

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some other courses in its
last minute shopping list.

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So this is to say, for those
of you who are joining us today

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for the first time,
that's totally fine.

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Nothing has been due yet.

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That's not till Friday, and last
week's two lectures are both

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available online at cs50.net.

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I didn't quite know how

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to interpret the tag
line that was up here.

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If you zoom in up top,
which I can't quite do

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with a keynote here, it says,

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"This last minute
shopping, bargain hunting."

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So, I'm not quite sure
what the takeaway there is

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but realize you are
perfectly welcome to shop

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and then still take the course.

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If you decide to opt
in by a pass fail,

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I just need to sign
your white study card

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or if you wanna decide
before the 5th Monday,

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just grab the pink slip and I
can sign that and you can turn

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that in at any point and I'm
happy to chat during break today

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or after class if you have
any questions or concerns.

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So, where is the support
structure thus far?

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We haven't sectioned yet.

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That will officially
start this Friday.

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So more details on sectioning
on Friday, but do realize

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that office hours are
very much in progress.

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Our 60 plus person staff has
been hanging out in a couple

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of dining halls on campus,

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namely the Cabot
and Quincy House.

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Moving forward, we'll
actually move some place other

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than the dining halls but we
thought this is a nice way

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to start the term in a more
casual communal environment.

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But just go to cs50.net,
click office hours

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and you'll see the
Google Calendar with all

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of the office hours that
are still happening tonight,

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as well as tomorrow and these
are just an opportunity to chat,

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to ask questions one on one
with the teaching fellows

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or course assistants or
frankly just a way to hang

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out during brain break
and work on projects

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and then see what some
of your friends might be

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tackling themselves.

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As for sections, there will be
what we call super sections this

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coming Sunday, Monday, Tuesday.

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CS50 sections are standardized
on Sundays, Mondays and Tuesdays

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but we won't officially assign
you to sections for a week or so

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but unofficially, well, there'll
be open sections this coming

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Sunday, Monday, Tuesday.

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The schedule is also on the
course's website under sections.

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And these are super
sections in just

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that they are superset
of the class.

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You can go to any one of
these times more comfortable,

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less comfortable, somewhere
in between, and the focus

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of these sections will be on the
material cover this week as well

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as potentially the start of
next week and with a focus

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on problem set 1, which
is the second problem set,

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0 index which will go
out via PDF this weekend.

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So realize the support structure
is now in place and in progress

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so do just let us
know at help@cs50.net

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if you have questions that
can't be addressed during

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office hours.

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So, we looked last week
at Scratch, very briefly,

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but hopefully you found
if you've dived in already

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that the interface
is fairly intuitive

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and it really just allows
you to implement ideas

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that you conjure
up in your mind.

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Well today we're
gonna transition

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to a more traditional
programming language,

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a lower level programming
language if you will

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where the ideas are
pretty much identical

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to what we've seen last week
in what you might have seen

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in prior courses, but what
I thought I'd do is try

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to set your minds
on the equivalence

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of the ideas we talked
about last week,

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the ideas we'll talk
about this week.

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And even though the syntax, the
characters you are about to see

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on the screen and in programs
today onward is a little more

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arcane, it's a little uglier
looking, you've got semicolons

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and parentheses and
fairly arcane syntax,

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realize that at the end

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of the day these are just
arbitrary human conventions,

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the ideas are identical
to this thing here.

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>> So this thing here, when
green flag clicked say, oh,

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hi world, this was perhaps the
simplest program we could write

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in Scratch.

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It's sort of the canonical
hello world program

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as computer scientists call
this very simple example.

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Well, starting today
in a language called C,

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this is going to look like this.

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So again immediately on first
glance, a little more arcane,

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there are some distracting
details, these curly braces,

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the semicolon, the
quotation marks.

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But if you avoid all of these
minutia and focus really just

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on the idea, it looks like
what was a blue say block,

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a so called statement,
is now going

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to be something called
printf where the F happens

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to stand for formatted.

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But the string, the sentence or
the words that we're displaying

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on the screen, previously
you clicked

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on a little white text
field and typed it in,

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well now you're gonna do the
same thing at the keyboard

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but you're gonna
have to sandwich it

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in between double
quotation marks.

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So it's these little differences

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that you'll very
quickly acclimate to

241
00:11:00,066 --> 00:11:02,156
but it's these stupid
little syntactic details

242
00:11:02,156 --> 00:11:04,456
that trips off, especially
those less comfortable early

243
00:11:04,456 --> 00:11:07,586
on because if you leave
out a single character,

244
00:11:07,586 --> 00:11:08,786
namely that semicolon

245
00:11:08,786 --> 00:11:10,866
on the right hand side,
nothing will work.

246
00:11:11,096 --> 00:11:14,436
Which is an unfortunate design
decision made many years ago

247
00:11:14,736 --> 00:11:15,966
with this particular language.

248
00:11:15,966 --> 00:11:18,066
More modern languages as
we'll see toward the tail end

249
00:11:18,066 --> 00:11:19,866
of the semester are a
little more generous

250
00:11:19,866 --> 00:11:20,976
with some of these details.

251
00:11:21,206 --> 00:11:23,686
Well, this statement here
say "oh hi world" is going

252
00:11:23,686 --> 00:11:26,846
to map very specifically
to this line here.

253
00:11:26,846 --> 00:11:28,746
What about some of the other
constructs we looked at?

254
00:11:28,746 --> 00:11:30,666
Well loops are something
we talked briefly about.

255
00:11:30,866 --> 00:11:32,266
There is something
you perhaps employed.

256
00:11:32,266 --> 00:11:35,036
Certainly in the cookie love
story was there a looping

257
00:11:35,036 --> 00:11:35,566
going on.

258
00:11:35,566 --> 00:11:37,796
The cookies were going back
and forth and back and forth

259
00:11:37,796 --> 00:11:40,396
and the hearts up and down,
well, this thing that we had

260
00:11:40,396 --> 00:11:42,806
that called a loop last
week with a forever block

261
00:11:42,996 --> 00:11:45,586
with a statement inside of it
is simply gonna look a little

262
00:11:45,636 --> 00:11:46,606
something like this.

263
00:11:47,226 --> 00:11:51,336
So while. And thankfully
the jargon that the authors

264
00:11:51,336 --> 00:11:54,596
of this language chose early
on conjures up the right ideas

265
00:11:54,596 --> 00:11:57,556
in your mind while
something is true.

266
00:11:57,896 --> 00:12:00,076
So here I've hardcoded
the word true

267
00:12:00,426 --> 00:12:03,206
which is actually deliberately
inducing what we called what

268
00:12:03,206 --> 00:12:03,706
last week.

269
00:12:05,146 --> 00:12:08,856
So a loop or an infinite loop,
because true is always true,

270
00:12:08,856 --> 00:12:10,816
there is no-- nothing
on the screen that seems

271
00:12:10,816 --> 00:12:12,366
to change this keyword true,

272
00:12:12,616 --> 00:12:14,766
so this is actually a
deliberate infinite loop,

273
00:12:15,026 --> 00:12:17,916
but so was that thing
up top to forever say hi

274
00:12:17,916 --> 00:12:19,406
or whatever it is
you wanted to do.

275
00:12:19,686 --> 00:12:22,176
So the translation here
is that whereas last week

276
00:12:22,176 --> 00:12:25,086
and in Scratch, the looping
structures are yellow

277
00:12:25,086 --> 00:12:27,006
and have kind of this
brace structure to them.

278
00:12:27,186 --> 00:12:30,016
Well in C and many similar
languages, you have to implement

279
00:12:30,016 --> 00:12:31,516
that same bracing structure

280
00:12:31,516 --> 00:12:33,746
with your keyboard using
an open curly brace

281
00:12:34,056 --> 00:12:36,556
and a closed curly brace,
so a tiny little detail.

282
00:12:36,826 --> 00:12:38,456
On last week too we saw--

283
00:12:39,026 --> 00:12:40,516
and in your projects you
might have seen a so-called

284
00:12:40,516 --> 00:12:41,206
repeat loop.

285
00:12:41,516 --> 00:12:42,186
Same idea.

286
00:12:42,436 --> 00:12:44,886
But whereas the other one
was obviously forever,

287
00:12:45,106 --> 00:12:47,896
this allows you to specify
a finite number of times

288
00:12:47,896 --> 00:12:48,906
that you wanna do something.

289
00:12:48,906 --> 00:12:50,636
Here I arbitrarily chose 10.

290
00:12:50,916 --> 00:12:52,546
How can you implement
that same idea?

291
00:12:52,796 --> 00:12:55,486
Well in C and other languages
that we'll see this semester,

292
00:12:55,706 --> 00:12:57,146
you use what's called
the for loop,

293
00:12:57,396 --> 00:12:59,596
and this as syntax isn't
quite as straightforward

294
00:12:59,596 --> 00:13:00,916
but once you know
what to look for,

295
00:13:00,916 --> 00:13:03,066
it's very easy to understand.

296
00:13:03,066 --> 00:13:05,916
So for, so for the
following definition.

297
00:13:06,176 --> 00:13:08,416
Now you have a set of
parentheses as we'll see

298
00:13:08,416 --> 00:13:12,306
in actual code and then
I have int I equal 0.

299
00:13:12,566 --> 00:13:15,236
So int is gonna stand for
integer, it's just a number.

300
00:13:15,236 --> 00:13:17,906
I is the generic
variable that you use

301
00:13:17,906 --> 00:13:19,366
when you really just
wanna count something

302
00:13:19,366 --> 00:13:21,026
and don't really care
what its name is.

303
00:13:21,356 --> 00:13:23,546
The semicolon just
separates this from the rest.

304
00:13:23,846 --> 00:13:27,186
The I less than 10
says do the following

305
00:13:27,186 --> 00:13:28,506
as long as I is less than 10.

306
00:13:28,506 --> 00:13:31,916
And then on the right hand side
I++, we saw that in pseudocode

307
00:13:31,916 --> 00:13:35,026
with our socks example last
week that just says increment I.

308
00:13:35,286 --> 00:13:38,036
So, long story short after
this fairly arcane syntax,

309
00:13:38,036 --> 00:13:40,426
the end result is
completely straightforward,

310
00:13:40,566 --> 00:13:42,256
do the following 10 times.

311
00:13:42,546 --> 00:13:44,396
But because we're now
programming a computer

312
00:13:44,396 --> 00:13:47,046
at a lower level and
because as we said last week,

313
00:13:47,276 --> 00:13:49,976
you really have to be
precise and then careful

314
00:13:49,976 --> 00:13:52,856
to instruct this machine,
this fairly dumb machine

315
00:13:52,856 --> 00:13:54,146
that will only do
what you tell it

316
00:13:54,146 --> 00:13:56,286
to do very specific
instructions,

317
00:13:56,556 --> 00:13:58,636
do we have to use a
more precise syntax

318
00:13:58,856 --> 00:14:02,016
than just a puzzle piece
might have previously allowed.

319
00:14:02,186 --> 00:14:03,326
Well what about variables.

320
00:14:03,326 --> 00:14:05,186
Well, this block here
on the left that I came

321
00:14:05,186 --> 00:14:08,106
up with arbitrarily sets a
variable called counter equal

322
00:14:08,106 --> 00:14:11,556
to 0 and then it forever
says that counters value

323
00:14:11,776 --> 00:14:13,506
and then it changes
the counter by 1.

324
00:14:13,686 --> 00:14:15,096
So this is like counter ++.

325
00:14:15,096 --> 00:14:17,516
Well, what is code like
that from the world

326
00:14:17,516 --> 00:14:18,586
of Scratch gonna look like?

327
00:14:18,806 --> 00:14:20,756
Well, you can implement
this in a few different ways

328
00:14:20,756 --> 00:14:22,506
but here's one approach
that uses some

329
00:14:22,506 --> 00:14:26,426
of the syntax we just looked
at, int counter equals 0.

330
00:14:26,646 --> 00:14:28,856
So declare as we'll
say a variable,

331
00:14:28,856 --> 00:14:30,066
a placeholder called counter.

332
00:14:30,306 --> 00:14:32,726
It's gonna hold integers so I
have to say int as we'll see

333
00:14:32,926 --> 00:14:34,106
and set it equal to 0.

334
00:14:34,316 --> 00:14:36,336
Then we have to steal
the code from earlier

335
00:14:36,336 --> 00:14:38,886
to do a forever loop,
a while true loop

336
00:14:38,886 --> 00:14:40,316
and then I'm gonna
go ahead and use

337
00:14:40,316 --> 00:14:41,726
that printf thing that we saw.

338
00:14:41,726 --> 00:14:43,446
So everything is kind
of coming together now

339
00:14:43,816 --> 00:14:45,256
with then the counter ++.

340
00:14:45,256 --> 00:14:47,716
So again, idea on the left,
fairly straightforward,

341
00:14:47,826 --> 00:14:50,896
idea on the right, same but
just looks a little different.

342
00:14:51,146 --> 00:14:53,286
And lastly with these things
called Boolean expressions

343
00:14:53,316 --> 00:14:56,126
that allow you to check if
something is true or false,

344
00:14:56,336 --> 00:14:59,036
well, we've seen this in
our loops just a moment ago

345
00:14:59,306 --> 00:15:01,096
but in Scratch we might
have asked the question,

346
00:15:01,096 --> 00:15:06,806
is X less than Y or is X less
than Y and Y is less than Z.

347
00:15:06,806 --> 00:15:09,176
So you can conjoin
these expressions

348
00:15:09,226 --> 00:15:12,036
into ended expressions
so to speak.

349
00:15:12,276 --> 00:15:14,416
Well, the syntax is
gonna be pretty similar.

350
00:15:14,586 --> 00:15:16,896
The green block is gonna
become a pair of parentheses.

351
00:15:17,146 --> 00:15:19,216
The less than side is
gonna be a less than sign,

352
00:15:19,426 --> 00:15:22,366
and then the word and is
going to become not one

353
00:15:22,436 --> 00:15:25,666
but two ampersands back to
back, and we'll see why it's two

354
00:15:25,666 --> 00:15:27,376
and not one before long.

355
00:15:27,376 --> 00:15:29,136
But that's it in terms
of the conversion.

356
00:15:29,366 --> 00:15:32,006
And with actually using those
Boolean expressions on the left,

357
00:15:32,346 --> 00:15:37,136
if X is less than Y say,
"X is less than Y" else

358
00:15:37,366 --> 00:15:39,756
if X is greater than Y say,

359
00:15:39,756 --> 00:15:43,326
"X is greater then Y" else
say, "X is equal to Y."

360
00:15:43,326 --> 00:15:44,796
So, fairly simple idea,

361
00:15:44,846 --> 00:15:47,816
who knows why we actually care
what the values of X or Y are

362
00:15:47,816 --> 00:15:49,286
at least in this out of context.

363
00:15:49,546 --> 00:15:51,956
But to translate this
now to a language C,

364
00:15:52,196 --> 00:15:55,066
you've got some curly braces,
you got an if, some parentheses,

365
00:15:55,296 --> 00:15:56,616
but now we're just
building on some

366
00:15:56,616 --> 00:15:59,076
of the syntax we
introduced before.

367
00:15:59,266 --> 00:16:02,196
So it's all fairly
well convertible

368
00:16:02,196 --> 00:16:03,156
from one to the other.

369
00:16:03,156 --> 00:16:06,806
And lastly, you might have
recalled the Fruit-craft RPG

370
00:16:06,806 --> 00:16:10,536
game that we played briefly with
the little man that moves up

371
00:16:10,536 --> 00:16:13,096
and down, left and right and
collects like oranges and apples

372
00:16:13,096 --> 00:16:14,586
and then brings them
into the store.

373
00:16:14,836 --> 00:16:16,686
Well, that program
was interesting

374
00:16:16,686 --> 00:16:20,166
because it had an inventory
which was implemented in Scratch

375
00:16:20,166 --> 00:16:22,326
with a list which is
kind of like a container

376
00:16:22,326 --> 00:16:24,356
that you can add things
to again and again.

377
00:16:24,546 --> 00:16:27,046
Well, C has things that
are similar to list.

378
00:16:27,386 --> 00:16:28,396
They're called arrays.

379
00:16:28,616 --> 00:16:30,796
We'll see they're not
quite as versatile

380
00:16:30,796 --> 00:16:33,816
as Scratch's little
inventory capability.

381
00:16:33,816 --> 00:16:36,026
You're gonna have to be a little
more careful when it comes

382
00:16:36,056 --> 00:16:38,596
to adding stuff to an
array and removing it from.

383
00:16:38,856 --> 00:16:41,726
But what looks like this orange
block as of now in Scratch,

384
00:16:41,806 --> 00:16:44,926
add orange to your so called
inventory, well, it's gonna take

385
00:16:44,926 --> 00:16:46,626
at least two lines of code in C

386
00:16:46,836 --> 00:16:49,916
to actually create the inventory
array with this first line

387
00:16:50,146 --> 00:16:53,216
and then put something like
the word orange inside of it.

388
00:16:53,216 --> 00:16:55,946
But at the end of the day,
the point of the slides is not

389
00:16:55,946 --> 00:16:59,416
to expect you to now go write
programs in C but really

390
00:16:59,416 --> 00:17:01,796
to just remind you that, hey,
if you ever get distracted

391
00:17:01,796 --> 00:17:04,486
from some of the minutia that
is the semicolons, parentheses,

392
00:17:04,486 --> 00:17:07,796
and what is very new to many of
you, it's really no different

393
00:17:07,896 --> 00:17:09,786
than the fun and
games we implemented

394
00:17:10,076 --> 00:17:11,416
with Scratch last week.

395
00:17:11,536 --> 00:17:13,616
So perhaps take some
comfort from that.

396
00:17:13,616 --> 00:17:16,176
So here is the simplest program

397
00:17:16,316 --> 00:17:20,216
which is perhaps an ironic
statement that you can implement

398
00:17:20,216 --> 00:17:23,326
with this language called C.
So it takes a few lines of code

399
00:17:23,326 --> 00:17:24,876
to actually get something
up and running

400
00:17:24,876 --> 00:17:27,146
and as you'll recall last
Friday, I didn't even get this

401
00:17:27,146 --> 00:17:29,146
up and running because
part of the process

402
00:17:29,146 --> 00:17:31,746
of writing a program
is to one, write it.

403
00:17:31,956 --> 00:17:34,366
So write the so called
source code which is

404
00:17:34,366 --> 00:17:35,516
in a specific language.

405
00:17:35,856 --> 00:17:38,916
Last week we did the socks thing
and that was in pseudocode.

406
00:17:38,916 --> 00:17:41,236
It's just some bogus language
we made up that's kind

407
00:17:41,236 --> 00:17:43,386
of English-like, kind
of look C-like now

408
00:17:43,386 --> 00:17:44,436
that you've seen this syntax,

409
00:17:44,826 --> 00:17:46,366
but it was just an
arbitrary language.

410
00:17:46,596 --> 00:17:49,076
Today, I'm gonna start
writing C code with you

411
00:17:49,326 --> 00:17:52,706
but I can't just write it
and expect it to do anything.

412
00:17:52,706 --> 00:17:55,876
What was the key step that I
said I needed to do on Friday

413
00:17:55,876 --> 00:17:57,856
but failed very publicly
to do so?

414
00:17:57,856 --> 00:17:57,923
[ Inaudible Remark ]

415
00:17:57,923 --> 00:18:00,166
>> Okay, I did a
lot of things wrong.

416
00:18:00,166 --> 00:18:01,486
Okay, so a compiler.

417
00:18:01,666 --> 00:18:04,276
So I tried running a
program in this little black

418
00:18:04,276 --> 00:18:05,326
and white window called GCC

419
00:18:05,326 --> 00:18:07,196
and we'll see it
today working properly

420
00:18:07,466 --> 00:18:09,106
that is a so called compiler.

421
00:18:09,326 --> 00:18:11,096
So writing a program at least

422
00:18:11,096 --> 00:18:13,056
in this language is
a two-step process.

423
00:18:13,056 --> 00:18:15,626
You write the source code,
the stuff that looks like this

424
00:18:15,626 --> 00:18:18,536
and then you save your file, and
now you just have a text file.

425
00:18:18,736 --> 00:18:21,356
The convention is not to
call it something dot txt

426
00:18:21,606 --> 00:18:23,876
but something dot C,
just a convention.

427
00:18:24,166 --> 00:18:26,286
But that's not enough
because computers at the end

428
00:18:26,286 --> 00:18:29,056
of the day only understand 0s
and 1s and what you [inaudible]

429
00:18:29,056 --> 00:18:31,756
and is clearly not a
collection of 0s and 1s.

430
00:18:31,896 --> 00:18:34,726
So you have to somehow
convert the source code,

431
00:18:34,866 --> 00:18:37,786
the C code into 0s
and 1s and for

432
00:18:37,786 --> 00:18:40,966
that process there exist
this tool called a compiler

433
00:18:40,966 --> 00:18:43,726
that someone else wrote that
takes this stuff as input

434
00:18:43,726 --> 00:18:45,926
and produces this
stuff as output.

435
00:18:46,136 --> 00:18:49,266
And it's the 0s and 1s, that
notice they're in patterns

436
00:18:49,266 --> 00:18:51,916
of 8 generally or chunks
of 8, the so called byte

437
00:18:52,256 --> 00:18:54,946
that the actual CPU understands.

438
00:18:54,946 --> 00:18:57,156
And later in the semester we
will tease apart just a little

439
00:18:57,156 --> 00:19:00,386
bit what these patterns are
and why they do what they do.

440
00:19:00,556 --> 00:19:02,066
But for those of
you who decide to go

441
00:19:02,066 --> 00:19:04,856
on in computer science realize
there is a whole interesting

442
00:19:04,856 --> 00:19:07,196
world that uses details
like these in the world

443
00:19:07,196 --> 00:19:08,946
of networking, in hardware,

444
00:19:09,126 --> 00:19:12,666
to actually solve some
real world problems.

445
00:19:12,666 --> 00:19:15,986
So with that said, let's
write our first program.

446
00:19:16,226 --> 00:19:19,026
Probably I shouldn't have said
it's a two-step process 'cause

447
00:19:19,026 --> 00:19:21,206
it's a three-step process to
actually write and run it,

448
00:19:21,386 --> 00:19:23,166
and the third step is just
going to be to run it.

449
00:19:23,226 --> 00:19:24,496
So, let's go ahead and do this.

450
00:19:24,916 --> 00:19:27,236
How am I gonna go about
writing a program?

451
00:19:27,236 --> 00:19:30,676
Well, it turns out you can write
a program on most any computer.

452
00:19:30,966 --> 00:19:35,616
For instance I have an Apple
Mac here but you can do this

453
00:19:35,616 --> 00:19:37,596
on Windows computers,
on Linux computers

454
00:19:37,866 --> 00:19:40,216
if you have a compiler
installed.

455
00:19:40,216 --> 00:19:42,736
It's pretty easy to do this
on Macintosh computers.

456
00:19:42,736 --> 00:19:43,876
You can just go to
Apple's website

457
00:19:43,876 --> 00:19:45,086
and download the
appropriate link.

458
00:19:45,396 --> 00:19:48,326
It's a little more involved
on a PC but it's all free,

459
00:19:48,326 --> 00:19:49,466
the software you can download,

460
00:19:49,696 --> 00:19:52,366
and Linux in similar operating
systems generally come

461
00:19:52,366 --> 00:19:54,546
with a compiler or you run
a simple command to get it.

462
00:19:54,796 --> 00:19:57,046
But long story short, all
of you who own laptops

463
00:19:57,046 --> 00:19:59,646
or desktops could probably
be writing programs,

464
00:19:59,976 --> 00:20:02,986
trivial though this one
is up until now even

465
00:20:03,196 --> 00:20:03,946
if you didn't actually know it.

466
00:20:04,656 --> 00:20:05,976
>> So this time I
had the foresight

467
00:20:05,976 --> 00:20:08,256
to install this compiler but
what I'm gonna do is this.

468
00:20:08,256 --> 00:20:10,026
I'm gonna run the
command called Nano

469
00:20:10,386 --> 00:20:11,966
which is a very simple program.

470
00:20:12,246 --> 00:20:14,216
It is similar in
spirit to Text Edit

471
00:20:14,216 --> 00:20:16,586
on a Mac or Notepad on a PC.

472
00:20:16,696 --> 00:20:20,416
I'm gonna call my file
hello.c. There's a little bit

473
00:20:20,416 --> 00:20:23,326
of distraction here but this
is because this is not a GUI,

474
00:20:23,326 --> 00:20:25,836
a graphical user
interface, this is a CLI,

475
00:20:26,236 --> 00:20:27,706
command-line interface,

476
00:20:27,996 --> 00:20:30,116
which just means there
are no windows and icons.

477
00:20:30,116 --> 00:20:31,786
It's really a text
based interface.

478
00:20:32,106 --> 00:20:33,726
Now just to get used
to the conventions,

479
00:20:33,756 --> 00:20:36,616
because this is the program
you'll use not for problems at 0

480
00:20:36,986 --> 00:20:39,966
but for 1 to start writing
programs, notice at the top

481
00:20:40,056 --> 00:20:42,116
in this window just reminds
you what you're working on.

482
00:20:42,116 --> 00:20:44,976
The top left tells you what
program you're actually using.

483
00:20:45,196 --> 00:20:47,086
The interesting stuff
is at the bottom.

484
00:20:47,086 --> 00:20:49,086
And most of these commands
you don't actually need

485
00:20:49,296 --> 00:20:51,566
but this little caret
symbol means control,

486
00:20:51,776 --> 00:20:54,756
so every keyboard these days has
a control key so you hold that

487
00:20:55,096 --> 00:20:57,446
and hit X for instance
to exit the program.

488
00:20:57,886 --> 00:21:01,886
You hit control O to
write out your program.

489
00:21:01,886 --> 00:21:04,126
I'm not sure why it's not
S for save but it's write

490
00:21:04,126 --> 00:21:05,326
out and that means save.

491
00:21:05,576 --> 00:21:07,606
And so frankly this is pretty
much all you need to know

492
00:21:07,606 --> 00:21:09,296
with Nano to get up and running.

493
00:21:09,516 --> 00:21:11,756
So now let me recreate
this program from memory.

494
00:21:11,756 --> 00:21:12,836
I think the first thing I saw

495
00:21:12,836 --> 00:21:15,686
on the slide was
include standard io.h,

496
00:21:16,116 --> 00:21:19,356
then I said int main, some
parentheses and curly braces.

497
00:21:19,356 --> 00:21:25,216
And then I said printf
"Oh hi world!".

498
00:21:25,216 --> 00:21:28,816
For some reason I had this
backslash N, a close quote,

499
00:21:28,816 --> 00:21:31,346
parenthesis, semicolon,
close curly brace.

500
00:21:31,626 --> 00:21:33,816
Alright, so maybe a
little underwhelming

501
00:21:33,816 --> 00:21:36,066
and we saw already what
this program is going to do

502
00:21:36,066 --> 00:21:38,426
but let's just go through this
process once before we start

503
00:21:38,426 --> 00:21:40,316
doing more interesting things
and then we'll come back

504
00:21:40,686 --> 00:21:42,366
and at least give you
a hint of what some

505
00:21:42,366 --> 00:21:44,526
of these syntactic
details are doing.

506
00:21:44,826 --> 00:21:48,446
So I hit control X. It's very
unnecessarily arcanely saying

507
00:21:48,486 --> 00:21:51,266
save modified buffer, and
then answering no will

508
00:21:51,266 --> 00:21:52,106
destroy changes.

509
00:21:52,136 --> 00:21:54,296
So that part is pretty
clear, so hit Y for yes.

510
00:21:54,886 --> 00:21:57,016
Then it's saying name
filename to write.

511
00:21:57,016 --> 00:21:58,966
It's just asking you if
you wanna change the name.

512
00:21:58,966 --> 00:22:00,436
I'm just gonna hit
enter to leave it alone,

513
00:22:00,686 --> 00:22:01,916
and now I'm back at the prompt.

514
00:22:02,166 --> 00:22:04,626
So generally, if you
don't get an error message

515
00:22:04,726 --> 00:22:07,336
in a command line environment
like this, that's good.

516
00:22:07,466 --> 00:22:10,676
If nothing bad happens, it means
what didn't happen is good.

517
00:22:10,976 --> 00:22:12,576
So with the fact that
I'm now just back

518
00:22:12,576 --> 00:22:14,736
at my prompt is a
good thing so now

519
00:22:14,736 --> 00:22:20,196
if I type this command
GCC hello.c,

520
00:22:20,456 --> 00:22:23,196
this is gonna run this program
that Apple wrote in this case

521
00:22:23,196 --> 00:22:25,136
or someone else wrote
called GCC.

522
00:22:25,136 --> 00:22:28,916
It's gonna take as input hello.c
and what it's gonna produce

523
00:22:28,916 --> 00:22:31,766
as output is something
called a.out.

524
00:22:33,416 --> 00:22:34,066
Nothing happened.

525
00:22:34,326 --> 00:22:35,716
Why? That's good.

526
00:22:35,716 --> 00:22:37,376
That means there is in
fact something there.

527
00:22:37,376 --> 00:22:39,446
So this has created
a file that just

528
00:22:39,516 --> 00:22:42,476
by convention happens
to be called a.out.

529
00:22:42,476 --> 00:22:44,836
Why? Well, out presumably
'cause it's output,

530
00:22:44,836 --> 00:22:46,536
a because it's the first
letter of the alphabet.

531
00:22:46,536 --> 00:22:47,736
It was really that creative.

532
00:22:48,106 --> 00:22:50,586
Now I'm typing dot
slash as a convention.

533
00:22:50,586 --> 00:22:52,036
So little tricks
you'll start to pick up.

534
00:22:52,296 --> 00:22:54,506
Because I know this
program was saved by default

535
00:22:54,506 --> 00:22:56,796
into my current folder,
my current directory,

536
00:22:57,056 --> 00:23:00,726
so dot slash means run the
program called a.out that's

537
00:23:00,726 --> 00:23:03,276
right here in my current
directory not in, like,

538
00:23:03,276 --> 00:23:05,226
my applications folder
or somewhere else.

539
00:23:05,526 --> 00:23:07,476
So now I hit enter and voila!

540
00:23:07,476 --> 00:23:08,736
I see, oh, hi world.

541
00:23:09,016 --> 00:23:10,476
So how did this actually work?

542
00:23:10,476 --> 00:23:11,656
Well, let's just
give a teaser of some

543
00:23:11,656 --> 00:23:12,946
of the details before we move

544
00:23:12,946 --> 00:23:15,316
on to actual interesting
functionality.

545
00:23:15,526 --> 00:23:18,126
So, we'll see in a moment that
that first line at the very top,

546
00:23:18,286 --> 00:23:22,326
sharp include, says include
some code, some source code

547
00:23:22,606 --> 00:23:23,666
that someone else wrote.

548
00:23:24,046 --> 00:23:26,986
Because what you don't see
in this file is any code

549
00:23:26,986 --> 00:23:29,716
that actually says put
a letter of the alphabet

550
00:23:29,716 --> 00:23:31,616
at the top left hand
corner of the screen

551
00:23:31,616 --> 00:23:33,826
and then put the next character
to the right in other words,

552
00:23:34,066 --> 00:23:36,216
I'm taking for granted
at this moment in time

553
00:23:36,466 --> 00:23:39,986
that my computer knows how to
print something to the screen.

554
00:23:39,986 --> 00:23:41,486
But that was not
always the case.

555
00:23:41,616 --> 00:23:43,916
Someone-- or someone
had to implement,

556
00:23:43,916 --> 00:23:49,716
had to write a tool called
printf that itself takes input

557
00:23:49,716 --> 00:23:51,636
between quotes and produces

558
00:23:51,636 --> 00:23:53,696
as output some characters
on the screen.

559
00:23:53,926 --> 00:23:56,416
Now, how can I use code
that someone else wrote?

560
00:23:56,536 --> 00:23:59,746
Well, I include what's called
a header file as we'll call it.

561
00:23:59,986 --> 00:24:03,936
This file here is called
standard I/O or stdio.h

562
00:24:04,166 --> 00:24:06,876
and this is just another text
file someone else wrote many

563
00:24:06,876 --> 00:24:09,236
years ago and by using
that line of code there,

564
00:24:09,236 --> 00:24:12,096
I'm telling the computer,
give me acces to this code

565
00:24:12,096 --> 00:24:14,436
that this other person
wrote that's in that file.

566
00:24:14,506 --> 00:24:15,376
What's in that file?

567
00:24:15,376 --> 00:24:17,316
Well, a whole bunch of stuff
but the only one I care

568
00:24:17,316 --> 00:24:20,176
about right now is a
tool called printf.

569
00:24:20,506 --> 00:24:22,636
And henceforth I'm gonna
start calling these tools,

570
00:24:22,856 --> 00:24:25,756
these monickers that I
can use as building blocks

571
00:24:25,756 --> 00:24:28,776
in my own program, it's gonna be
called a function as we'll see.

572
00:24:29,046 --> 00:24:31,296
Now this other stuff here,
int, we'll come back to that.

573
00:24:31,476 --> 00:24:33,996
Main is relevant because when
you start writing a program

574
00:24:33,996 --> 00:24:36,946
in Scratch, you used what block
as the very first usually?

575
00:24:36,946 --> 00:24:38,276
[ Inaudible Remark ]

576
00:24:38,276 --> 00:24:40,166
>> Yeah, so when green flag
clicked, and that's kind

577
00:24:40,166 --> 00:24:41,616
of the very explicit
way of saying

578
00:24:41,826 --> 00:24:44,436
when you start this program
by clicking in the green flag,

579
00:24:44,676 --> 00:24:46,556
here is what's gonna
get executed first.

580
00:24:46,676 --> 00:24:49,126
Well in C, the counterpart
is main.

581
00:24:49,176 --> 00:24:52,076
And for now I'm just doing main
open paren and close paren.

582
00:24:52,296 --> 00:24:54,866
Before long we'll see that you
can actually put things inside

583
00:24:54,866 --> 00:24:57,586
those parentheses which make
your programs even more useful.

584
00:24:57,856 --> 00:24:59,656
The curly brace is
kind of similar

585
00:24:59,656 --> 00:25:01,926
to Scratch's interlocking
mechanism

586
00:25:02,136 --> 00:25:04,046
that you really see
explicitly with for loops.

587
00:25:04,346 --> 00:25:07,436
Well, for a thing called main
which itself is a function

588
00:25:07,756 --> 00:25:11,276
but a function I wrote, a tool
I am writing at the moment not

589
00:25:11,276 --> 00:25:14,456
like printf which someone else
wrote long ago, I have to say

590
00:25:14,636 --> 00:25:16,516
where does main start,
where does main end,

591
00:25:16,696 --> 00:25:18,666
and I do that by using
these curly braces.

592
00:25:18,716 --> 00:25:21,696
Everything inside of the curly
braces is the code I am writing.

593
00:25:21,966 --> 00:25:23,306
Finally, what code did I write?

594
00:25:23,306 --> 00:25:25,576
Well, just this line
of code here.

595
00:25:25,826 --> 00:25:28,596
Now, you can probably guess
what this piece of syntax does.

596
00:25:28,596 --> 00:25:30,896
So the double quote says
here comes the input.

597
00:25:31,076 --> 00:25:33,186
This is what you typed into
the white box in Scratch

598
00:25:33,486 --> 00:25:34,946
and anyone perhaps
already familiar,

599
00:25:34,946 --> 00:25:37,626
the backslash N is the
shorthand way of saying what?

600
00:25:37,626 --> 00:25:38,516
[ Inaudible Remark ]

601
00:25:38,516 --> 00:25:40,016
>> So it's a new line character.

602
00:25:40,016 --> 00:25:42,536
So things break if you try--

603
00:25:42,536 --> 00:25:44,666
if the computer tries to
take you too literally

604
00:25:44,846 --> 00:25:46,646
so it would not be
correct generally

605
00:25:46,646 --> 00:25:48,716
to do something like that.

606
00:25:48,716 --> 00:25:52,376
Even though this looks
like what the out--

607
00:25:52,376 --> 00:25:54,896
this looks like the output
you want, "Oh, hi world"

608
00:25:54,896 --> 00:25:57,136
and then everything else
should go on the next line.

609
00:25:57,436 --> 00:26:00,106
Things generally break and
this is also not very readable.

610
00:26:00,166 --> 00:26:02,536
You can imagine having many
lines of code where all

611
00:26:02,536 --> 00:26:03,726
of a sudden your closed quote is

612
00:26:03,726 --> 00:26:05,156
on the next line all
the way on the left.

613
00:26:05,156 --> 00:26:06,756
Things very quickly get ugly

614
00:26:06,756 --> 00:26:09,036
and you might even already be
feeling this tension in Scratch

615
00:26:09,036 --> 00:26:11,126
if you're dragging and dropping
and moving things around.

616
00:26:11,326 --> 00:26:14,446
Once your programs get fairly
complex, it becomes harder

617
00:26:14,446 --> 00:26:16,686
and harder to find
things and to tweak things

618
00:26:16,686 --> 00:26:18,006
if you're not being organized.

619
00:26:18,006 --> 00:26:20,416
So you have what are
called escape characters

620
00:26:20,456 --> 00:26:23,196
like backslash N that is
the shorthand notation

621
00:26:23,196 --> 00:26:26,606
of telling the computer put
a new line character here.

622
00:26:26,606 --> 00:26:28,366
So I'm gonna go ahead
and save this again.

623
00:26:28,366 --> 00:26:30,426
And to recap, we
just did three steps.

624
00:26:30,426 --> 00:26:33,236
We did Nano for hello.c
or whatever I called it,

625
00:26:33,556 --> 00:26:36,126
GCC to actually compile
into 0s and 1,

626
00:26:36,336 --> 00:26:38,566
and then step three
is to actually run it.

627
00:26:38,956 --> 00:26:42,136
Well a.out is a pretty stupid
name for a program, right?

628
00:26:42,136 --> 00:26:44,336
It would kind of be
unfortunate if every program

629
00:26:44,336 --> 00:26:45,896
that Microsoft made
and Apple made

630
00:26:45,896 --> 00:26:47,046
by default was called a.out.

631
00:26:47,366 --> 00:26:48,696
You couldn't really
distinguish them,

632
00:26:48,986 --> 00:26:51,786
so certainly it has the worlds
come up with names for programs.

633
00:26:51,956 --> 00:26:53,776
Well, how do you
do this with GCC?

634
00:26:53,776 --> 00:26:57,136
Well, you can specify what's
called a switch or a flag.

635
00:26:57,806 --> 00:27:00,056
So in general, when you're
running command line programs,

636
00:27:00,326 --> 00:27:02,546
they can take input or arguments

637
00:27:02,546 --> 00:27:04,736
or parameters or
switches or flags.

638
00:27:04,916 --> 00:27:06,566
There are so many
synonyms for these things.

639
00:27:06,566 --> 00:27:08,076
It's just hard to keep
track of them at first

640
00:27:08,326 --> 00:27:10,046
but they all generally
mean the same thing,

641
00:27:10,326 --> 00:27:13,696
provide additional input
to this program called GCC

642
00:27:13,866 --> 00:27:15,866
by just specifying it
on the command line.

643
00:27:15,866 --> 00:27:18,626
So you see two similarities
between the first version,

644
00:27:19,026 --> 00:27:21,686
GCC hello.c, and
the second version

645
00:27:21,726 --> 00:27:25,306
but obviously I've just added
which part for that second line.

646
00:27:25,306 --> 00:27:26,796
[ Inaudible Remark ]

647
00:27:26,796 --> 00:27:28,286
>> Yeah, dash O hello.

648
00:27:28,286 --> 00:27:32,286
And this is arbitrary, but
dash O probably means output.

649
00:27:32,436 --> 00:27:34,486
So output, what do
you wanna output?

650
00:27:34,736 --> 00:27:37,746
A file called hello and
not the default a.out.

651
00:27:37,976 --> 00:27:41,296
So, let's try this, let me
go ahead and run GCC again

652
00:27:41,586 --> 00:27:44,436
on hello.c but let me
move my cursor over

653
00:27:44,436 --> 00:27:45,906
and say dash little o,

654
00:27:45,906 --> 00:27:48,686
capitalization is usually
important, and hello,

655
00:27:48,926 --> 00:27:51,686
then hit enter and now
I can do dot slash,

656
00:27:51,686 --> 00:27:54,706
just say run the hello program
I put right here by default,

657
00:27:54,706 --> 00:27:58,016
it's not elsewhere on my hard
drive, hello, enter and there

658
00:27:58,016 --> 00:27:59,326
in fact is that program.

659
00:27:59,326 --> 00:28:01,836
So, it's getting more
interesting but we'll soon see

660
00:28:01,836 --> 00:28:05,146
that things don't
always compile as easily

661
00:28:05,146 --> 00:28:06,236
as this command suggests.

662
00:28:06,516 --> 00:28:10,966
The lines of commands we have to
write get increasingly complex.

663
00:28:11,246 --> 00:28:14,016
So it turns out a really nice
shortcut on most systems, oops,

664
00:28:14,076 --> 00:28:17,066
is to say if you have
a file called hello.c

665
00:28:17,306 --> 00:28:19,936
and you wanna turn it into a
program called hello, well,

666
00:28:19,936 --> 00:28:22,866
just write in the command
makeHello and it will do all

667
00:28:22,866 --> 00:28:24,266
of that for you automatically

668
00:28:24,266 --> 00:28:25,786
and hand you a program
called hello.

669
00:28:26,016 --> 00:28:27,806
And we'll see why this
is increasingly useful

670
00:28:27,806 --> 00:28:30,836
as our programs get more
and more interesting.

671
00:28:31,136 --> 00:28:34,476
So with that said, where
are you actually going

672
00:28:34,476 --> 00:28:36,526
to run these programs?

673
00:28:36,526 --> 00:28:38,046
So, I'm doing this on my Mac.

674
00:28:38,096 --> 00:28:39,476
You might have a Mac,
you might have a PC,

675
00:28:39,476 --> 00:28:42,456
you might have a Linux machine
or any number of other devices

676
00:28:42,456 --> 00:28:45,166
and this very quickly
becomes a bit of a nightmare

677
00:28:45,166 --> 00:28:47,316
because everyone has a slightly
different configuration.

678
00:28:47,316 --> 00:28:48,926
It's hard to answer
questions and whatnot

679
00:28:49,086 --> 00:28:50,566
and you don't necessarily
have access

680
00:28:50,566 --> 00:28:53,276
to the same tools unless you
yourself go through the process

681
00:28:53,276 --> 00:28:54,996
of configuring your
machine manually

682
00:28:55,246 --> 00:28:57,506
by following many long
sequences of steps.

683
00:28:57,506 --> 00:29:00,936
So, an alternative to that
that is incredibly common

684
00:29:00,936 --> 00:29:03,466
in just the programming
world and in industry is

685
00:29:03,466 --> 00:29:06,456
to actually not write programs
on your own personal computer

686
00:29:06,656 --> 00:29:08,846
but to actually write
and run them on a server,

687
00:29:08,846 --> 00:29:11,106
someplace where the
software is all standardized.

688
00:29:11,266 --> 00:29:12,336
It is internet accessible

689
00:29:12,336 --> 00:29:15,606
so you can access it
most anywhere anytime

690
00:29:15,606 --> 00:29:18,696
and you can also access
tools and software and tricks

691
00:29:18,916 --> 00:29:20,366
that the staff have
made available.

692
00:29:20,676 --> 00:29:24,716
So what you'll see in problem
set 0 or problem set 1 is

693
00:29:24,716 --> 00:29:28,966
that you'll soon have access
to what we call the CS50 cloud

694
00:29:29,026 --> 00:29:32,356
which is a bunch of servers
that live on campus and on

695
00:29:32,356 --> 00:29:34,666
which you will all have
a username and password.

696
00:29:34,666 --> 00:29:37,616
By default, it's gonna be
the same username as you got

697
00:29:37,616 --> 00:29:41,306
for your FAS accounts because
FAS also has its own systems

698
00:29:41,306 --> 00:29:43,676
and you use those usernames
and passwords to log

699
00:29:43,676 --> 00:29:44,836
into the labs and whatnot.

700
00:29:45,026 --> 00:29:46,596
But just realize
as a teaser coming

701
00:29:46,596 --> 00:29:49,176
for problem set 1 will
be instructions on how

702
00:29:49,176 --> 00:29:51,216
to activate your so called
cloud account and it's

703
00:29:51,216 --> 00:29:53,636
on the CS50 cloud that you'll
be writing your programs,

704
00:29:53,636 --> 00:29:56,316
compiling your programs,
testing your programs,

705
00:29:56,316 --> 00:29:57,926
running your websites
ultimately.

706
00:29:58,156 --> 00:29:59,706
Just to fast forward to
the end of the course

707
00:29:59,706 --> 00:30:02,616
with a little teaser, a lot
of students for final projects

708
00:30:02,616 --> 00:30:06,336
in the CS50 fair like to
host their final projects,

709
00:30:06,336 --> 00:30:08,646
if they're websites,
at a real domain name,

710
00:30:08,686 --> 00:30:11,056
for instance isawyouharvard.com
or any--

711
00:30:11,116 --> 00:30:14,606
or shuttleboy.com or whatever
name you actually find available

712
00:30:14,606 --> 00:30:17,936
and you can buy for like
9.99 from various websites.

713
00:30:18,256 --> 00:30:20,736
>> Well, what we'll be able to
do this fall for you because all

714
00:30:20,736 --> 00:30:22,666
of the code will live
centrally on this cluster

715
00:30:22,666 --> 00:30:26,036
of systems called the cloud is
if you do decide, mostly for fun

716
00:30:26,036 --> 00:30:27,716
or for entrepreneurial reasons

717
00:30:27,716 --> 00:30:30,406
that you wanna go buy your own
domain name, it's very easy

718
00:30:30,406 --> 00:30:33,086
to map that domain
name to our servers

719
00:30:33,276 --> 00:30:35,396
so that even though the server
itself is gonna be called

720
00:30:35,396 --> 00:30:38,726
cloud.cs50.net, your
users, your friends,

721
00:30:38,726 --> 00:30:42,656
your family can actually
visit isawyouharvard.com

722
00:30:42,846 --> 00:30:45,786
and the internet will know
that isawyouhavard.com

723
00:30:45,786 --> 00:30:48,216
or your own domain name
actually lives here.

724
00:30:48,216 --> 00:30:51,906
So, we'll do all of that hosting
for you which will be a nice way

725
00:30:51,906 --> 00:30:54,396
of giving a little more personal
touch to your final projects

726
00:30:54,486 --> 00:30:56,706
if you decide to implement
something web based and would

727
00:30:56,706 --> 00:30:58,716
like your own so called domain.

728
00:30:59,346 --> 00:31:02,736
Alright, so let's now start
doing more interesting things

729
00:31:02,736 --> 00:31:05,276
than just saying hello
ad nauseam to ourselves.

730
00:31:05,276 --> 00:31:06,836
So, the jargon with that will--

731
00:31:06,946 --> 00:31:08,076
with which we'll
proceed is this.

732
00:31:08,076 --> 00:31:09,846
So a function is
just a little tool.

733
00:31:09,846 --> 00:31:12,256
It's like a miniature program
that you can write or use

734
00:31:12,256 --> 00:31:14,976
that someone else wrote and the
first one we ourselves wrote is

735
00:31:14,976 --> 00:31:17,636
main and henceforth in
this language called C just

736
00:31:17,636 --> 00:31:19,176
like every Scratch
project begins

737
00:31:19,176 --> 00:31:22,906
with one green flag clicked so
our program is gonna be called--

738
00:31:22,906 --> 00:31:24,076
start with this thing
called main.

739
00:31:24,426 --> 00:31:26,526
The standard library,
that first line of code,

740
00:31:26,526 --> 00:31:31,376
it's not strictly necessary but
any time you use a function,

741
00:31:31,506 --> 00:31:34,526
a tool that someone else
wrote that lives in that

742
00:31:34,526 --> 00:31:37,786
so called library, you do need
that include line as we'll see.

743
00:31:37,786 --> 00:31:39,816
And if you don't
include it, GCC,

744
00:31:39,816 --> 00:31:41,206
the compiler will yell at you.

745
00:31:41,206 --> 00:31:43,816
And we'll see innumerable
error messages today and onward

746
00:31:43,816 --> 00:31:46,086
as with you working at home ends

747
00:31:46,086 --> 00:31:48,026
on the problem sets 'cause
this is the way you learn.

748
00:31:48,026 --> 00:31:49,736
And it's actually a
useful feedback mechanism.

749
00:31:50,136 --> 00:31:54,196
But because C is a fairly old
language and fairly low level,

750
00:31:54,376 --> 00:31:56,826
it's actually hard
to do easy things.

751
00:31:56,826 --> 00:31:58,996
Even those of you who've
taken AP computer science

752
00:31:58,996 --> 00:32:00,766
or something similar
perhaps recall

753
00:32:00,766 --> 00:32:02,826
that even just getting
a piece of input

754
00:32:02,826 --> 00:32:05,356
from the user is
ridiculously not obvious.

755
00:32:05,356 --> 00:32:07,336
You need multiple lines of
code, you need these days

756
00:32:07,336 --> 00:32:08,876
to use something
called a scanner class.

757
00:32:08,876 --> 00:32:11,436
You have to jump through hoops
just to ask the user for a piece

758
00:32:11,436 --> 00:32:13,296
of input which should
be really the thing a

759
00:32:13,296 --> 00:32:14,666
language prioritizes.

760
00:32:14,666 --> 00:32:16,856
So, what we do at the
start of the semester,

761
00:32:16,856 --> 00:32:18,986
and these are essentially
training wheels we very quickly

762
00:32:18,986 --> 00:32:20,616
take off once you
get more comfortable,

763
00:32:20,926 --> 00:32:23,096
we provide you with
our own library.

764
00:32:23,326 --> 00:32:27,166
It's not called standard io.h,
it's called cs50.h. That's

765
00:32:27,166 --> 00:32:29,886
where the code we wrote for you
will live and then we'll see how

766
00:32:29,886 --> 00:32:32,576
to do this in examples
today onward but we wrote

767
00:32:32,576 --> 00:32:34,896
for you some tools,
some functions

768
00:32:35,146 --> 00:32:37,666
that calls GetChar
for get a character.

769
00:32:37,666 --> 00:32:40,436
If you just want a single
letter from the user, GetDouble.

770
00:32:40,486 --> 00:32:42,236
This is a type of number
that we'll see in a bit.

771
00:32:42,276 --> 00:32:44,656
GetFloating-- GetFloat for
a floating point number.

772
00:32:44,656 --> 00:32:46,826
GetInt, GetLong long,
we'll see what

773
00:32:46,826 --> 00:32:48,626
that redundancy means,
and GetString.

774
00:32:48,626 --> 00:32:51,656
And the last is perhaps the most
useful at least at first glance.

775
00:32:51,816 --> 00:32:54,156
So, let's go ahead and do a
little something with these.

776
00:32:54,186 --> 00:32:56,686
I'm gonna go away from my
own Mac and I'm gonna go

777
00:32:56,686 --> 00:32:58,096
to the standard environment.

778
00:32:58,136 --> 00:33:01,826
And the means by which I connect
to the so called cloud is

779
00:33:01,826 --> 00:33:04,126
with a command line
program or GUI program.

780
00:33:04,546 --> 00:33:07,296
You'll see how to do this
via some tutorials online

781
00:33:07,296 --> 00:33:08,186
for problem set 1.

782
00:33:08,466 --> 00:33:11,006
But on the Mac, I'm gonna run
this program called Terminal.

783
00:33:11,286 --> 00:33:12,896
It comes with all
Macs by default.

784
00:33:12,896 --> 00:33:14,976
PC users will wanna use
something called PuTTY

785
00:33:15,036 --> 00:33:18,066
which is free or secureCRT
with Harvard has available.

786
00:33:18,066 --> 00:33:19,936
But again, more on
that in problem set 1.

787
00:33:20,336 --> 00:33:22,446
I'm gonna go ahead and
run this command, SSH,

788
00:33:22,536 --> 00:33:25,086
which stands for Secure Shell.

789
00:33:25,086 --> 00:33:26,756
This just means making
encrypted connection

790
00:33:26,756 --> 00:33:27,926
to someone else's server.

791
00:33:28,216 --> 00:33:32,216
Malan@cloud.cs50.net, enter.

792
00:33:32,536 --> 00:33:33,996
It's asking me now
for my password.

793
00:33:34,256 --> 00:33:38,136
And now I met a prompt that says
who I am as a little reminder.

794
00:33:38,196 --> 00:33:38,646
Where I am?

795
00:33:38,646 --> 00:33:40,896
And then in the parentheses
we'll see in a moment,

796
00:33:40,896 --> 00:33:42,886
it tells me what
folder I am inside of.

797
00:33:43,226 --> 00:33:45,636
So let's go ahead and
write a slightly more

798
00:33:45,636 --> 00:33:46,786
interesting program.

799
00:33:46,786 --> 00:33:48,006
Among your printouts today,

800
00:33:48,236 --> 00:33:50,186
even though I generally
won't bother printing slides

801
00:33:50,186 --> 00:33:51,276
since they're fairly minimalist,

802
00:33:51,476 --> 00:33:53,036
we will typically
print source codes.

803
00:33:53,036 --> 00:33:54,406
You can actually take
notes and don't have

804
00:33:54,406 --> 00:33:56,596
to scribble everything down
that you see on the screen.

805
00:33:57,166 --> 00:33:59,376
Every file or program
that you see here

806
00:33:59,376 --> 00:34:01,406
in this packet is
alphabetically listed.

807
00:34:01,406 --> 00:34:05,286
So if I say hello.c which
actually doesn't exist in here

808
00:34:05,416 --> 00:34:08,016
but hi1.c does, you'll
find it alphabetically.

809
00:34:08,016 --> 00:34:09,096
So let's take a look at this,

810
00:34:09,646 --> 00:34:14,766
hi2.c. Actually I'm
gonna use a different--

811
00:34:15,256 --> 00:34:17,436
I'm gonna do a small
cheat today.

812
00:34:17,436 --> 00:34:19,096
Rather than use nano
on this computer,

813
00:34:19,096 --> 00:34:20,586
I'm gonna use a program
called Vim,

814
00:34:20,906 --> 00:34:22,596
which is just another
text editor.

815
00:34:22,596 --> 00:34:25,646
It's more like a word processor
than it is a simple text editor.

816
00:34:26,036 --> 00:34:27,676
So, let's go ahead and
open in your printouts

817
00:34:27,676 --> 00:34:30,336
if you would hi2.c.
So first of all,

818
00:34:30,546 --> 00:34:31,996
there're a few more
lines of code here.

819
00:34:31,996 --> 00:34:34,816
There's a whole bunch of stuff
at the top which is wrapping

820
00:34:34,816 --> 00:34:36,986
in an ugly way 'cause
I've made my font so big.

821
00:34:37,276 --> 00:34:38,886
But there're a few
details to take away.

822
00:34:39,246 --> 00:34:41,386
So when you're writing a
program in this language

823
00:34:41,386 --> 00:34:43,916
and many others, if you wanna
make a comment to yourself

824
00:34:44,146 --> 00:34:46,546
or for other people who are
reading this with their TF

825
00:34:46,546 --> 00:34:48,966
or fellow colleagues, you
use what are called comments.

826
00:34:48,966 --> 00:34:51,246
So at top left we have
a slash and a star

827
00:34:51,506 --> 00:34:53,946
and that says everything
that follows that--

828
00:34:54,116 --> 00:34:55,906
those symbols is just a comment.

829
00:34:55,906 --> 00:34:58,286
It's not going to get
converted into 0s and 1s.

830
00:34:58,496 --> 00:34:59,496
It's just for humans.

831
00:34:59,766 --> 00:35:02,286
The fact that I have this
star here, this star here,

832
00:35:02,286 --> 00:35:04,716
this star here, is really
just 'cause I'm a little anal

833
00:35:04,716 --> 00:35:06,766
and it looks pretty to make a
little box out of everything

834
00:35:06,766 --> 00:35:08,066
but it's not strictly necessary.

835
00:35:08,066 --> 00:35:10,686
The only thing that matters
is the slash star here

836
00:35:10,976 --> 00:35:13,196
and the slash star at
the bottom of this.

837
00:35:13,196 --> 00:35:14,256
All of those are comments.

838
00:35:14,256 --> 00:35:16,936
So let's actually scroll down
to the interesting stuff here.

839
00:35:17,216 --> 00:35:18,976
And as an aside,
those in the room

840
00:35:18,976 --> 00:35:20,536
who are among those
more comfortable,

841
00:35:20,686 --> 00:35:22,556
even if we're not quite
filling in blanks today,

842
00:35:22,556 --> 00:35:26,446
realize we will very quickly
entice you with details

843
00:35:26,476 --> 00:35:27,726
that you don't already know.

844
00:35:27,726 --> 00:35:30,316
Today is meant to level the
playing field so to speak.

845
00:35:30,606 --> 00:35:33,426
So here we have the code
you have printed before you.

846
00:35:33,426 --> 00:35:36,336
I'm gonna zoom in a little more
and notice it's pretty much 1,

847
00:35:36,336 --> 00:35:41,516
2, 3, 4, 5, 6, 7, 8, 9 lines of
code and that's kind of rounding

848
00:35:41,516 --> 00:35:42,796
up because of the white space.

849
00:35:43,116 --> 00:35:44,076
So what's going on?

850
00:35:44,076 --> 00:35:47,296
Well, the first line of code,
I'll fix the purple next time

851
00:35:47,296 --> 00:35:48,716
around so it's more legible.

852
00:35:48,966 --> 00:35:52,676
But this says include and this
says include a file called

853
00:35:52,816 --> 00:35:56,586
cs50.h that lives in some
special place on the server

854
00:35:56,856 --> 00:35:58,626
that the cs50 staff wrote.

855
00:35:59,006 --> 00:36:02,776
The next line of code says
include standard io.h which just

856
00:36:02,806 --> 00:36:05,306
as a little sanity check
contains among other things,

857
00:36:06,166 --> 00:36:06,426
printf.

858
00:36:06,806 --> 00:36:09,436
So not the actual function
as we'll see printf.

859
00:36:09,846 --> 00:36:13,376
Just the .h file as we'll see
is obviously named different

860
00:36:13,376 --> 00:36:14,326
than a .c file.

861
00:36:14,586 --> 00:36:16,616
In a .h file, it's
generally the names

862
00:36:16,616 --> 00:36:17,966
of functions other people wrote.

863
00:36:18,146 --> 00:36:20,166
In .c files are the actual co--

864
00:36:20,166 --> 00:36:21,966
is the actual code that
other people wrote.

865
00:36:22,196 --> 00:36:24,786
All you have to include in your
own programs is the so-called

866
00:36:24,786 --> 00:36:25,526
header file.

867
00:36:25,526 --> 00:36:27,266
And that's what cs50.h and--

868
00:36:28,096 --> 00:36:30,456
standard io.h are
but we'll see those

869
00:36:30,456 --> 00:36:31,706
in more detail before long.

870
00:36:31,956 --> 00:36:34,056
So for now, just take it
on faith that you'll have

871
00:36:34,056 --> 00:36:36,166
to say int before you say main.

872
00:36:36,426 --> 00:36:38,176
Then you say main,
open parenthesis,

873
00:36:38,176 --> 00:36:40,216
close parenthesis,
open curly brace.

874
00:36:40,616 --> 00:36:43,676
So now after all these stupid
setup, we can finally dive

875
00:36:43,676 --> 00:36:45,066
in to the actual program.

876
00:36:45,376 --> 00:36:47,006
Now this program's
a little different

877
00:36:47,006 --> 00:36:49,666
from what we saw before because
it's obviously not gonna say oh,

878
00:36:49,666 --> 00:36:50,366
hi world.

879
00:36:50,366 --> 00:36:53,706
It's going to say oh,
hi David presumably.

880
00:36:53,706 --> 00:36:55,986
That part is uninteresting
but how is it doing this?

881
00:36:55,986 --> 00:36:57,046
Well, a few details,

882
00:36:57,046 --> 00:36:58,756
a few building blocks
we can start using

883
00:36:58,756 --> 00:36:59,826
in more compelling ways.

884
00:37:00,186 --> 00:37:03,906
So previously in the slides,
I said there's gonna be things

885
00:37:03,906 --> 00:37:05,286
like ints or integers.

886
00:37:05,486 --> 00:37:07,056
Well, ints are only so useful.

887
00:37:07,056 --> 00:37:09,476
Sometimes you actually
want words and characters.

888
00:37:09,476 --> 00:37:11,976
Fast forward mid-semester when
we talk about cryptography

889
00:37:11,976 --> 00:37:14,266
and scrambling information,
we need the ability

890
00:37:14,266 --> 00:37:16,406
to express ourselves in
words, not just numbers,

891
00:37:16,616 --> 00:37:17,766
and we have that already.

892
00:37:18,096 --> 00:37:22,636
So if I want to use a string as
we'll call it which is a word

893
00:37:22,636 --> 00:37:25,556
or phrase generally, they
always go between double quotes,

894
00:37:25,646 --> 00:37:27,136
hence the quotes around David,

895
00:37:27,136 --> 00:37:29,686
and they are double quotes not
single quotes intentionally.

896
00:37:30,126 --> 00:37:33,146
On the left hand side
string name equals, well,

897
00:37:33,236 --> 00:37:36,496
what would we call this thing
called name, in general,

898
00:37:36,496 --> 00:37:38,116
using last week's nomenclature?

899
00:37:38,736 --> 00:37:40,036
That's just a variable, right?

900
00:37:40,036 --> 00:37:42,586
It's more interestingly
named an X or Y or Z.

901
00:37:42,626 --> 00:37:45,376
If we named it based on
what it is, it's a name.

902
00:37:45,826 --> 00:37:48,086
So it's-- the name of
the variable is name.

903
00:37:48,286 --> 00:37:50,016
Now, what type of
variable is this?

904
00:37:50,016 --> 00:37:53,056
Well, unfortunately in C, you
have to be ever so specific

905
00:37:53,056 --> 00:37:54,816
as to what you wanna
put in the variable.

906
00:37:55,076 --> 00:37:56,646
So whereas before
I mentioned int,

907
00:37:56,896 --> 00:37:59,266
turns out if you wanna put
a string in a variable,

908
00:37:59,306 --> 00:38:01,986
you have to say this
is a string.

909
00:38:02,246 --> 00:38:05,786
Now at this point in the story,
I have a variable called name,

910
00:38:06,096 --> 00:38:09,666
stored inside of that is
the string, literally David.

911
00:38:09,916 --> 00:38:11,786
What do I now want to
do with this variable?

912
00:38:11,786 --> 00:38:15,206
Well, here is the F in printf
for formatted printing.

913
00:38:15,546 --> 00:38:18,436
Well, before I just
hardcoded a string inside

914
00:38:18,436 --> 00:38:19,746
of printf's double quotes.

915
00:38:20,056 --> 00:38:21,746
This time, I've done
the same thing

916
00:38:21,936 --> 00:38:23,876
but there's a little
curiosity here.

917
00:38:24,146 --> 00:38:25,946
There's this percent S sign.

918
00:38:26,236 --> 00:38:28,196
So this is a so-called
format string.

919
00:38:28,246 --> 00:38:31,296
It's just a placeholder
that says I don't

920
00:38:31,296 --> 00:38:33,666
yet know what I wanna put here
but I wanna put something,

921
00:38:33,666 --> 00:38:35,116
let me get back to
you in just a moment.

922
00:38:35,366 --> 00:38:37,816
Well, in just a moment, it
happens right after the comma.

923
00:38:38,136 --> 00:38:41,056
Notice I close the quotes
after I'm putting a new line--

924
00:38:41,296 --> 00:38:42,676
after I'm putting this new line,

925
00:38:42,886 --> 00:38:45,056
but then to the right
hand side is comma name.

926
00:38:45,356 --> 00:38:48,906
So before we use printf,
a function, and we passed

927
00:38:48,906 --> 00:38:51,396
in one input and input
henceforth we're gonna call

928
00:38:51,546 --> 00:38:53,106
arguments or parameters.

929
00:38:53,146 --> 00:38:54,636
It's just an input
to a function,

930
00:38:54,846 --> 00:38:56,086
the thing inside parentheses.

931
00:38:56,366 --> 00:38:58,796
But sometimes you can pass
multiple things inside those

932
00:38:58,796 --> 00:39:01,056
parentheses, and so you
separate them by commas.

933
00:39:01,256 --> 00:39:02,696
What do I wanna pass in name?

934
00:39:02,976 --> 00:39:05,716
And so now if you follow
the intuition here,

935
00:39:05,936 --> 00:39:07,436
percent S is the placeholder,

936
00:39:07,686 --> 00:39:09,406
name is what I want
to place there.

937
00:39:09,646 --> 00:39:12,106
So the end result, because
someone wrote this function

938
00:39:12,106 --> 00:39:15,526
years ago is that printf takes
this thing, takes this thing,

939
00:39:15,786 --> 00:39:19,326
plops David inside the middle
of that formatted string

940
00:39:19,326 --> 00:39:21,276
and then renders
the whole result.

941
00:39:21,626 --> 00:39:23,776
So let me go ahead and
quit from this program.

942
00:39:24,576 --> 00:39:30,786
And let me go ahead and
run-- how do I compile this?

943
00:39:31,236 --> 00:39:32,796
GCC, hi2.c, enter.

944
00:39:32,796 --> 00:39:34,076
It seems to work.

945
00:39:34,076 --> 00:39:36,246
A.out enter and oh, hi David.

946
00:39:36,456 --> 00:39:37,826
Alright, so now let's
make things a little

947
00:39:37,826 --> 00:39:38,456
more interesting.

948
00:39:38,456 --> 00:39:42,716
Let's go into hi3.c. So again
I have some comments up top.

949
00:39:42,716 --> 00:39:44,986
As a matter of convention
and good style

950
00:39:44,986 --> 00:39:47,126
which is another theme of the
course that we'll focus more

951
00:39:47,126 --> 00:39:49,786
on on Friday onward,
it's generally good to--

952
00:39:50,116 --> 00:39:52,196
good style to actually describe

953
00:39:52,196 --> 00:39:54,616
in English even succinct
English,

954
00:39:54,876 --> 00:39:56,166
what your program is doing.

955
00:39:56,166 --> 00:39:58,236
Because as you'll soon find,
when you're writing programs

956
00:39:58,236 --> 00:40:01,736
that aren't just four lines
long but are 40 or 400

957
00:40:01,736 --> 00:40:04,366
which very quickly becomes easy,

958
00:40:04,366 --> 00:40:07,076
you'll find that even you the
next morning have no idea what

959
00:40:07,076 --> 00:40:09,156
this code does for
whatever reason,

960
00:40:09,156 --> 00:40:12,356
and just having some English
comments is a huge boon

961
00:40:12,356 --> 00:40:13,976
to comprehension for
yourself and others.

962
00:40:14,546 --> 00:40:17,476
>> So now we can finally do
something more interactive.

963
00:40:17,536 --> 00:40:20,666
Right now, we're finally
getting to the point of scratch

964
00:40:20,666 --> 00:40:22,506
which can take some
input from the user.

965
00:40:22,506 --> 00:40:23,766
It's not nearly as graphical

966
00:40:24,036 --> 00:40:26,096
but at least we're not
hardcoding programs

967
00:40:26,356 --> 00:40:28,066
that are otherwise
uninteresting.

968
00:40:28,066 --> 00:40:28,716
So here we go.

969
00:40:28,976 --> 00:40:30,666
The top of this program
is the same.

970
00:40:30,666 --> 00:40:33,126
Include our library,
include the standard library.

971
00:40:33,416 --> 00:40:35,546
Here's my pro-- here's
my function called main.

972
00:40:35,786 --> 00:40:36,856
What do I now wanna do?

973
00:40:36,856 --> 00:40:38,836
Well, I don't have to
call printf just once.

974
00:40:38,886 --> 00:40:41,076
If I wanna say something,
do something,

975
00:40:41,296 --> 00:40:43,776
then say something else,
I just call printf twice.

976
00:40:44,156 --> 00:40:45,636
So notice here I'm printing

977
00:40:45,636 --> 00:40:48,686
out state your name,
a colon and a space.

978
00:40:48,686 --> 00:40:50,936
No new line just 'cause
I made an aesthetic call

979
00:40:50,936 --> 00:40:51,906
that this looks kinda nice

980
00:40:51,906 --> 00:40:53,496
if the prompt stays
on the same line.

981
00:40:53,926 --> 00:40:55,066
Now, I'm using this thing.

982
00:40:55,366 --> 00:40:58,056
So unlike printf which
someone else wrote and is

983
00:40:58,056 --> 00:41:00,586
in the standard library, as
we-- I mentioned a moment ago,

984
00:41:00,586 --> 00:41:03,536
GetString is in the--
says 50 library.

985
00:41:03,536 --> 00:41:05,966
So this is again part of
the package we provide you

986
00:41:05,966 --> 00:41:09,596
within the server for the first
few weeks to just make this kind

987
00:41:09,596 --> 00:41:12,936
of intuitively easy stuff,
mechanically easy as well.

988
00:41:13,196 --> 00:41:14,566
So let's see what's
going on here.

989
00:41:14,566 --> 00:41:17,416
Well, printf's job in
like is to print something

990
00:41:17,416 --> 00:41:18,776
in the screen-- to the screen.

991
00:41:19,166 --> 00:41:21,846
But GetString as its
name implies is supposed

992
00:41:21,846 --> 00:41:23,766
to get something from the user.

993
00:41:24,046 --> 00:41:27,336
But odds are it's not sufficient
just to get it from the user, I,

994
00:41:27,336 --> 00:41:31,306
the human need to be handed
whatever this function got

995
00:41:31,306 --> 00:41:32,856
from the user in some form.

996
00:41:33,216 --> 00:41:36,826
Odds are when this function
called GetString gets a string

997
00:41:36,826 --> 00:41:39,546
from the user by expecting them
to manipulate their keyboard

998
00:41:39,546 --> 00:41:41,236
with their fingers, I need to--

999
00:41:41,236 --> 00:41:44,126
initiatively I'm gonna get
back a sentence or a word

1000
00:41:44,126 --> 00:41:45,436
or a phrase or something.

1001
00:41:45,806 --> 00:41:46,866
And the reason that we now

1002
00:41:46,866 --> 00:41:49,036
on the second line
have the equal sign is

1003
00:41:49,036 --> 00:41:51,696
because this function
GetString has what's called a

1004
00:41:51,866 --> 00:41:52,776
return value.

1005
00:41:53,026 --> 00:41:55,226
It not only does
something, as we'll see,

1006
00:41:55,436 --> 00:41:58,046
it also gets something and
it has to hand it back to me.

1007
00:41:58,276 --> 00:42:00,646
Otherwise, what was the point of
getting a string from the user

1008
00:42:00,646 --> 00:42:02,096
if I'm not handed the result

1009
00:42:02,096 --> 00:42:04,086
to actually do something
interesting with.

1010
00:42:04,246 --> 00:42:08,856
So, thankfully I know I have
storage space for return values.

1011
00:42:08,856 --> 00:42:10,246
I can just whip out a variable,

1012
00:42:10,526 --> 00:42:12,746
this time it's gonna be
dynamically stored there.

1013
00:42:12,746 --> 00:42:15,296
I'm not hardcoding
in D-A-V-I-D anymore

1014
00:42:15,556 --> 00:42:16,826
but I can still use a variable.

1015
00:42:17,096 --> 00:42:18,896
So long as I say I'm
gonna put a string here,

1016
00:42:18,896 --> 00:42:22,046
it doesn't matter what string it
is, GetString is gonna take care

1017
00:42:22,206 --> 00:42:23,876
of the job of getting
it from the user.

1018
00:42:24,096 --> 00:42:26,286
And then I'm gonna use the
same printf line after that

1019
00:42:26,716 --> 00:42:29,776
to actually plop it into
this formatted string.

1020
00:42:30,026 --> 00:42:32,506
Let me go ahead and
run this now.

1021
00:42:32,506 --> 00:42:38,276
So, GCC hi3.c and I'm getting
a little tired of a.out.

1022
00:42:38,276 --> 00:42:41,476
So if I wanna give this
thing a name I do, you know,

1023
00:42:41,476 --> 00:42:43,876
dash oh hi 3 or whatever
I wanna call it.

1024
00:42:43,876 --> 00:42:49,216
Enter. Problem, first of many,
but let's solve it together.

1025
00:42:49,446 --> 00:42:52,196
And then remember how you solve
this 'cause otherwise completely

1026
00:42:52,196 --> 00:42:52,996
useless exercise.

1027
00:42:53,586 --> 00:42:54,686
So what's the error here?

1028
00:42:54,686 --> 00:42:55,646
Well, the font is big.

1029
00:42:55,646 --> 00:42:57,296
Normally the errors don't
look this complicated

1030
00:42:57,296 --> 00:42:58,706
but they still look
fairly arcane.

1031
00:42:58,706 --> 00:43:02,626
So, on the one hand, I've run
this command here, GCC dash O

1032
00:43:03,506 --> 00:43:07,146
and this projector I'm
highlighting things all day long

1033
00:43:07,146 --> 00:43:08,346
on the screen here and
you're not seeing any

1034
00:43:08,346 --> 00:43:09,996
of it unfortunately it
seems on this projector.

1035
00:43:10,386 --> 00:43:14,226
But what I ran was GCC and
then I get back this big

1036
00:43:14,226 --> 00:43:14,966
error message.

1037
00:43:14,966 --> 00:43:17,706
So this thing here that
starts with slash temp,

1038
00:43:17,706 --> 00:43:19,376
that's pretty useless
information.

1039
00:43:19,566 --> 00:43:21,076
It's just that GCC

1040
00:43:21,076 --> 00:43:24,146
when compiling your program was
apparently using some temporary

1041
00:43:24,146 --> 00:43:25,826
storage space in a
folder called temp,

1042
00:43:26,196 --> 00:43:27,746
not of real consequence to me.

1043
00:43:27,986 --> 00:43:29,666
But now the error
message gets interesting.

1044
00:43:29,666 --> 00:43:34,956
It says undefined reference to
GetString then collect to LD,

1045
00:43:34,956 --> 00:43:38,826
LD stands for loader and
that will be a useful detail

1046
00:43:38,826 --> 00:43:39,926
to keep in mind later on.

1047
00:43:40,316 --> 00:43:42,496
Loader returned 1 exit status.

1048
00:43:42,636 --> 00:43:45,876
So, what is it sound like is
the problem intuitively here?

1049
00:43:46,056 --> 00:43:47,306
So if you know the answer

1050
00:43:47,306 --> 00:43:49,066
and know what the fix
is don't answer this.

1051
00:43:49,176 --> 00:43:50,796
But if intuitively,
you just have a sense

1052
00:43:50,796 --> 00:43:51,656
of what the problem is.

1053
00:43:52,326 --> 00:43:53,736
>> You can't find GetString.

1054
00:43:53,846 --> 00:43:55,266
>> It can't find
GetString right?

1055
00:43:55,336 --> 00:43:56,136
Someone else-- thank you.

1056
00:43:56,136 --> 00:43:58,636
Someone else wrote
GetString namely CS50.

1057
00:43:58,996 --> 00:44:03,236
I have included it
using the cs50.h

1058
00:44:03,756 --> 00:44:06,866
but that only gives me
access in the source code

1059
00:44:06,866 --> 00:44:08,656
to actually using it by name.

1060
00:44:09,046 --> 00:44:10,936
The bits, the zeros and ones

1061
00:44:10,976 --> 00:44:13,416
that compose the
library are actually

1062
00:44:13,416 --> 00:44:17,696
in that file called cs50.c
but those bits live or--

1063
00:44:17,696 --> 00:44:18,646
even that's a white lie.

1064
00:44:18,726 --> 00:44:19,936
They technically live in a file,

1065
00:44:19,936 --> 00:44:22,856
as we'll eventually see
called cs50.o. But for now,

1066
00:44:22,856 --> 00:44:26,336
I have made no mentioned of
cs50.c. I've made no mentioned

1067
00:44:26,336 --> 00:44:29,996
of cs50.o. So, the problem
at hand really reduces

1068
00:44:30,026 --> 00:44:33,296
to GSS knowing that I wanna
use a function called GetString

1069
00:44:33,616 --> 00:44:35,636
but it doesn't know where
the zeros and ones are

1070
00:44:35,636 --> 00:44:38,756
so how can it possibly
integrate our zeros and ones

1071
00:44:38,756 --> 00:44:41,186
that the staff wrote into
the zeros and ones you wrote

1072
00:44:41,376 --> 00:44:43,126
to produce one collective whole.

1073
00:44:43,406 --> 00:44:45,856
So it turns out that
there's a two-step process.

1074
00:44:45,856 --> 00:44:48,206
When you want to use
a library like CS50's,

1075
00:44:48,426 --> 00:44:51,066
you can't just run
GCC on your program.

1076
00:44:51,296 --> 00:44:54,096
You also have to tell the
compiler at the very end

1077
00:44:54,096 --> 00:44:57,566
of your command-- you know what,
let me use the library with the

1078
00:44:57,616 --> 00:45:00,936
so called switch/l so
that's an L not a 1.

1079
00:45:01,146 --> 00:45:04,646
So a dash lowercase L
and then what library?

1080
00:45:04,726 --> 00:45:05,816
The CS50 library.

1081
00:45:06,246 --> 00:45:11,306
So this is pretty-- similarly
might we say this dash L STD

1082
00:45:11,306 --> 00:45:12,916
for standard, for Standard I/O.

1083
00:45:13,126 --> 00:45:16,056
But it turns out that because
the Standard I/O Library is

1084
00:45:16,056 --> 00:45:19,486
so common they don't force
people to type this all day long

1085
00:45:19,486 --> 00:45:21,066
because it's just
so commonly used.

1086
00:45:21,226 --> 00:45:23,856
Whereas CS50 is not nearly as
popular a library in the world,

1087
00:45:24,016 --> 00:45:26,726
and so you'll have to
explicitly say dash L CS50.

1088
00:45:26,986 --> 00:45:27,996
Why is there no space?

1089
00:45:28,366 --> 00:45:30,606
Stupid convention, but
you have to do dash L

1090
00:45:30,606 --> 00:45:31,666
and then the name
of the library.

1091
00:45:31,916 --> 00:45:32,866
So now let's hit Enter.

1092
00:45:33,616 --> 00:45:37,486
Better! So no error messages
means success is probably what

1093
00:45:37,486 --> 00:45:38,066
has happened.

1094
00:45:38,556 --> 00:45:40,146
HI3 Enter.

1095
00:45:41,096 --> 00:45:44,736
Oh-oh. Command not found.

1096
00:45:44,906 --> 00:45:46,176
Why is this now a problem?

1097
00:45:46,446 --> 00:45:48,016
I did call it hi3 right?

1098
00:45:48,016 --> 00:45:51,646
I did use the dash O
and I said hi3 dash O.

1099
00:45:51,646 --> 00:45:54,216
So why is the command not found?

1100
00:45:55,506 --> 00:45:55,626
Yeah?

1101
00:45:55,746 --> 00:45:56,756
>> It didn't know where hi3 is.

1102
00:45:57,136 --> 00:45:59,016
>> So it doesn't
know where hi3 is.

1103
00:45:59,056 --> 00:46:01,746
Now this is-- again, testament
to the fact that computers,

1104
00:46:01,966 --> 00:46:04,836
you know, sophisticated or scary
as they might seem to some,

1105
00:46:05,036 --> 00:46:06,196
they're pretty stupid devices.

1106
00:46:06,196 --> 00:46:07,126
Unless you tell them what

1107
00:46:07,126 --> 00:46:08,556
to do they're not gonna
know how to do it.

1108
00:46:08,766 --> 00:46:12,946
And by default a Mac and a PC
generally only know programs

1109
00:46:13,166 --> 00:46:16,176
that happen to live in your C
colon backslash programs file

1110
00:46:16,406 --> 00:46:18,796
or in the Mac on
slash applications.

1111
00:46:19,066 --> 00:46:22,556
Whereas Hi3, I just
created, it happens to live

1112
00:46:22,556 --> 00:46:24,456
in this directory but
my computer doesn't know

1113
00:46:24,456 --> 00:46:27,656
that so you do dot slash
which says current directory.

1114
00:46:27,656 --> 00:46:32,356
Dot is my current directory,
slash means Hi3 is located here.

1115
00:46:32,586 --> 00:46:34,016
So now my program's running.

1116
00:46:34,016 --> 00:46:36,556
State your name, excellent.

1117
00:46:36,666 --> 00:46:40,366
We have just recreated what I
could've implemented moments ago

1118
00:46:40,586 --> 00:46:41,606
with something more dynamic.

1119
00:46:41,746 --> 00:46:42,986
Well-- or with something static.

1120
00:46:42,986 --> 00:46:43,866
Let's do this once more.

1121
00:46:44,086 --> 00:46:44,896
So, state your name.

1122
00:46:44,896 --> 00:46:46,686
Let's do Cansu, enter.

1123
00:46:47,346 --> 00:46:50,586
An S in the side so Cansu,
that was spelled C-A-N-S-U.

1124
00:46:50,586 --> 00:46:53,256
Her name is pronounced Cansu so
she will appreciate your keeping

1125
00:46:53,256 --> 00:46:55,416
that in mind when you say
hello to her in person,

1126
00:46:55,416 --> 00:46:56,616
Cansu being our head TF.

1127
00:46:56,686 --> 00:46:57,776
So Cansu, oh hi Cansu!

1128
00:46:57,776 --> 00:46:58,736
And we can do this all day long.

1129
00:46:59,106 --> 00:47:01,736
So we can provide any
form of input here

1130
00:47:01,816 --> 00:47:03,206
and the program will
just keep working.

1131
00:47:03,206 --> 00:47:05,476
But we can also get a
little manipulative.

1132
00:47:05,516 --> 00:47:08,696
So, it turns out and
this is a hint at some

1133
00:47:08,696 --> 00:47:10,336
of the security issues
we'll discuss.

1134
00:47:10,606 --> 00:47:12,196
Sometimes you can mess
with the computer,

1135
00:47:12,196 --> 00:47:13,116
for better or for worse.

1136
00:47:13,116 --> 00:47:14,516
And give it for instance
nothing.

1137
00:47:15,166 --> 00:47:15,976
Well, this is weird.

1138
00:47:16,246 --> 00:47:20,276
This time I did nothing and
so it just said oh, hi null.

1139
00:47:20,276 --> 00:47:21,696
So that's kind of interesting.

1140
00:47:21,696 --> 00:47:23,216
And it turns out--
a little teaser

1141
00:47:23,216 --> 00:47:26,266
for our security discussion
as time passes that a lot

1142
00:47:26,266 --> 00:47:28,916
of the times when software is
compromised and you download

1143
00:47:28,916 --> 00:47:30,466
like a cracked version
of Photoshop

1144
00:47:30,466 --> 00:47:31,316
or something like that.

1145
00:47:31,566 --> 00:47:33,596
Or a lot of the times when
a website is compromised

1146
00:47:33,596 --> 00:47:35,136
and you're able to steal
someone's usernames

1147
00:47:35,136 --> 00:47:37,476
and passwords, very
often are the result

1148
00:47:37,476 --> 00:47:39,826
of simple little
things like this.

1149
00:47:39,826 --> 00:47:43,276
Notice in this program I
have not checked whether

1150
00:47:43,276 --> 00:47:45,986
or not the user actually
provided input.

1151
00:47:45,986 --> 00:47:48,446
I have not checked the length
of the variable called nan.

1152
00:47:48,676 --> 00:47:49,356
I have not checked

1153
00:47:49,356 --> 00:47:51,416
that something actually
came back to GetString.

1154
00:47:51,416 --> 00:47:53,996
In fact, if I don't hit the
right key on the keyboard

1155
00:47:53,996 --> 00:47:57,406
like D-A-V-I-D and enter but
instead hit a special command

1156
00:47:57,456 --> 00:47:59,546
that says, I'm not gonna
give you a string at all.

1157
00:48:00,066 --> 00:48:03,026
Weird things, bad things
can happen and so one

1158
00:48:03,026 --> 00:48:05,926
of the lessons too that will be
repeated throughout the course

1159
00:48:05,926 --> 00:48:08,926
is to actually rigorously
error check and make sure

1160
00:48:09,086 --> 00:48:12,666
that you're always thinking
about your user being a bad guy.

1161
00:48:12,856 --> 00:48:14,576
So an adversary who's
actually trying

1162
00:48:14,576 --> 00:48:17,216
to break your programs 'cause
frankly too much software these

1163
00:48:17,216 --> 00:48:20,886
days is not written
with a paranoia in mind.

1164
00:48:21,116 --> 00:48:26,856
So with that said, any questions
on some the basic thus far?

1165
00:48:27,066 --> 00:48:27,216
>> Yes.

1166
00:48:27,216 --> 00:48:27,506
>> Yeah.

1167
00:48:27,506 --> 00:48:28,956
>> Okay. Now, I mean.

1168
00:48:29,026 --> 00:48:31,046
I got string and it
went along [inaudible]

1169
00:48:31,046 --> 00:48:34,556
but what does that
actually mean?

1170
00:48:34,556 --> 00:48:36,096
Is it is just like a
string of words or--

1171
00:48:36,096 --> 00:48:37,176
>> What does-- what
does what mean?

1172
00:48:37,176 --> 00:48:37,596
A string?

1173
00:48:37,656 --> 00:48:39,466
>> Like what is that
actually behind it?

1174
00:48:39,466 --> 00:48:40,446
>> So a good question.

1175
00:48:40,446 --> 00:48:43,426
So just-- and we will come
back to this and flush out some

1176
00:48:43,426 --> 00:48:44,696
of these lower level details.

1177
00:48:45,116 --> 00:48:47,336
But for now, today's
teaser will be this.

1178
00:48:47,336 --> 00:48:50,846
We know already and we know
last week from our demonstration

1179
00:48:50,846 --> 00:48:53,496
of the floppy disks and such
that there're various forms

1180
00:48:53,496 --> 00:48:54,726
of memory inside of a computer.

1181
00:48:54,726 --> 00:48:55,846
We know that there're hard disks

1182
00:48:55,846 --> 00:48:57,196
for instance but
there's also RAM.

1183
00:48:57,196 --> 00:48:58,746
RAM being the one-- the memory

1184
00:48:58,746 --> 00:49:00,386
where things are
stored temporarily

1185
00:49:00,386 --> 00:49:01,376
when you're running a program.

1186
00:49:01,716 --> 00:49:03,476
Well, RAM you might
have a gigabyte

1187
00:49:03,476 --> 00:49:05,136
of RAM these days
or two gigabytes.

1188
00:49:05,406 --> 00:49:07,076
Now the byte is the
key word there.

1189
00:49:07,076 --> 00:49:09,696
Even though we say gigabyte
that really means billion bytes.

1190
00:49:10,106 --> 00:49:12,806
So that means if you-- whether
you own a Mac, PC or whatever,

1191
00:49:13,026 --> 00:49:15,896
you have a whole chunk of
RAM memory which you can draw

1192
00:49:15,896 --> 00:49:18,146
for instance as a rectangle,
if you can't quite see.

1193
00:49:18,146 --> 00:49:18,956
It's just a rectangle.

1194
00:49:19,316 --> 00:49:23,366
And because there are a billion
bytes accessible to us, that's--

1195
00:49:23,516 --> 00:49:27,216
essentially means well we can
just call that byte number zero.

1196
00:49:27,416 --> 00:49:28,896
We can call this
byte number one,

1197
00:49:28,896 --> 00:49:30,436
byte number two,
byte number three.

1198
00:49:30,436 --> 00:49:31,936
Again, for those-- for
whom this is blocked,

1199
00:49:31,936 --> 00:49:34,646
I'm just dividing the rectangle
into smaller squares which is

1200
00:49:34,646 --> 00:49:36,136
to say that memory
is addressable.

1201
00:49:36,176 --> 00:49:37,816
You can number every
single byte just

1202
00:49:37,816 --> 00:49:39,376
like you can number
every house on a street.

1203
00:49:39,806 --> 00:49:42,966
So now, in a program and you're
asking the user for a string.

1204
00:49:43,236 --> 00:49:45,906
A string is just a
sequence of characters.

1205
00:49:46,026 --> 00:49:49,136
In this case, it's a
sequence of ASCII characters.

1206
00:49:49,136 --> 00:49:51,446
And ASCII recalls the code
that we talked about last week.

1207
00:49:51,676 --> 00:49:54,556
Capital A is number 65.

1208
00:49:54,556 --> 00:49:56,706
So we talked about how the
computers represent characters.

1209
00:49:56,936 --> 00:49:59,256
So when you get a
string like D-A-V-I-D,

1210
00:49:59,466 --> 00:50:02,486
what's really happening is
inside your computer's RAM is

1211
00:50:02,486 --> 00:50:04,936
going a byte where there's
some zeros and ones.

1212
00:50:05,046 --> 00:50:06,636
>> But if you actually
do the math, those zeros

1213
00:50:06,636 --> 00:50:09,916
and ones translate to an
ASCII character which is D

1214
00:50:09,916 --> 00:50:14,666
which is gonna be a 65, 66,
67, 68 would be the letter D.

1215
00:50:14,936 --> 00:50:17,506
So similarly would there
be the number 65 next

1216
00:50:17,506 --> 00:50:20,946
to this then the
corresponding number for V-I-D

1217
00:50:20,946 --> 00:50:23,346
and then there's generally a
special character at the end

1218
00:50:23,616 --> 00:50:25,926
that looks like a zero that
says end of string here.

1219
00:50:26,256 --> 00:50:28,956
So when we call GetString,
the function

1220
00:50:28,956 --> 00:50:30,336
that the cs50 staff wrote.

1221
00:50:30,566 --> 00:50:33,656
What we are doing is
figuring out how to get input

1222
00:50:33,656 --> 00:50:35,136
from the user at the keyboard

1223
00:50:35,276 --> 00:50:37,176
into individual chunks
of memory like that.

1224
00:50:37,586 --> 00:50:39,046
And when we return the string

1225
00:50:39,046 --> 00:50:41,646
to you we're essentially
telling you

1226
00:50:41,646 --> 00:50:43,786
where this chunk of memory is.

1227
00:50:44,046 --> 00:50:45,946
So I can now proceed
to print it, change it,

1228
00:50:45,946 --> 00:50:49,746
manipulate it, or what now.

1229
00:50:49,746 --> 00:50:50,386
[ Inaudible Remark ]

1230
00:50:50,386 --> 00:50:52,486
>> Those bytes in
the row are called--

1231
00:50:52,486 --> 00:50:52,766
>> A string.

1232
00:50:52,766 --> 00:50:53,146
>> Exactly.

1233
00:50:53,146 --> 00:50:55,066
A string is a contiguous
sequence

1234
00:50:55,066 --> 00:50:56,806
of characters aka bytes.

1235
00:50:57,936 --> 00:50:58,496
Other questions?

1236
00:50:59,236 --> 00:50:59,396
Yes?

1237
00:51:00,836 --> 00:51:03,406
>> What exactly is the
difference with this system

1238
00:51:03,406 --> 00:51:04,936
that we have here of compiling

1239
00:51:04,936 --> 00:51:08,096
and say another compiler
for one like the C++?

1240
00:51:08,296 --> 00:51:08,796
>> Good question.

1241
00:51:08,796 --> 00:51:11,116
What is the difference between
this program here at GCC

1242
00:51:11,116 --> 00:51:14,556
and for instance various other
compilers DEV++, Visual Studio,

1243
00:51:15,366 --> 00:51:17,656
Eclipse is another one,
NetBeans and so forth?

1244
00:51:17,906 --> 00:51:19,506
Fundamentally, very little.

1245
00:51:19,596 --> 00:51:21,196
So generally and
those of you coming

1246
00:51:21,196 --> 00:51:23,756
from high school computer
science background probably used

1247
00:51:23,966 --> 00:51:24,856
what's called an IDE.

1248
00:51:24,856 --> 00:51:26,556
An Integrated Development
Environment

1249
00:51:26,716 --> 00:51:29,446
which actually is a gooey
with menus and icons and such.

1250
00:51:29,446 --> 00:51:30,786
And you can write
your source code in it

1251
00:51:30,996 --> 00:51:33,306
but to compile something you
don't run a command like GCC

1252
00:51:33,336 --> 00:51:35,846
and then the name of the file,
you instead click a button.

1253
00:51:36,106 --> 00:51:37,886
We'll actually get
back to that point.

1254
00:51:38,186 --> 00:51:40,506
But as part of a more
rigorous introduction

1255
00:51:40,506 --> 00:51:41,946
to understanding how
the machines work

1256
00:51:41,946 --> 00:51:43,496
and writing programs
at this lower level.

1257
00:51:43,656 --> 00:51:45,936
We start at this
lower level with GCC.

1258
00:51:46,246 --> 00:51:47,966
So really it's just
a matter of features.

1259
00:51:47,966 --> 00:51:51,516
GCC is probably the
most popular compiler

1260
00:51:51,766 --> 00:51:54,176
in the world these days 'cause
it's been around so long

1261
00:51:54,176 --> 00:51:55,986
and a lot of tools
are based on it.

1262
00:51:56,076 --> 00:51:58,676
But it's more limited
in its feature set

1263
00:51:58,886 --> 00:52:00,856
but again will build
backup to those features.

1264
00:52:01,336 --> 00:52:02,036
Other questions?

1265
00:52:02,586 --> 00:52:02,696
Yeah?

1266
00:52:02,696 --> 00:52:04,946
>> Before we had the Makefile?

1267
00:52:04,946 --> 00:52:05,216
>> Uh hmm.

1268
00:52:05,376 --> 00:52:09,846
>> What's the difference between
a compiler and a Makefile?

1269
00:52:09,846 --> 00:52:11,786
>> Good question, so we
didn't have a Makefile yet,

1270
00:52:11,786 --> 00:52:12,736
more on that in the future.

1271
00:52:12,736 --> 00:52:14,016
But I do use the command make.

1272
00:52:14,456 --> 00:52:17,076
Essentially, this command
make is just another program

1273
00:52:17,296 --> 00:52:20,356
that uses GCC to
compile my program for me

1274
00:52:20,726 --> 00:52:22,486
but make is configurable.

1275
00:52:22,666 --> 00:52:25,196
So where as I currently
am manually typing

1276
00:52:25,196 --> 00:52:28,166
like dash L cs50
and dash O hello.

1277
00:52:28,326 --> 00:52:29,786
All of this-- pain
in the neck right?

1278
00:52:29,786 --> 00:52:30,766
If you're writing a program

1279
00:52:30,766 --> 00:52:32,376
and you're testing
it and recompiling.

1280
00:52:32,586 --> 00:52:33,906
This very quickly gets tedious.

1281
00:52:33,946 --> 00:52:35,526
Typing the same commands
again and again

1282
00:52:35,526 --> 00:52:38,426
and again make can be
configured, as you'll soon see

1283
00:52:38,426 --> 00:52:41,296
in problem set 2 or 3
to do all these for you.

1284
00:52:41,296 --> 00:52:43,716
So that we take very
quickly the tedium out of--

1285
00:52:44,016 --> 00:52:47,086
what is otherwise-- could be
a very repetitive process.

1286
00:52:47,896 --> 00:52:48,876
One more question?

1287
00:52:50,226 --> 00:52:52,606
Alright, so just to-- yup?

1288
00:52:53,066 --> 00:52:57,106
>> So the dot slash
the A out thing--

1289
00:52:57,406 --> 00:52:57,516
>> Uh hmm.

1290
00:52:57,756 --> 00:53:02,506
>> They only use that when the
program that you're running--

1291
00:53:03,976 --> 00:53:05,576
the program that
you're referring

1292
00:53:05,576 --> 00:53:06,486
to is going to your computer?

1293
00:53:06,486 --> 00:53:06,986
>> That's correct.

1294
00:53:07,076 --> 00:53:09,676
When the program is on your
computer you say dot slash

1295
00:53:10,706 --> 00:53:12,816
and this means on my computer.

1296
00:53:12,816 --> 00:53:13,646
>> And that's the
[inaudible] this other stuff

1297
00:53:13,646 --> 00:53:14,786
that you would use,
the GCC oh hi--

1298
00:53:14,786 --> 00:53:15,816
>> Exactly.

1299
00:53:15,816 --> 00:53:17,536
Now GCC was installed
by a special installer.

1300
00:53:17,536 --> 00:53:19,106
It's elsewhere on my hardware.

1301
00:53:19,106 --> 00:53:20,916
It's not in my current folder
but because I installed it

1302
00:53:20,916 --> 00:53:22,806
with like a normal installer,
double click and all of that.

1303
00:53:23,036 --> 00:53:25,816
It configured the Mac or in this
case, the Linux server to put it

1304
00:53:25,816 --> 00:53:28,276
into my so-called path
which just means I know

1305
00:53:28,276 --> 00:53:29,446
where it is automatically.

1306
00:53:30,566 --> 00:53:32,756
But we'll pull back these
layers too before long.

1307
00:53:32,986 --> 00:53:37,456
So a couple of commands and
then an interesting code

1308
00:53:37,886 --> 00:53:40,166
and then the teaser,
that's a teaser.

1309
00:53:40,166 --> 00:53:41,486
So I just type the command LS.

1310
00:53:41,866 --> 00:53:44,886
So at first this might
seem very limited in terms

1311
00:53:44,886 --> 00:53:47,076
of functionality for you but
when I first logged in to

1312
00:53:47,076 --> 00:53:49,416
that cloud, recall that my
prompt looked like this.

1313
00:53:49,766 --> 00:53:52,236
LS is the list command,
those of you who played

1314
00:53:52,236 --> 00:53:54,316
with DOS before might
recall DIR.

1315
00:53:54,596 --> 00:53:57,086
When you type LS it just lists
what's in your current folder,

1316
00:53:57,336 --> 00:54:00,746
my current folder or my
so-called home directory.

1317
00:54:00,826 --> 00:54:04,106
The folder that I own on
the cloud or that I own

1318
00:54:04,106 --> 00:54:08,126
on my own Mac or my own PC
apparently has two folders

1319
00:54:08,126 --> 00:54:08,866
in it at the moment.

1320
00:54:09,076 --> 00:54:12,876
One is called SRC for Source
and one is called SRC.back

1321
00:54:13,166 --> 00:54:15,726
which was my backup of today's
source code in case I screw up.

1322
00:54:16,076 --> 00:54:19,046
If I want to move into a
directory and you'll see--

1323
00:54:19,046 --> 00:54:20,236
all of these will be walked

1324
00:54:20,236 --> 00:54:21,386
through in the first
problem set.

1325
00:54:21,386 --> 00:54:24,286
So realize if it doesn't quite
sink in today, not a problems

1326
00:54:24,286 --> 00:54:25,766
in terms of these
low level details.

1327
00:54:26,216 --> 00:54:27,826
CD is Change Directory.

1328
00:54:28,036 --> 00:54:30,666
So, I'm gonna go ahead
and type CD source, enter.

1329
00:54:30,836 --> 00:54:33,076
And now notice my prompt
changed a little bit.

1330
00:54:33,076 --> 00:54:35,926
In parenthesis now is a tilde
which is shorthand notation

1331
00:54:35,926 --> 00:54:37,346
for My Home Directory.

1332
00:54:37,726 --> 00:54:40,516
And then Source which is the
current directory I'm in.

1333
00:54:40,736 --> 00:54:42,956
So now if I type LS
again and hit Enter,

1334
00:54:43,166 --> 00:54:44,566
I see a whole bunch of stuff.

1335
00:54:44,566 --> 00:54:48,186
And what's in this output are
all the C files that we happened

1336
00:54:48,186 --> 00:54:50,116
to have printouts of for
this particular week.

1337
00:54:50,116 --> 00:54:52,126
This is just the folder
containing all of that stuff

1338
00:54:52,366 --> 00:54:54,936
that we created a PDF
out of ultimately.

1339
00:54:55,146 --> 00:54:58,646
Now, the things in green are
special things that I ran.

1340
00:54:58,646 --> 00:55:01,456
So what's nice if you configure
your command line environment

1341
00:55:01,456 --> 00:55:04,276
appropriately is that things
that are so-called executable,

1342
00:55:04,346 --> 00:55:05,316
programs that you wrote

1343
00:55:05,316 --> 00:55:07,456
like a.out happened
to show up as green.

1344
00:55:07,506 --> 00:55:08,756
Hi3 is green.

1345
00:55:08,926 --> 00:55:11,476
And then all of these other
white files are just source code

1346
00:55:11,476 --> 00:55:12,746
files, dot C file.

1347
00:55:12,966 --> 00:55:16,476
And I thought I would tease
you with this one here.

1348
00:55:16,826 --> 00:55:20,036
This is a file called
Holloway.c. This is--

1349
00:55:20,036 --> 00:55:21,566
to be clear, not a program.

1350
00:55:21,566 --> 00:55:23,606
We'd expect you to write
now or perhaps ever

1351
00:55:23,926 --> 00:55:25,836
since this program
was written as part

1352
00:55:25,836 --> 00:55:28,026
of an obfuscated C contest.

1353
00:55:28,176 --> 00:55:31,096
So there're a lot of geeks in
this world that like to compete

1354
00:55:31,096 --> 00:55:33,736
against each other by writing
the most complicated looking

1355
00:55:33,736 --> 00:55:35,916
compact programs possible.

1356
00:55:36,236 --> 00:55:38,126
Even though it would
take a computer

1357
00:55:38,126 --> 00:55:39,526
to even understand
how they work.

1358
00:55:40,046 --> 00:55:41,056
So here is a program.

1359
00:55:41,056 --> 00:55:43,096
It's not necessary
very well formatted

1360
00:55:43,096 --> 00:55:44,966
but it looks pretty
complex, right?

1361
00:55:44,966 --> 00:55:46,776
I will be happy to
admit that I read

1362
00:55:46,776 --> 00:55:48,726
through this I probably cannot
tell you what it did unless I

1363
00:55:48,726 --> 00:55:50,666
really obsessed over
these details.

1364
00:55:50,666 --> 00:55:52,566
And there's some syntax
we haven't seen before.

1365
00:55:52,566 --> 00:55:54,906
This define keyword is
something we'll see later on.

1366
00:55:54,906 --> 00:55:56,906
It turns out long is
another keyword we'll see.

1367
00:55:57,086 --> 00:55:58,776
These curly-- these
square brackets,

1368
00:55:59,066 --> 00:56:01,186
this is a pretty
complicated looking program.

1369
00:56:01,666 --> 00:56:06,196
So for those more
comfortable what does it do?

1370
00:56:06,426 --> 00:56:06,786
Could you tell?

1371
00:56:07,486 --> 00:56:11,056
So the winner of
this year's contest,

1372
00:56:11,056 --> 00:56:12,326
this is like 2003 or so.

1373
00:56:12,326 --> 00:56:13,476
I included the length,
let me go ahead

1374
00:56:13,476 --> 00:56:16,466
and compile this program
called Holloway.c. No errors

1375
00:56:16,466 --> 00:56:17,266
which means that's good.

1376
00:56:17,266 --> 00:56:21,526
I'm gonna run a.out and
your teaser for the kinds

1377
00:56:21,526 --> 00:56:24,196
of programs you can write
after years and years and years

1378
00:56:24,196 --> 00:56:27,016
of practice will give
you output like this.

1379
00:56:27,886 --> 00:56:31,356
Let's take a break.

1380
00:56:31,356 --> 00:56:31,423
[ Laughter ]

1381
00:56:31,423 --> 00:56:34,366
>> Let's take five-minute break.

1382
00:56:35,826 --> 00:56:39,096
Alright, so we are back.

1383
00:56:39,096 --> 00:56:41,906
Just to plant a couple of seeds
so that you've seen it before

1384
00:56:41,906 --> 00:56:44,346
and then we can revisit
in more useful context.

1385
00:56:44,646 --> 00:56:47,286
Printf, again it's a function,
it's a tool someone else wrote.

1386
00:56:47,326 --> 00:56:50,716
You can invoke it with zero or
more arguments or parameters.

1387
00:56:50,716 --> 00:56:53,546
The result-- or one or more
arguments or parameters

1388
00:56:53,576 --> 00:56:56,656
that result of which is-- that
will influence its behavior.

1389
00:56:57,216 --> 00:56:59,246
So these format strings
we saw earlier.

1390
00:56:59,246 --> 00:57:01,526
We saw percent S which is
a placeholder for a string,

1391
00:57:01,816 --> 00:57:02,756
turns out as you'll see

1392
00:57:02,756 --> 00:57:04,336
over time there are
other placeholders.

1393
00:57:04,336 --> 00:57:06,536
If you know you're not
gonna output a full string

1394
00:57:06,806 --> 00:57:10,066
but a single character, percent
C will get that job done.

1395
00:57:10,296 --> 00:57:14,286
Percent D is for a digit, like a
number or-- one or more digits.

1396
00:57:14,286 --> 00:57:17,086
If you want to output an integer
or other types as we'll see.

1397
00:57:17,486 --> 00:57:20,036
F is for floating point
value which is the fancy way

1398
00:57:20,036 --> 00:57:21,116
of saying real number.

1399
00:57:21,116 --> 00:57:23,426
Something with a decimal
point and numbers thereafter.

1400
00:57:23,706 --> 00:57:25,526
And then LLD, we'll
see in a moment,

1401
00:57:25,526 --> 00:57:28,016
this is for particularly
long numbers

1402
00:57:28,016 --> 00:57:30,606
which will become relevant
especially as you're trying

1403
00:57:30,606 --> 00:57:32,056
to process lots of information.

1404
00:57:32,366 --> 00:57:34,736
This backlash N, it's also a
recurring theme and not just

1405
00:57:34,736 --> 00:57:37,806
in C but in PHP, JavaScript,
other languages as well.

1406
00:57:38,446 --> 00:57:40,566
If you want to escape
certain characters,

1407
00:57:40,796 --> 00:57:43,016
you need to use these
so-called escape sequences.

1408
00:57:43,086 --> 00:57:45,666
So this is a way of
experiencing various succinctly.

1409
00:57:45,746 --> 00:57:47,596
Usually with just two,
maybe three characters,

1410
00:57:47,936 --> 00:57:50,726
a character that you
can't really see as well

1411
00:57:50,726 --> 00:57:53,196
on the screen, otherwise,
so a new line character.

1412
00:57:53,376 --> 00:57:55,306
You can certainly hit
Enter but as we saw--

1413
00:57:55,436 --> 00:57:57,776
seen that very quickly makes
a mess of your code and such

1414
00:57:57,776 --> 00:57:59,496
and so backslash N is new line.

1415
00:57:59,816 --> 00:58:04,076
There's also this backslash R.
Anyone know what backslash R is?

1416
00:58:04,526 --> 00:58:06,286
Yeah, so this is
a carriage return.

1417
00:58:06,626 --> 00:58:09,406
So if you've ever seen or
used an old school typewriter,

1418
00:58:09,656 --> 00:58:11,646
you might recall that when
you're typing something

1419
00:58:11,646 --> 00:58:13,656
out on an old mechanical
typewriter,

1420
00:58:13,946 --> 00:58:15,556
when you like hit the char--

1421
00:58:15,556 --> 00:58:18,776
when you hit the enter
key to move the head back

1422
00:58:18,816 --> 00:58:21,346
to the left hand side to the
next line or you would pull

1423
00:58:21,346 --> 00:58:24,026
down some lever, it was
really a two part process.

1424
00:58:24,026 --> 00:58:27,146
Pulling this lever or pushing
the button would actually rotate

1425
00:58:27,146 --> 00:58:29,496
the paper enough so that
you'd be on a new line

1426
00:58:29,596 --> 00:58:31,626
and it would also move
the little imprinting head

1427
00:58:31,626 --> 00:58:33,026
that prints the letters
of the alphabet

1428
00:58:33,026 --> 00:58:34,486
and such all the
way to the left.

1429
00:58:34,486 --> 00:58:38,326
So you can think of a new line
as being rolling the paper

1430
00:58:38,326 --> 00:58:42,016
up so you're on a new line but a
carriage return moves the cursor

1431
00:58:42,016 --> 00:58:44,676
or moves that writing head
all the way back to the left.

1432
00:58:45,316 --> 00:58:49,496
Now, in the world of Linux
and Unix more generally,

1433
00:58:49,496 --> 00:58:52,486
both of which are our
operating systems that had been

1434
00:58:52,486 --> 00:58:54,766
around for many, many
years, Mac OS is based

1435
00:58:54,766 --> 00:58:56,636
on one of-- one version thereof.

1436
00:58:56,636 --> 00:58:58,516
I mean, you'll play
with it in great--

1437
00:58:59,026 --> 00:59:03,186
with great delight this
semester, you will see

1438
00:59:03,186 --> 00:59:06,116
that backslash N is the
standard in any text file

1439
00:59:06,496 --> 00:59:10,196
for creating what we know
as new line characters.

1440
00:59:10,196 --> 00:59:12,006
In other words, if you
pull up on a Linux system,

1441
00:59:12,256 --> 00:59:14,696
the equivalent nano or
this thing called Vim

1442
00:59:14,916 --> 00:59:17,646
or the equivalent in general
of Text Edit or Notepad

1443
00:59:17,646 --> 00:59:19,576
and you save that
text file, what is--

1444
00:59:19,576 --> 00:59:22,796
and anytime you hit the enter
key, what's really being stored

1445
00:59:22,796 --> 00:59:25,836
in memory, what's really being
stored in one of these bytes

1446
00:59:26,076 --> 00:59:30,756
on disk or in RAM is
backslash N. Unfortunately,

1447
00:59:30,786 --> 00:59:33,336
in the Windows world,
they decided years ago

1448
00:59:33,336 --> 00:59:36,486
that they would store not
only backslash N but much

1449
00:59:36,486 --> 00:59:39,956
like a typewriter they would
store backslash N, backslash--

1450
00:59:40,126 --> 00:59:45,766
backslash, sorry, backslash R,
backslash N. They store 2 bytes

1451
00:59:45,936 --> 00:59:47,206
at the end of every line.

1452
00:59:47,206 --> 00:59:50,706
Every time you hit the enter key
in Notepad or in Microsoft Word,

1453
00:59:50,706 --> 00:59:53,046
small white lie, those
two characters end

1454
00:59:53,046 --> 00:59:54,106
up getting saved in the file.

1455
00:59:54,416 --> 00:59:56,006
Now, for who knows what reason,

1456
00:59:56,186 --> 01:00:00,146
Apple decided they would use
not backslash N, not backslash

1457
01:00:00,146 --> 01:00:03,476
or backslash N, they would
just use backslash R. This is a

1458
01:00:03,476 --> 01:00:06,786
complete pain in the neck and
at some point in your life,

1459
01:00:07,226 --> 01:00:09,246
even if it's after the
course working in some lab

1460
01:00:09,246 --> 01:00:11,246
or in some project where
you're processing files

1461
01:00:11,246 --> 01:00:12,756
or just opening files
in a program,

1462
01:00:12,976 --> 01:00:14,716
you'll run into this
stupid issue

1463
01:00:14,926 --> 01:00:17,566
that really is just a result
of humans not agreeing

1464
01:00:17,566 --> 01:00:18,976
on some standard or just
doing things differently.

1465
01:00:19,106 --> 01:00:22,286
>> You can see this frankly
in the consumer world.

1466
01:00:22,486 --> 01:00:25,256
If you've ever noticed,
Mac OS has its Apple menu

1467
01:00:25,256 --> 01:00:27,426
on the top left and
Windows has its start menu

1468
01:00:27,426 --> 01:00:30,406
in the bottom left, really
for no compelling reason.

1469
01:00:30,406 --> 01:00:32,226
In the other context,
you know in a Window

1470
01:00:32,226 --> 01:00:34,346
and a Windows machine,
you close it with the X

1471
01:00:34,346 --> 01:00:35,286
on the top right hand corner,

1472
01:00:35,526 --> 01:00:37,456
where do you close
it in the Mac?

1473
01:00:37,456 --> 01:00:39,346
Top left hand corner, it's
completely stupid, right?

1474
01:00:39,346 --> 01:00:41,186
It's just different for the
sake of being different.

1475
01:00:41,186 --> 01:00:42,656
Whoever got there
first, who cares,

1476
01:00:43,046 --> 01:00:44,366
same issue with these
things here.

1477
01:00:44,366 --> 01:00:46,606
Now I mention this
only because over time,

1478
01:00:46,886 --> 01:00:49,176
this will become important
when you're not reading

1479
01:00:49,176 --> 01:00:51,796
in files necessarily
but writing files.

1480
01:00:51,796 --> 01:00:54,266
When we get to the
spell checking,

1481
01:00:54,266 --> 01:00:55,306
the dictionary assignment,

1482
01:00:55,466 --> 01:00:57,056
when you're actually
manipulating text files,

1483
01:00:57,056 --> 01:00:59,626
maybe writing some output,
these lower level details

1484
01:00:59,626 --> 01:01:02,956
that actually empower you
to solve problems properly.

1485
01:01:02,956 --> 01:01:08,006
But there're others, backslash
T. Folks in the consulting world

1486
01:01:08,006 --> 01:01:10,766
and finance world like to
share files in Excel format

1487
01:01:10,766 --> 01:01:12,676
or in Text format
that looks like Excel.

1488
01:01:12,966 --> 01:01:16,016
A CSV file, if you've ever
opened a dot CSV file,

1489
01:01:16,306 --> 01:01:19,566
comma-separated values, that
just means it's rows and columns

1490
01:01:19,566 --> 01:01:21,886
where each of the columns
is represented with a comma

1491
01:01:22,056 --> 01:01:23,866
but you can open them
in things like Excel.

1492
01:01:24,096 --> 01:01:27,936
A TSV file, tab-separated
values uses the same thing

1493
01:01:27,936 --> 01:01:28,936
but a tab character,

1494
01:01:29,136 --> 01:01:31,436
if you wanna output an
actual tab character,

1495
01:01:31,436 --> 01:01:32,876
the result of hitting
the tab key,

1496
01:01:33,126 --> 01:01:34,446
you do use backslash T. That's

1497
01:01:34,446 --> 01:01:35,746
so those kinds of
files are made.

1498
01:01:36,096 --> 01:01:38,186
And then sometimes
we'll see you need

1499
01:01:38,186 --> 01:01:40,446
to escape even more
useful characters.

1500
01:01:40,446 --> 01:01:43,116
Sometimes you need to escape
a single quote as we'll see.

1501
01:01:43,436 --> 01:01:45,676
Sometimes you need to
escape a double quote.

1502
01:01:45,976 --> 01:01:49,296
And just intuitively, when might
you need to put a backslash

1503
01:01:49,606 --> 01:01:53,006
in front of a double quote
given the examples we've seen

1504
01:01:53,006 --> 01:01:53,506
thus far?

1505
01:01:53,506 --> 01:01:54,086
[ Inaudible Remark ]

1506
01:01:54,086 --> 01:01:56,376
>> What's that?

1507
01:01:56,946 --> 01:01:58,056
Someone speak up more boldly.

1508
01:01:58,056 --> 01:01:58,736
[ Inaudible Remark ]

1509
01:01:58,736 --> 01:02:01,466
>> If it's in the
middle of the string.

1510
01:02:01,466 --> 01:02:02,946
So thus far, the things I put

1511
01:02:02,946 --> 01:02:05,396
between quotes are just
simple things like David

1512
01:02:05,396 --> 01:02:07,836
or David backslash
N, but what if I want

1513
01:02:07,836 --> 01:02:10,636
to do call my self
David in quotes, right?

1514
01:02:10,636 --> 01:02:12,976
I would need to surround
my self with quotes just

1515
01:02:12,976 --> 01:02:14,706
to pass my self into printf.

1516
01:02:14,996 --> 01:02:17,646
But if I want those quotes
to also appear in the output,

1517
01:02:17,876 --> 01:02:21,226
I now need to also include not
another pair of double quotes

1518
01:02:21,486 --> 01:02:22,926
but escaped double quotes.

1519
01:02:22,926 --> 01:02:26,236
The reason being, again,
computers are not very bright.

1520
01:02:26,326 --> 01:02:30,216
If I do this and this but then
I also put some other quotes

1521
01:02:30,216 --> 01:02:33,226
inside, what's the computer
potentially gonna think?

1522
01:02:33,856 --> 01:02:35,956
That it's actually
two strings, right?

1523
01:02:35,956 --> 01:02:37,186
One starts here, one ends here,

1524
01:02:37,186 --> 01:02:38,446
then there's another
and then another.

1525
01:02:38,576 --> 01:02:39,466
But this is not legit.

1526
01:02:39,466 --> 01:02:41,796
You can't just take a
quoted string back to back

1527
01:02:41,896 --> 01:02:42,896
with another quoted string

1528
01:02:43,146 --> 01:02:44,756
so you'd get some kind
of compiler error.

1529
01:02:44,756 --> 01:02:46,466
GCC would yell at
you for some reason.

1530
01:02:46,746 --> 01:02:51,676
But if I instead put backslash
there and backslash there,

1531
01:02:51,936 --> 01:02:54,466
so now I have double quote,
backslash double quote,

1532
01:02:54,466 --> 01:02:56,796
backslash double
quote, double quote,

1533
01:02:57,116 --> 01:02:58,606
this will actually
print properly.

1534
01:02:58,606 --> 01:03:00,746
So, when in doubt, if you're
having some weird character

1535
01:03:00,746 --> 01:03:02,706
issue, odds are you need
to escape it in some way.

1536
01:03:02,906 --> 01:03:05,966
It gets really kind of trippy
if you wanna output a backslash,

1537
01:03:06,126 --> 01:03:07,416
you can't just use
the backslash,

1538
01:03:07,416 --> 01:03:09,066
you need to use backslash
backslash

1539
01:03:09,316 --> 01:03:11,936
because otherwise a single
backslash is confused with the

1540
01:03:11,936 --> 01:03:13,126
so called escape character.

1541
01:03:13,306 --> 01:03:15,016
And we'll also see backslash 0

1542
01:03:15,276 --> 01:03:17,736
which is actually the special
character that's used at the end

1543
01:03:17,736 --> 01:03:19,726
of a string to say
no more string

1544
01:03:19,726 --> 01:03:21,096
after this point in memory.

1545
01:03:21,366 --> 01:03:22,766
Okay, not to dwell
too much on math

1546
01:03:22,986 --> 01:03:26,166
because you can very quickly
bore someone in computer science

1547
01:03:26,166 --> 01:03:28,606
by dwelling too much on
math but it's useful even

1548
01:03:28,606 --> 01:03:31,326
for logical constructs
as we'll see with loops.

1549
01:03:31,386 --> 01:03:33,406
You do have mathematical
operators in C

1550
01:03:33,406 --> 01:03:34,416
and most every language.

1551
01:03:34,696 --> 01:03:37,356
Most of these are probably
self explanatory, addition,

1552
01:03:37,476 --> 01:03:40,456
subtraction, multiplication,
division, and anyone?

1553
01:03:40,456 --> 01:03:41,806
[ Inaudible Remark ]

1554
01:03:41,806 --> 01:03:43,486
>> Mod or remainder.

1555
01:03:43,486 --> 01:03:44,936
So, there's subtle
difference there.

1556
01:03:44,936 --> 01:03:46,166
But we'll use this over time

1557
01:03:46,416 --> 01:03:48,806
but the percent sign essentially
gives you the remainder.

1558
01:03:48,806 --> 01:03:52,886
So if you do for instance 11
divided by 10 in grade school,

1559
01:03:53,126 --> 01:03:55,306
that's 1 with the
remainder of 1.

1560
01:03:55,686 --> 01:03:59,236
So if you actually do in
a program 11 percent 10,

1561
01:03:59,606 --> 01:04:02,446
what you'll get back
is 1 because that is

1562
01:04:02,446 --> 01:04:05,296
in fact the remainder of
that arithmetic operation.

1563
01:04:05,296 --> 01:04:07,926
So, there is more
useful purposes than that

1564
01:04:07,926 --> 01:04:08,926
than just taking remainders.

1565
01:04:09,106 --> 01:04:10,546
We'll actually see
it's a wonderful way

1566
01:04:10,546 --> 01:04:13,716
of effectively looping yourself
back around, for instance

1567
01:04:13,716 --> 01:04:14,936
if you're at the
end of the alphabet

1568
01:04:15,266 --> 01:04:17,296
or for other tricks altogether.

1569
01:04:17,606 --> 01:04:18,936
So what about those variables?

1570
01:04:18,936 --> 01:04:20,896
Well, it turns out C,
like a lot of languages,

1571
01:04:21,136 --> 01:04:24,296
comes out of the box
with a whole bunch

1572
01:04:24,296 --> 01:04:26,646
of built-in variable types.

1573
01:04:27,176 --> 01:04:31,026
So in C, this is not going to be
the case as we'll see with PHP

1574
01:04:31,026 --> 01:04:33,696
and in JavaScript, at
least not as rigorously.

1575
01:04:33,906 --> 01:04:36,966
In C, you really have to be
specific as to what data type,

1576
01:04:37,286 --> 01:04:40,256
what type of data you're
gonna put in a variable partly

1577
01:04:40,256 --> 01:04:43,586
because C gives us
so low level access

1578
01:04:43,626 --> 01:04:46,026
to your computer's hardware
and memory that you have

1579
01:04:46,066 --> 01:04:48,646
to tell your program what kind
of variable you're gonna use

1580
01:04:48,886 --> 01:04:50,926
so that the operating
system knows how much memory

1581
01:04:50,926 --> 01:04:52,056
to actually give you.

1582
01:04:52,276 --> 01:04:55,266
And with this low level power as
we'll see, as I hinted before,

1583
01:04:55,356 --> 01:04:58,666
comes a lot of fun, interesting,
scary security implications.

1584
01:04:58,666 --> 01:05:00,736
'Cause when you can actually
manipulate a computer's memory

1585
01:05:00,736 --> 01:05:03,686
at this low level, you can
steal people's passwords,

1586
01:05:03,686 --> 01:05:05,366
you can steal their
data if you know how

1587
01:05:05,366 --> 01:05:06,306
that memory is laid out.

1588
01:05:06,476 --> 01:05:08,616
So we'll-- more on
that as time passes.

1589
01:05:08,886 --> 01:05:12,506
So the types that come
built into C are several

1590
01:05:12,686 --> 01:05:14,826
but the most common ones
or the most useful ones

1591
01:05:14,826 --> 01:05:16,106
for our purposes are char.

1592
01:05:16,376 --> 01:05:18,246
If you just wanna store
a single character

1593
01:05:18,446 --> 01:05:19,566
that maybe the user typed in,

1594
01:05:19,566 --> 01:05:21,906
you just call the
variable of type char.

1595
01:05:22,266 --> 01:05:25,726
Float is a floating point number
which is like a real number,

1596
01:05:25,726 --> 01:05:27,736
something with the digits
after the decimal point,

1597
01:05:28,026 --> 01:05:31,896
and it's fairly small, it's
what we're gonna call 32 bits.

1598
01:05:32,046 --> 01:05:35,046
A double is also a
float but it has twice

1599
01:05:35,226 --> 01:05:36,836
as much storage capacity.

1600
01:05:37,076 --> 01:05:39,216
So here is one of the
interesting takeaways, a char,

1601
01:05:39,216 --> 01:05:42,756
we've already spoiled this
answer, takes up how many bits.

1602
01:05:43,896 --> 01:05:45,086
Here is the char,
here is the char,

1603
01:05:45,086 --> 01:05:46,856
how many bits did we say
each of these squares is?

1604
01:05:47,556 --> 01:05:47,876
>> 8.

1605
01:05:47,876 --> 01:05:49,736
>> So 8 bits, aka 1 byte.

1606
01:05:50,116 --> 01:05:53,386
So a char size is
1 byte or 8 bits.

1607
01:05:53,386 --> 01:05:55,886
This is convention,
so on almost--

1608
01:05:55,886 --> 01:05:58,276
and most any computer you
overuse, if you're writing code

1609
01:05:58,486 --> 01:06:01,776
that uses a char, you're gonna
be handed 8 bits or 1 byte

1610
01:06:01,826 --> 01:06:03,266
to store that particular char.

1611
01:06:03,466 --> 01:06:07,016
This is useful especially when
we get to details like forensics

1612
01:06:07,126 --> 01:06:08,706
and looking at data
on a hard drive,

1613
01:06:08,896 --> 01:06:10,266
'cause if you don't
know how much store--

1614
01:06:10,266 --> 01:06:13,756
how many bits are composing your
file, you're never gonna recover

1615
01:06:13,806 --> 01:06:16,276
that data or be able to access
what you're looking for.

1616
01:06:16,486 --> 01:06:22,276
Now a float is not 1 byte, it's
4 bytes, or 4 times 8, 32 bits.

1617
01:06:22,766 --> 01:06:28,576
So, a 32 bit of float gives
you a bunch of precision.

1618
01:06:28,926 --> 01:06:31,086
So a floating point
value again is a number

1619
01:06:31,086 --> 01:06:32,716
with a decimal point,
so it depends.

1620
01:06:32,856 --> 01:06:33,506
It's a tradeoff.

1621
01:06:33,506 --> 01:06:35,526
You can either have a big
number and a small decimal point

1622
01:06:35,756 --> 01:06:37,486
or small number and
a big decimal point.

1623
01:06:37,606 --> 01:06:39,536
But because you only have
a finite number of bits,

1624
01:06:39,536 --> 01:06:42,906
the takeaway for today is that
there's only so much precision.

1625
01:06:42,906 --> 01:06:45,406
With the computer today,
you can, for the most part,

1626
01:06:45,406 --> 01:06:48,676
not represent every
possible number

1627
01:06:48,676 --> 01:06:50,056
that mankind can think of.

1628
01:06:50,116 --> 01:06:52,496
Because you can come up with
an infinite number of values

1629
01:06:52,656 --> 01:06:54,746
but if you have a
finite amount of space

1630
01:06:54,776 --> 01:06:57,276
and certainly a computer has
a finite amount of disk space

1631
01:06:57,276 --> 01:06:59,396
or RAM, you have to
make approximations.

1632
01:06:59,666 --> 01:07:00,406
And that turns out,

1633
01:07:00,406 --> 01:07:02,596
approximations might
sometimes be fine,

1634
01:07:02,596 --> 01:07:04,606
certainly for the small
programs we've seen thus far.

1635
01:07:04,836 --> 01:07:06,676
But when you start doing
scientific research

1636
01:07:06,716 --> 01:07:09,216
or financial work, imprecision

1637
01:07:09,216 --> 01:07:11,836
and rounding errors
start to add up.

1638
01:07:11,836 --> 01:07:16,076
Those of you may have seen
Superman 3 or Office Space,

1639
01:07:16,076 --> 01:07:19,216
this little scam where they make
money in the financial markets

1640
01:07:19,216 --> 01:07:21,136
by scheming fractions
of pennies off.

1641
01:07:21,526 --> 01:07:23,616
That kind of problem
tends to arise

1642
01:07:23,866 --> 01:07:25,916
because they're using a
language or a computer

1643
01:07:25,916 --> 01:07:27,266
that only has finite precision

1644
01:07:27,266 --> 01:07:28,436
and so there are
rounding errors.

1645
01:07:28,436 --> 01:07:31,386
And so the guys in Office Space
took that money and they--

1646
01:07:31,386 --> 01:07:32,956
their for loop--
way too fast is--

1647
01:07:33,036 --> 01:07:35,556
was the takeaway in that
movie, when they freaked

1648
01:07:35,556 --> 01:07:37,326
out because they
had million dollars

1649
01:07:37,326 --> 01:07:38,426
or something like
that overnight.

1650
01:07:38,746 --> 01:07:40,046
Okay, too long of a digression.

1651
01:07:40,326 --> 01:07:43,576
So, primitive type besides
those three types, we have ints

1652
01:07:43,576 --> 01:07:46,316
which are just integers
which are similarly 32 bits.

1653
01:07:46,406 --> 01:07:48,786
And just to plant this
seed with 32 bits,

1654
01:07:48,786 --> 01:07:50,746
what's the biggest
number you can represent?

1655
01:07:50,876 --> 01:07:56,426
So it's 2 to the 32, 'cause
remember a bit is a 0 or 1.

1656
01:07:56,426 --> 01:07:59,196
So if I have 32 bits, each
of which can be a 0 or 1,

1657
01:07:59,446 --> 01:08:01,286
that's two possibilities
for every place,

1658
01:08:01,286 --> 01:08:03,346
so 2 to the 32, that's
4 billion.

1659
01:08:03,696 --> 01:08:07,536
So with an integer, an int data
type, you can store any number

1660
01:08:07,536 --> 01:08:09,856
between 0 and 4 billion roughly.

1661
01:08:10,066 --> 01:08:13,186
Or if you want negative numbers,
between negative 2 billion

1662
01:08:13,186 --> 01:08:15,736
and positive 2 billion,
but it's a finite range.

1663
01:08:15,736 --> 01:08:18,506
And as we'll see, this can
in fact be problematic.

1664
01:08:18,766 --> 01:08:22,586
In fact, when Facebook
first debuted, random aside,

1665
01:08:23,036 --> 01:08:26,766
they used I believe an int to
represent your profile IDs.

1666
01:08:26,946 --> 01:08:28,886
But if you've looked at
more recent [inaudible]

1667
01:08:29,046 --> 01:08:32,356
or if you've looked at friends
who relatively recently signed

1668
01:08:32,356 --> 01:08:34,976
up for Facebook, their profile
IDs which you sometimes see

1669
01:08:34,976 --> 01:08:37,116
in the URL are actually
pretty long and that's

1670
01:08:37,146 --> 01:08:39,366
because I believe at some
point they did transition

1671
01:08:39,366 --> 01:08:44,226
to 64-bit values which tend
to be called long longs.

1672
01:08:44,226 --> 01:08:47,936
And this is so that they could
have more possible user IDs not

1673
01:08:47,936 --> 01:08:51,666
because they have billions
and billions of users

1674
01:08:51,886 --> 01:08:53,976
but surely people create
accounts to lead accounts

1675
01:08:53,976 --> 01:08:55,956
and they don't necessarily
have to reuse numbers this way.

1676
01:08:56,206 --> 01:08:57,986
So, that might be a complete lie

1677
01:08:58,086 --> 01:08:59,866
but I think the numbers
are now 64 bits,

1678
01:09:00,026 --> 01:09:01,786
so we'll see, I'll take a look.

1679
01:09:02,266 --> 01:09:04,766
Alright, so it turns out
there're two other useful types

1680
01:09:04,766 --> 01:09:06,556
that you don't get for
free out of the box.

1681
01:09:06,556 --> 01:09:07,456
One is a bool.

1682
01:09:07,626 --> 01:09:10,266
So we talked about Mr. Bool
briefly last week in this idea

1683
01:09:10,266 --> 01:09:13,686
of true or false, but in C, you
don't have a Boolean data type.

1684
01:09:13,686 --> 01:09:15,706
There is no such thing
as true or false.

1685
01:09:15,986 --> 01:09:18,646
You have to fake
it by using 0 or 1.

1686
01:09:18,886 --> 01:09:20,596
This is just a little
inelegant and so one

1687
01:09:20,596 --> 01:09:22,486
of the things we'll introduce
early on is this idea

1688
01:09:22,486 --> 01:09:26,756
of abstraction or design whereby
if you wanna represent the ideas

1689
01:09:26,756 --> 01:09:29,526
of 0 and 1, true or false, well,

1690
01:09:29,526 --> 01:09:31,906
let's give them a synonym
like true or false.

1691
01:09:31,956 --> 01:09:34,666
Let's not just arbitrarily
say the number 0 is false,

1692
01:09:34,706 --> 01:09:35,776
the number 1 is true.

1693
01:09:36,096 --> 01:09:39,736
You can actually declare in C
synonyms using special syntax

1694
01:09:39,736 --> 01:09:41,996
that says, you know what,
even though yes at the end

1695
01:09:41,996 --> 01:09:45,866
of the day I'm implementing
false as a 0 and true as a 1,

1696
01:09:46,146 --> 01:09:48,886
I can abstract away from
that detail and use words

1697
01:09:48,886 --> 01:09:51,316
that are more intuitive
to me like true and false.

1698
01:09:51,566 --> 01:09:54,136
So earlier today when I
showed these little snippets

1699
01:09:54,136 --> 01:09:56,126
of infinite loops
with while true,

1700
01:09:56,456 --> 01:09:59,526
true is we've inserted
it ourselves

1701
01:09:59,566 --> 01:10:01,956
because we're assuming you're
using the CS50 library.

1702
01:10:02,206 --> 01:10:03,846
>> In other languages, C++,

1703
01:10:03,846 --> 01:10:06,566
Java have the bool
data type built in.

1704
01:10:06,786 --> 01:10:08,926
Strings, this also is
a little white lie.

1705
01:10:08,926 --> 01:10:11,696
We've been using strings thus
far to get input from user

1706
01:10:11,696 --> 01:10:12,656
and store it in variables.

1707
01:10:12,936 --> 01:10:14,916
String is actually
not a data type

1708
01:10:14,916 --> 01:10:20,916
in C. It actually is
something called a char star

1709
01:10:20,916 --> 01:10:22,886
and we'll come back
to this before long.

1710
01:10:22,886 --> 01:10:24,066
But a char we've seen,

1711
01:10:24,436 --> 01:10:25,986
a character being
a single character.

1712
01:10:26,196 --> 01:10:28,816
A star is something called
a pointer so it turns

1713
01:10:28,816 --> 01:10:33,926
out a string is not quite what
I'm describing it to be here.

1714
01:10:33,926 --> 01:10:36,356
It turns out when we
call string a string,

1715
01:10:36,636 --> 01:10:38,866
we're really referring
not to those characters

1716
01:10:39,126 --> 01:10:43,016
but to the address, the location
and memory of those characters.

1717
01:10:43,056 --> 01:10:46,686
But, uninteresting for now but
a teaser of details to come.

1718
01:10:46,986 --> 01:10:50,866
This chart, very boring and
uninteresting but useful

1719
01:10:50,866 --> 01:10:52,046
over time and that is

1720
01:10:52,086 --> 01:10:55,386
to say there are very
well defined rules as to

1721
01:10:55,386 --> 01:10:57,796
when you're writing
code, what operators,

1722
01:10:57,856 --> 01:10:59,986
like what syntax should
get evaluated first.

1723
01:10:59,986 --> 01:11:02,036
You might recall from grade
school sometimes you put things

1724
01:11:02,036 --> 01:11:04,386
in parentheses so that your
addition happens first,

1725
01:11:04,386 --> 01:11:05,536
then your multiplication,

1726
01:11:05,536 --> 01:11:07,756
otherwise multiplication
would happen by default first.

1727
01:11:07,986 --> 01:11:10,586
Realize in programming
languages like C, there is a set

1728
01:11:10,586 --> 01:11:12,136
of very well-defined rules,

1729
01:11:12,136 --> 01:11:13,426
and this is in the
recommended books.

1730
01:11:13,426 --> 01:11:14,736
There're various
resources online.

1731
01:11:14,956 --> 01:11:17,416
I just want to make mention of
this in case there's ever doubt,

1732
01:11:17,946 --> 01:11:20,766
generally parentheses do solve
your problem as an arithmetic

1733
01:11:21,056 --> 01:11:24,136
but do realize this is some
lower level details we'll come

1734
01:11:24,136 --> 01:11:25,646
back to over time.

1735
01:11:26,026 --> 01:11:29,686
So we looked at a number
of fairly small programs

1736
01:11:30,176 --> 01:11:32,886
that at the end of the
day did very little more

1737
01:11:32,886 --> 01:11:34,736
than print hello
world and the like.

1738
01:11:34,736 --> 01:11:36,526
There are so many other
constructs like loops

1739
01:11:36,526 --> 01:11:39,086
and conditions and such,
so what I thought we do

1740
01:11:39,396 --> 01:11:41,496
in conclusion today is
look at this program.

1741
01:11:41,986 --> 01:11:45,356
So, we'll talk about this other
axis of style on the course,

1742
01:11:45,356 --> 01:11:46,976
writing code that looks pretty

1743
01:11:46,976 --> 01:11:49,846
and as well aesthetically
laid out.

1744
01:11:50,186 --> 01:11:54,166
So this is kind of a
misinterpretation of that idea.

1745
01:11:54,516 --> 01:11:56,606
Writing code that just looks--

1746
01:11:56,746 --> 01:11:58,546
I'm not even sure what this
is supposed to look like,

1747
01:11:58,546 --> 01:12:00,646
a little flower or pinwheel
or something like that.

1748
01:12:00,936 --> 01:12:02,946
It is in fact syntactically
valid.

1749
01:12:02,946 --> 01:12:06,426
The semicolons are where
they could be or should be.

1750
01:12:06,736 --> 01:12:11,076
I'm not quite sure what
this is but it is--

1751
01:12:11,076 --> 01:12:13,446
notice the familiar constructs,
there's some ifs, some whiles,

1752
01:12:13,446 --> 01:12:15,766
some fors, these are details
we'll certainly start playing

1753
01:12:15,766 --> 01:12:16,616
with in great detail.

1754
01:12:16,796 --> 01:12:17,996
Let me go ahead and
compile this.

1755
01:12:18,026 --> 01:12:22,416
This two is from that obfuscated
C contest so this is not

1756
01:12:22,416 --> 01:12:24,536
where we're going but this is
where some people choose to go.

1757
01:12:24,896 --> 01:12:26,826
Let me go ahead and
compile this.

1758
01:12:26,826 --> 01:12:29,166
Lots of errors but we've
seen this error before.

1759
01:12:29,166 --> 01:12:31,516
Undefined reference, what
was the solution last time?

1760
01:12:32,056 --> 01:12:32,396
>> Library.

1761
01:12:32,636 --> 01:12:33,446
>> So we need a library.

1762
01:12:33,446 --> 01:12:35,486
It turns out there's other
libraries we'll see over time.

1763
01:12:35,486 --> 01:12:36,766
One is the math library.

1764
01:12:36,986 --> 01:12:40,066
You might wanna say dash L math
but it turns out it's so common

1765
01:12:40,066 --> 01:12:41,286
that people just say dash LM.

1766
01:12:41,856 --> 01:12:44,136
And then for this stuff
up here, it turns out

1767
01:12:44,136 --> 01:12:45,486
and I know this is
just from experience,

1768
01:12:45,736 --> 01:12:47,656
this is called the
Ncurses library.

1769
01:12:47,656 --> 01:12:51,456
This is a-- somewhat graphical
library that allows me

1770
01:12:51,456 --> 01:12:53,056
to display graphics
on the screen.

1771
01:12:53,276 --> 01:12:57,836
So if I link in both of those
libraries, now it compiles okay.

1772
01:12:57,836 --> 01:13:00,626
If I now run a.out, your teaser

1773
01:13:00,626 --> 01:13:03,496
for this Friday's
lecture is this climax.

1774
01:13:03,496 --> 01:13:04,746
[ Inaudible Remark ]

1775
01:13:04,746 --> 01:13:06,286
>> I know.

1776
01:13:06,436 --> 01:13:16,836
See you on Friday.

1777
01:13:16,836 --> 01:13:18,768
[ Inaudible Discussion ]

1778
01:13:18,768 --> 01:13:20,700
[ Music ]