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>> All right, welcome
back to CS-50.

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This is the start of week 1,
so it's this week that we dive

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into actual programming with
a modern programming language

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and begin to tease apart some
of the basic building blocks

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that frankly will carry us
through the whole semester.

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More on that in a moment
as to what that means,

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but quickly one, you do
have a printout today

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of all the source code

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that we'll use today
and also on Friday.

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You'll find that they
lines are numbered,

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so if you've ever heard

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of a language called basic
that's actually a language

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from yesteryear where you
actually as the human how

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to type line 10,
line 20, line 30,

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and then you wrote your
code to the right of this.

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So that was kind of a humanities
clever approach to leaving room

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in case in the future you wanted
to add some more lines of code.

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You could put it
between lines 11 and 19,

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didn't really scale
particularly well.

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These are just printouts,
line numbers,

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so you won't be writing
those numbers there

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but I'll often refer to them.

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Office hours have
been in progress.

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If you would like to come
hang out in either the Quad

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or Quincy Hall dining
hall this evening

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or next evening, do drop by.

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The teaching fellows will
be there with the CAs

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to just chit chat
about the course,

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help you with your scratch
projects or just generally hang

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out and spend the evening in
more of a communal environment.

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And then sections, we will
have the sectioning process

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officially begin
tonight around midnight,

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so check CS50.net's home page

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and we will have an announcement
there as to how you section.

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But so that we have
some breathing room

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and we don't start off the
semester on too rushed a foot,

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what we're going to do is not
start in intimate sections

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where 12 to 15 students and
one CF this coming week,

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we're going to start
those in two weeks time

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and this first week, so this
coming Monday Tuesday Wednesday

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we're going to have super
sections led by pairs of TFs.

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There's just going to be larger
sections for the first week

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that you're welcome to
attend any of those.

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The locations will be announced,
so go to CS50.net/sections

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to see the current
Google calendar for that.

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And then finally we do have
a bunch of sensor boards

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if you would like to take on
the hacker edition of PSET-0,

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but do realize that will
always be the case we expect,

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we encourage almost
everyone in the course

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to pursue the standard edition,
so by no means should you feel

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like you are falling
behind or cutting corners

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by just doing the standard
edition that's meant

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for most folks.

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So with that said,
we spent and a number

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of you have spent already
some time playing with scratch

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and we actually have
kept an eye,

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partly just out of
curiosity, on what kinds

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of submissions have
been coming in already.

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Now, not to worry if you think
it's a little premature for --

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little bug sometimes, the
mouse stops responding so see

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if the screen starts
responding again.

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There we go.

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All right.

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So I went ahead and dug up this
so if only for curiosity's sake,

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only about 10 percent of the
class has submitted already.

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So here too don't worry

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that you're already behind given
how quickly some folks have

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started submitting.

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About 30 people submitted.

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We found this is slightly
statistically interesting.

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So as might be expected,
most of you,

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many of you are running Windows.

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What was kind of interesting was
that neon of you have gone out

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and got and installed this
brand new shiny operating system

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by Apple, Snow Leopard,
and we'll see if some

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of those numbers change.

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Or maybe someone back
there, so we'll see

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when those numbers come in.

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Some of these questions
you'll find

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in PSET-0 survey are really
just for statistical purposes.

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We'll try to do some fun things

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with the data throughout
the semester.

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We don't fundamentally need to
know what city you were born in,

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but it's kind of interesting
if we can later visualize

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that with something
like a Google map.

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So don't be too thrown
off by those.

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If you're worried that everyone
in this room has an i-Phone,

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very much not the case.

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Most of you have, or most of you

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that have submitted already
have a normal mobile phone,

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which is perfect because it
actually means the time Willy

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Chow CA and myself has
spent on shuttle boy voice

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of late will actually get used.

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It's kind of a retro approach

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to making an application
using touchtone prompt,

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but indeed if you
call 617-BUG-CS50

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that service is now live.

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You can get out of here
quickly to the Quad or to matter

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after class where it
operates 24/7 as well.

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And coming soon are going to be
a whole bunch of other features.

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So right now it's just
option 1 for shuttle boy,

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but coming soon will be an
option for wakeup calls,

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which we suspect will
be kind of useful

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around mid-term time, exam time.

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We're mildly nervous because
I think this could backfire

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completely if we only discover
bugs the morning you have

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midterms but we'll
try to avoid that.

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Coming soon too will
be dining hall menus.

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If you want to find out
what is for lunch today,

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what is for dinner today, thanks
to a former teaching fellow,

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Kato Yuchiama [assumed
spelling],

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we'll have an option
online there as well,

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and then soon you'll
be able to find

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out when the next CS-50
office hours actually are.

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Well, people tend to
like where they live.

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This is a little slide
that gives you a sense

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of where PSETs have
been submitted from.

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Freshmen are kind of
slacking it seems thus far,

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although Holeworthy
and Matthews seem

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to have some decent
contingencies.

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And then here among the
houses what's striking is

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that the only house that is
apparently doing other things

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this week has been Leverett,
which ironically is one

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of the largest houses and last
year the data was completely

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the opposite.

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We had more students from
Leverett house than any

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so perhaps we're now
paying that price.

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So just random fun
facts here from CS-50.

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So let's now dive in.

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So this week and
beyond is largely

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about this language called
C and when you start playing

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with C you're playing
with a real,

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a traditional programming
language.

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so PSET 1, which will
be realized this Friday

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on the course's website, will
have you tackle something

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that may be unfamiliar
to many of you,

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this operating system called
Linux [assumed spelling],

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and also this language
called C and we'll spend much

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of the semester on C. What's
fun I think particularly

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about this language
is that unlike Java

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and unlike scripting languages
that some of you might know,

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PHP, Python, buzzwords like
this, it's really low level.

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It's just shy of something
called assembly language,

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which we'll glance at
later in the semester,

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and by that I mean it really
gives you find-grained control

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over where stuff
is going in memory,

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like where in RAM are you
actually using strings

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and numbers and databases
and stuff like that.

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And it allows you at the same
time to do really bad things.

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So many of these
exploits that you read

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about if you read these kinds
of publications or websites

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that involve websites being
hacked over servers being taken

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over by hackers and so
forth, to this day a lot

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of those compromises
are either the result

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of just stupid mistakes
by humans

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or sometimes more
subtle mistakes made

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because the website or the
program, whatever it is,

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was written in C or C++ or
some related languages as well

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because with this power comes as
they say great responsibility.

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You can do a lot of
damage to your own data,

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to your own security by coding
improperly and not being mindful

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of certain security issues.

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And what's actually fun,
I think, as a result,

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about talking about
this language

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for the next several weeks is
you really understand just some

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societal issues like that, like
why did this website get hacked?

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How could someone
possibly do that

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and then what are more
intelligent solutions to some

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of the problems we
ourselves will see.

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So scratch meets
C. so hopefully,

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even if you haven't sat down to
do PSET 0 yet, which is fine,

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still plenty of time, you
hopefully got a sense last week

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of what some of the
basic building blocks are

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of programming.

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There's like a statement,
do this,

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whether it's say something,
think something, move somewhere.

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There's conditions.

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Do this if the following
is true.

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There's variables if you need
to keep around a piece of data,

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and then we saw some slightly
more sophisticated topics

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like threads, multiples
scripts and events, this ability

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to send messages from one script

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to another while all these
same building blocks exist in C

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as they do in most languages.

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And even though what you're
going to start writing

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for PSET 1 onward is going
to look more like the below

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and less like the top,
it's really the same ideas.

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And if you take one important
point away from today,

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realize that you will, when
you first sit down to program,

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maybe for the very first
time and most likely with C

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for the first time, you will
make some stupid mistake

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and you'll stress over
some meaningless detail

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like why is my code not
working, why is not compiling,

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and odds are it's going to
boil down to something stupid

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like see that semicolon
on line 4 there,

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like you'll accidentally
have omitted that

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or you will have left off
a single quote somewhere

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or double quote.

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And frankly this is one

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of the reasons we start off
the course just last week

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with scratch, right?

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You don't have to worry about
this uninteresting minutiae

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but realize now that we're going
to be programming real computers

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and we have to be ever more
precise and it's this level

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of precision, this
level of nitpickiness

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that actually does matter.

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Because you'll find

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that a computer will do
literally what you tell it

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to do, and if you don't
say semicolon there

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in this particular language,
you're not telling the computer

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that this is the end
of the statement,

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and you're making
things ambiguous.

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So ambiguity tends to be bad
in programs as we will see.

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So what are these things going
to start looking like today?

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Well, a quick teaser, a
statement that last week looked

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like this, oh, hi world,
with the little purple block

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and C onward is going to
look a little more cryptic

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but at the tense of the day
the same exact idea, Print F,

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which means a formatted printing
mechanism, Ohi kind of a world

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and then backslash N, I
think we said last week,

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what does that mean?

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New line. So you have to
be even ever so explicit

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about where you want
the enter key to be hit

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and /N represents the act of
hitting enter on the keyboard.

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So that's what the statement's
going to start looking

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like shortly, a bouillon
expression,

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so this was something that could
evaluate to true or false 1

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or 0 on or off, however
you want to view the world.

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Well now what used to
look like orange and green

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up top there is instead going

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to look a little more
high school math,

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but parenthesis X less
than Y, closed parenthesis.

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And then if you want to join
these things together we're

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going to see things like
ampersand ampersand,

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just happens to be
the nomenclature in C

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for the bouillon idea of and.

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And the notion of or, if you
want to say this or this,

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it's not going to
be or it's going

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to be a vertical
bar a vertical bar.

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And it's stupid things
like this too,

241
00:09:58,356 --> 00:10:01,156
where we'll inevitably
get an FAQ this semester

242
00:10:01,156 --> 00:10:03,236
like where is the vertical
bar on my keyboard?

243
00:10:03,236 --> 00:10:05,356
This is not a character
many of you usually use,

244
00:10:05,666 --> 00:10:08,566
it's usually shift and
above the enter key

245
00:10:08,566 --> 00:10:10,416
on the typical keyboard
that would vary.

246
00:10:10,416 --> 00:10:12,756
So again don't get hung
up early in the semester

247
00:10:12,756 --> 00:10:16,486
on stupid details like this
because they are stupid

248
00:10:16,486 --> 00:10:17,906
and they're intellectually
uninteresting

249
00:10:18,136 --> 00:10:20,876
so just accept the
fact that this is what

250
00:10:20,876 --> 00:10:23,746
at least initially some
languages are like.

251
00:10:23,746 --> 00:10:25,576
Well, what about more
interesting constructs,

252
00:10:25,646 --> 00:10:26,176
conditions.

253
00:10:26,226 --> 00:10:29,786
If X is less than Y last week
we wanted to say something.

254
00:10:30,086 --> 00:10:31,986
Alt we wanted to
say something else.

255
00:10:32,226 --> 00:10:35,366
Well, what looks like this
nested puzzle piece construction

256
00:10:35,366 --> 00:10:37,486
at left this week
onward is going to look

257
00:10:37,486 --> 00:10:38,516
like the stuff at right.

258
00:10:38,516 --> 00:10:40,216
And again it's getting
a little messier

259
00:10:40,216 --> 00:10:42,436
but it's still basic
building blocks,

260
00:10:42,476 --> 00:10:46,416
if space open parenthesis, X
less than Y close parentheses,

261
00:10:46,416 --> 00:10:49,086
curly braces, essentially
represent what scratch does

262
00:10:49,086 --> 00:10:51,786
graphically by embracing
puzzle pieces similarly

263
00:10:51,786 --> 00:10:55,236
to embrace chunks or lines of
code with these curly braces.

264
00:10:55,546 --> 00:10:57,376
Print up is again
formatted print.

265
00:10:57,416 --> 00:11:00,246
We've got some familiar quotes
now, backslash and semicolon.

266
00:11:00,286 --> 00:11:02,026
So already this is old hat.

267
00:11:02,026 --> 00:11:04,866
Else if now we have a
second branch here, else,

268
00:11:05,036 --> 00:11:07,806
a third branch, and notice
again the symmetry throughout

269
00:11:07,806 --> 00:11:08,276
this code.

270
00:11:08,276 --> 00:11:11,396
If we open a curly brace
we close it and things seem

271
00:11:11,396 --> 00:11:12,976
to be done symmetrically.

272
00:11:13,036 --> 00:11:14,486
So that will become
important as well.

273
00:11:14,876 --> 00:11:16,936
So loops, we saw
things like forever

274
00:11:16,936 --> 00:11:18,346
and repeat some number of times.

275
00:11:18,596 --> 00:11:20,616
Well, these two look a
little weird at first glance

276
00:11:20,666 --> 00:11:24,216
but at the top right there we
have while 1 and this is kind

277
00:11:24,216 --> 00:11:27,286
of a stupid loop in a
sense because while 1,

278
00:11:27,286 --> 00:11:28,076
well what does that mean?

279
00:11:28,076 --> 00:11:30,746
Well, 1 is synonymous
so far as we've seen

280
00:11:30,746 --> 00:11:32,036
with the notion of true.

281
00:11:32,266 --> 00:11:36,186
So while true what does this
suggest is going to happen

282
00:11:36,236 --> 00:11:38,776
with that loop at top right?

283
00:11:38,886 --> 00:11:41,506
It's going to go forever,
right because true is true,

284
00:11:41,506 --> 00:11:42,916
it's not going to change.

285
00:11:43,006 --> 00:11:44,526
One is not a variable,
it's not X,

286
00:11:44,736 --> 00:11:46,526
it's not Y. It's not
foo it's not far.

287
00:11:46,526 --> 00:11:49,796
It's not some placeholder, it
is literally a value of truth

288
00:11:50,116 --> 00:11:52,956
so this is an intentional
infinite loop.

289
00:11:53,236 --> 00:11:55,306
Now probably not the best use

290
00:11:55,306 --> 00:11:57,506
of an infinite loop here
unless you literally want

291
00:11:57,506 --> 00:11:59,836
to write a program that
when double-clicked or run

292
00:11:59,836 --> 00:12:04,716
and a prompt just says
ohi ad nauseum forever,

293
00:12:04,716 --> 00:12:06,956
literally until the cord is
pulled from the computer.

294
00:12:06,956 --> 00:12:10,416
So sometimes we'll see
infinitive loops are good

295
00:12:10,416 --> 00:12:12,786
but infinite loops if you
can somehow get out of them

296
00:12:12,786 --> 00:12:15,296
and the only way to get out of
this is to do the equivalent

297
00:12:15,296 --> 00:12:17,146
in Linux will see
of force quitting

298
00:12:17,146 --> 00:12:18,236
that particular program.

299
00:12:18,236 --> 00:12:20,686
And to be clear, these
are not real programs,

300
00:12:20,966 --> 00:12:22,566
just as the left-hand
blocks don't have

301
00:12:22,566 --> 00:12:24,796
that when green flag
clicked block

302
00:12:24,796 --> 00:12:26,706
which started a script
last week,

303
00:12:27,036 --> 00:12:28,496
these are not complete programs,

304
00:12:28,496 --> 00:12:29,786
these are just snippets
thus far.

305
00:12:29,786 --> 00:12:31,046
We'll see what we
need to wrap them

306
00:12:31,046 --> 00:12:32,356
in to make them real programs.

307
00:12:32,596 --> 00:12:34,416
Now bottom right's a
little more cryptic still

308
00:12:34,416 --> 00:12:36,906
but it's a pattern you're
going to get very familiar

309
00:12:36,906 --> 00:12:38,266
with because it's
terribly useful.

310
00:12:38,446 --> 00:12:40,666
Four int I gets 0.

311
00:12:40,666 --> 00:12:43,896
So this says declare variable
called I, initialize it to 0.

312
00:12:44,186 --> 00:12:48,536
Do the following so long
as I is less than 10,

313
00:12:48,536 --> 00:12:49,716
so that middle piece there

314
00:12:49,716 --> 00:12:52,676
between the two semicolons
says it's a condition,

315
00:12:52,996 --> 00:12:56,006
so it's similar in spirit to
what's in parentheses up above,

316
00:12:56,006 --> 00:12:57,786
uninteresting though
that parenthetical was,

317
00:12:58,056 --> 00:13:01,396
so while I is less than
10, do the following

318
00:13:01,446 --> 00:13:03,846
and then you recall from last
week what plus plus tends

319
00:13:03,846 --> 00:13:04,306
to denote?

320
00:13:05,406 --> 00:13:08,746
Increment, so the effect in
English of this bottom loop is

321
00:13:08,746 --> 00:13:10,116
to do what in layman's terms?

322
00:13:10,566 --> 00:13:13,606
Print ohi 10 times.

323
00:13:14,556 --> 00:13:16,016
Right. It's not an
interesting program,

324
00:13:16,016 --> 00:13:19,236
but again what's otherwise
cryptic looking syntax just gets

325
00:13:19,236 --> 00:13:20,496
a very simple job done.

326
00:13:20,496 --> 00:13:22,706
And so that will become
now a new building block.

327
00:13:22,706 --> 00:13:23,856
Well what about variables?

328
00:13:23,856 --> 00:13:25,926
Well, this snippet
of scratch code

329
00:13:25,926 --> 00:13:28,316
at top declares a
variable called counter.

330
00:13:28,316 --> 00:13:29,766
It initializes it to zero

331
00:13:30,006 --> 00:13:33,136
and then apparently this scratch
code forever said the value

332
00:13:33,136 --> 00:13:35,606
of counter and this little
cartoon bubble on the screen

333
00:13:35,606 --> 00:13:37,396
and then it changed
counter by one

334
00:13:37,396 --> 00:13:39,576
and we said last week that's
the equivalent of plus plus.

335
00:13:39,886 --> 00:13:43,396
So that loop up there, that
scratch construct has the effect

336
00:13:43,776 --> 00:13:45,646
of counting from 0 on up.

337
00:13:45,716 --> 00:13:47,706
Well, how do we implement
that same idea in C?

338
00:13:47,706 --> 00:13:49,216
Well, it's going to look a
little something like this.

339
00:13:49,306 --> 00:13:54,136
To declare a variable in C we
shall see will be a relatively

340
00:13:54,176 --> 00:13:56,296
simple saying what kind
of variable do you want?

341
00:13:56,536 --> 00:13:58,516
I want an integer, aka int.

342
00:13:58,816 --> 00:14:00,756
what do you want to
give its name as?

343
00:14:00,756 --> 00:14:04,266
Counter. What do you want to
assign it a default value of?

344
00:14:04,926 --> 00:14:07,226
Gets 0 so = 0 semicolon.

345
00:14:07,306 --> 00:14:09,926
Declares a variable called
counter, it says I'm going

346
00:14:09,926 --> 00:14:11,786
to put an int inside of
this thing, what Int?

347
00:14:11,786 --> 00:14:13,976
I'm going to put 0
inside of it initially.

348
00:14:14,216 --> 00:14:17,196
Now this loop we saw before
while 1 means do this ad

349
00:14:17,196 --> 00:14:20,076
infinitum, print F. Now
this is a little interesting

350
00:14:20,076 --> 00:14:23,876
and we'll look at more of this
today and on Friday, percent D,

351
00:14:24,536 --> 00:14:26,946
this is what the
F is in Print F.

352
00:14:27,166 --> 00:14:30,146
When we say Print F it means
print a formatted string,

353
00:14:30,476 --> 00:14:32,036
well what does it mean
to format a string?

354
00:14:32,286 --> 00:14:35,016
It means to tell the computer
display this value somewhat

355
00:14:35,016 --> 00:14:37,466
differently based on
what type of value it is.

356
00:14:37,726 --> 00:14:39,436
Now this is kind of
an uninteresting case

357
00:14:39,436 --> 00:14:40,516
because what do we
want to print?

358
00:14:40,856 --> 00:14:43,076
Just that value of
counter, because I want

359
00:14:43,076 --> 00:14:45,486
to essentially implement
the scratch thing up above,

360
00:14:45,776 --> 00:14:49,516
so percent D henceforth,
decimal stands for integer.

361
00:14:49,666 --> 00:14:50,896
Put in integer here.

362
00:14:51,136 --> 00:14:51,866
What integer?

363
00:14:52,106 --> 00:14:55,756
Well, put whatever integer
is inside of the variable

364
00:14:55,756 --> 00:14:57,246
that comes after the comma.

365
00:14:57,596 --> 00:14:59,056
So again, basic building blocks.

366
00:14:59,396 --> 00:15:01,466
You're certainly welcome to take
little notes on things like this

367
00:15:01,466 --> 00:15:02,926
but it will recur so frequently

368
00:15:03,146 --> 00:15:05,086
that this will very
quickly become old hat.

369
00:15:05,086 --> 00:15:08,146
And finally counter plus plus,
and that simply means increment

370
00:15:08,146 --> 00:15:09,466
that counter and do it again.

371
00:15:09,706 --> 00:15:11,056
So we have scratch, we have C,

372
00:15:11,286 --> 00:15:12,786
functionally they're
exactly the same.

373
00:15:12,786 --> 00:15:15,056
I think you'll find in
week zero and for PSET 0,

374
00:15:15,236 --> 00:15:17,056
just a lot more fun
to get something up

375
00:15:17,056 --> 00:15:20,396
and running using the approach
up top, but you're going to run,

376
00:15:20,516 --> 00:15:23,806
some of you, into sort
of ceilings with scratch,

377
00:15:23,806 --> 00:15:25,436
things you just can't really do

378
00:15:25,436 --> 00:15:27,486
because it's not expressive
enough of language.

379
00:15:27,686 --> 00:15:29,986
We'll be able to do much
more powerful things with C,

380
00:15:30,336 --> 00:15:32,936
much more powerful things
with PHP and with java script,

381
00:15:33,236 --> 00:15:35,906
so we are on, as you shall
see, an upward trajectory.

382
00:15:36,046 --> 00:15:37,846
And finally we have this
thing called an inventory.

383
00:15:37,846 --> 00:15:41,266
What program was it last week
for which I used an inventory?

384
00:15:41,266 --> 00:15:44,846
Yeah, it was that RPG,
the role-playing game

385
00:15:44,846 --> 00:15:46,636
where I moved the little
guy up, down, left,

386
00:15:46,636 --> 00:15:48,656
right and collected fruit
and then presented it

387
00:15:48,656 --> 00:15:49,606
to the guy at the counter.

388
00:15:49,606 --> 00:15:54,026
Well, I was putting fruit
into an array, aka a vector,

389
00:15:54,026 --> 00:15:56,876
aka a list, although technically
we'll see this semester those

390
00:15:56,876 --> 00:15:59,296
actually have different
useful meanings.

391
00:15:59,456 --> 00:16:02,016
In scratch we just have a
variable called inventory.

392
00:16:02,016 --> 00:16:05,676
We can add a string, like
orange, and when I say string,

393
00:16:05,676 --> 00:16:08,676
incidentally, this is computer
science speak for a sequence

394
00:16:08,676 --> 00:16:11,866
of characters that represent
a word, a sentence, whatever.

395
00:16:12,576 --> 00:16:14,426
So that's going to
be represented in C,

396
00:16:14,426 --> 00:16:18,566
perhaps most cryptically
still is char star inventory,

397
00:16:18,686 --> 00:16:21,386
open bracket size
closed bracket semicolon.

398
00:16:21,386 --> 00:16:23,836
And we won't spend much
time on arrays this week,

399
00:16:24,106 --> 00:16:27,156
but in C because you're so close
to the hardware, so to speak,

400
00:16:27,226 --> 00:16:29,586
because you have so
much control over RAM

401
00:16:29,586 --> 00:16:31,166
and where the computer
puts stuff,

402
00:16:31,576 --> 00:16:34,136
you have to be more explicit
in C than in scratch.

403
00:16:34,136 --> 00:16:37,136
You have to say I want
an array of what size?

404
00:16:37,356 --> 00:16:39,286
Well, you tell the
computer by way

405
00:16:39,286 --> 00:16:42,676
of this bracket notation how
big you want the array to be,

406
00:16:42,886 --> 00:16:46,276
so size is actually a variable
for some constant value

407
00:16:46,496 --> 00:16:48,906
that must be defined
somewhere else in my program.

408
00:16:48,906 --> 00:16:50,046
It's simply not on screen.

409
00:16:50,326 --> 00:16:53,976
But then when you want to put
something at a specific location

410
00:16:53,976 --> 00:16:56,666
in an array, the IFE [sounds
like] location, we'll say,

411
00:16:56,666 --> 00:16:58,116
I'll stop using finger
quotes today

412
00:16:58,116 --> 00:16:59,846
because all these terms
are new, I'm sure.

413
00:16:59,846 --> 00:17:04,206
So the IFE location in
inventory is expressed

414
00:17:04,206 --> 00:17:08,416
with inventory bracket I
closed bracket gets or equals

415
00:17:08,586 --> 00:17:10,716
and then whatever value
I want to put in there.

416
00:17:10,986 --> 00:17:14,956
and now notice I'm trying even
today, this day zero of week 1

417
00:17:14,956 --> 00:17:18,406
to be careful with terms like
equals because we'll find

418
00:17:18,406 --> 00:17:20,236
that this equals sign
that you're familiar

419
00:17:20,236 --> 00:17:23,546
with from algebra in most
programming languages is not

420
00:17:23,546 --> 00:17:26,116
equal, it's not equality,
it's assignment.

421
00:17:26,476 --> 00:17:28,936
It means take what's on
the left and put it inside

422
00:17:28,936 --> 00:17:30,036
of the thing on the left.

423
00:17:30,266 --> 00:17:32,796
Wait, take what's on the
right and put it inside

424
00:17:32,796 --> 00:17:34,016
of the thing on the left.

425
00:17:34,176 --> 00:17:35,916
And that's why we've
seen this pattern array.

426
00:17:36,226 --> 00:17:37,766
So this begs an interesting
question

427
00:17:37,766 --> 00:17:39,526
and invites an interesting bug.

428
00:17:39,826 --> 00:17:41,526
How do you compare two values?

429
00:17:41,646 --> 00:17:43,376
Is X equal to Y?

430
00:17:43,666 --> 00:17:46,356
Well, you can't say X equals
Y because that would seem

431
00:17:46,356 --> 00:17:47,666
to put Y's value in X,

432
00:17:47,806 --> 00:17:50,606
so hopefully there will
be a solution to that.

433
00:17:51,416 --> 00:17:56,646
So with that said, here comes
C. when you write a C program,

434
00:17:56,696 --> 00:17:59,946
as I did on the fly on my Mac
last week, recall that I opened

435
00:17:59,946 --> 00:18:02,496
that like terminal window, it
was this little blinking cursor.

436
00:18:02,646 --> 00:18:05,696
I quickly whipped up the
code, the source code,

437
00:18:05,696 --> 00:18:07,706
at top left there, and
then I ran a command.

438
00:18:08,116 --> 00:18:11,786
Well, trivia what
was that command?

439
00:18:12,306 --> 00:18:14,936
GCC. So this is a term you will
not forget after term's end

440
00:18:14,936 --> 00:18:16,396
because you're going to
be running it every day

441
00:18:16,756 --> 00:18:17,856
that you're tackling problems.

442
00:18:17,856 --> 00:18:20,176
That's GCC, is a compiler.

443
00:18:20,586 --> 00:18:21,946
A compiler is a program

444
00:18:22,016 --> 00:18:23,636
that takes something
called source code

445
00:18:23,866 --> 00:18:25,896
and turns it into object code.

446
00:18:26,016 --> 00:18:26,816
Now what does that mean?

447
00:18:27,076 --> 00:18:29,646
Well, nicely enough
this course is not

448
00:18:29,706 --> 00:18:31,886
about writing stuff like this.

449
00:18:32,246 --> 00:18:34,016
There was a time
and there were --

450
00:18:34,016 --> 00:18:36,556
there are people still
roaming this campus who had

451
00:18:36,556 --> 00:18:40,746
to essentially write programs
at the level of zeros and ones,

452
00:18:40,746 --> 00:18:42,576
or if you've heard of this
thing called a punch card,

453
00:18:42,576 --> 00:18:44,816
very similar in spirit,
and there was a time

454
00:18:44,816 --> 00:18:48,026
when the science center
was a center of science.

455
00:18:48,026 --> 00:18:49,826
You would go into the
basement, you would hand

456
00:18:49,826 --> 00:18:54,096
in your punch cards and then
the person running the mainframe

457
00:18:54,096 --> 00:18:55,006
computer in the basement

458
00:18:55,006 --> 00:18:57,496
of the science center would
essentially run your homework

459
00:18:57,496 --> 00:19:00,236
for you overnight, because it
would usually take a while,

460
00:19:00,236 --> 00:19:03,426
by literally putting a stack of
cards somewhere into a machine.

461
00:19:03,736 --> 00:19:05,816
So not the most efficient
process, and I've talked

462
00:19:05,816 --> 00:19:10,356
to current faculty who grew
up this way on a campus,

463
00:19:10,586 --> 00:19:14,416
not the best time of day to find
out that your program is buggy,

464
00:19:14,556 --> 00:19:15,976
it has some mistake
in it, right?

465
00:19:15,976 --> 00:19:17,146
Because that's probably
happening

466
00:19:17,146 --> 00:19:19,646
at 9 a.m. the next morning
when you go back to the center

467
00:19:19,646 --> 00:19:21,996
of science and pick up the
results of your program

468
00:19:22,176 --> 00:19:23,676
and it's just one
big error message.

469
00:19:23,676 --> 00:19:26,676
So fortunately we've come a long
way, and all of your fighting

470
00:19:26,676 --> 00:19:29,426
with bugs and implementing cool
things will happen much more

471
00:19:29,426 --> 00:19:32,316
interactively within seconds
in front of your own computer

472
00:19:32,316 --> 00:19:35,696
or the lab computer, but the
process is essentially still

473
00:19:35,696 --> 00:19:36,126
the same.

474
00:19:36,216 --> 00:19:39,236
At the end of the day a
computer, whether it's a PC

475
00:19:39,236 --> 00:19:41,016
or a Mac or whatever
you guys own at home,

476
00:19:41,326 --> 00:19:43,596
it only understands 0s and 1s.

477
00:19:43,596 --> 00:19:46,126
Now the world has done some
really clever things with 0s

478
00:19:46,126 --> 00:19:49,626
and 1s but you still have to
turn your programs into 0s

479
00:19:49,626 --> 00:19:51,966
and 1s and a compiler
does exactly that.

480
00:19:52,176 --> 00:19:54,476
So you'll be writing stuff that
looks like this at top left.

481
00:19:54,786 --> 00:19:57,416
You'll run that file, which
is really just a text file,

482
00:19:57,506 --> 00:19:58,916
through a program
called a compiler,

483
00:19:59,116 --> 00:20:01,556
and that compiler will take
care of the task of turning it

484
00:20:01,556 --> 00:20:03,976
into 0s and 1s like this.

485
00:20:04,886 --> 00:20:06,016
So where do you do this?

486
00:20:06,016 --> 00:20:10,116
Well, last week I went
ahead and ran this program,

487
00:20:10,116 --> 00:20:12,136
so just a quick show of hands
then we'll have the data

488
00:20:12,136 --> 00:20:13,856
for sure, how many of you
have Macs in the room?

489
00:20:14,036 --> 00:20:17,106
All right, so that's actually
statistically kind of a lot,

490
00:20:17,556 --> 00:20:19,446
so it looks like
more than half of you

491
00:20:19,446 --> 00:20:21,266
in the room this semester,
and that's pretty consistent

492
00:20:21,266 --> 00:20:24,626
with terms past, what's neat is
that all the stuff we're going

493
00:20:24,626 --> 00:20:25,926
to start doing this
week and beyond,

494
00:20:26,176 --> 00:20:28,706
you actually could have been
doing on your own computers,

495
00:20:28,706 --> 00:20:30,656
your own Macs, this whole time.

496
00:20:30,656 --> 00:20:33,156
Now terminal is not a
program that's usually

497
00:20:33,386 --> 00:20:35,676
in the little dock
here as it is in mine

498
00:20:35,676 --> 00:20:38,006
because most users don't
need to know or care

499
00:20:38,006 --> 00:20:39,016
about this little terminal.

500
00:20:39,276 --> 00:20:40,796
But you may at least
know anecdotally

501
00:20:40,796 --> 00:20:42,986
that Mac OS these days is based

502
00:20:42,986 --> 00:20:44,576
on an operating system
called UNIX.

503
00:20:44,796 --> 00:20:48,626
And back in the day UNIX was
essentially a command line

504
00:20:48,686 --> 00:20:51,096
interface, CLI is
the buzzword there,

505
00:20:51,096 --> 00:20:54,356
and this means you don't double
click on icons to run programs,

506
00:20:54,586 --> 00:20:57,106
you don't go to pretty
menus that have sub menus

507
00:20:57,106 --> 00:20:59,806
and graphical icons and all of
this fanciness that developed

508
00:20:59,806 --> 00:21:02,186
over the years, you if
you want to run a program,

509
00:21:02,396 --> 00:21:04,796
you type the name of the
program and hit enter.

510
00:21:05,026 --> 00:21:07,726
And if you want to influence
the behavior of that program,

511
00:21:07,726 --> 00:21:11,216
you provide it with commands
line argument, words or numbers

512
00:21:11,486 --> 00:21:13,736
after the program's
name, and then hit enter.

513
00:21:13,736 --> 00:21:16,246
If you've ever seen DOS
on a computer back home,

514
00:21:16,416 --> 00:21:17,746
it's very similar in spirit.

515
00:21:17,826 --> 00:21:20,636
Usually a black screen, white
text, and you can't do much

516
00:21:20,706 --> 00:21:23,926
of interest with it unless you
know what's there underneath.

517
00:21:23,926 --> 00:21:26,226
So you're not going to write
for the most part programs

518
00:21:26,226 --> 00:21:29,746
in this course on your own
Macs or on your own PCs per se,

519
00:21:29,966 --> 00:21:34,346
because it's quite hard for us
to standardize the configuration

520
00:21:34,346 --> 00:21:36,176
of 300 plus people's laptops.

521
00:21:36,406 --> 00:21:38,276
So instead what we do
is provide everyone

522
00:21:38,276 --> 00:21:40,676
with a uniform environment
throughout the semester.

523
00:21:40,926 --> 00:21:42,936
Initially this is a cluster

524
00:21:43,146 --> 00:21:48,036
of servers called
nice.fas.harvard.edu.

525
00:21:48,106 --> 00:21:50,246
And this nice cluster
does so happen to live

526
00:21:50,246 --> 00:21:52,306
in this very idyllic
building here.

527
00:21:52,646 --> 00:21:54,246
So it's in the basement,
unfortunately,

528
00:21:54,246 --> 00:21:55,206
the cluster of servers.

529
00:21:55,206 --> 00:21:57,216
It's also a bit of a misnomer

530
00:21:57,216 --> 00:22:01,366
because NICE new instructional
computing environments has been

531
00:22:01,446 --> 00:22:05,486
NICE for about 10 years now, and
there isn't even an old version

532
00:22:05,486 --> 00:22:07,846
so they've kind of painted
themselves into a corner here.

533
00:22:07,906 --> 00:22:10,006
We're using the same
NICE cluster as always.

534
00:22:10,006 --> 00:22:13,416
So nice.fas.harvard.edu,
it's not a website,

535
00:22:13,416 --> 00:22:15,906
it's not something you type
into a browser, but it's going

536
00:22:15,906 --> 00:22:18,966
to be the name of a server
that you access by way

537
00:22:18,966 --> 00:22:21,766
of your own Mac or by
way of your own PC.

538
00:22:21,766 --> 00:22:22,686
Well, what do I mean by that?

539
00:22:23,086 --> 00:22:26,216
well, a lot of you these
days have college.harvard.edu

540
00:22:26,216 --> 00:22:28,776
addresses, but you also have
the so-called FAS accounts,

541
00:22:28,776 --> 00:22:31,756
and these are the user names
and passwords you use to login

542
00:22:31,756 --> 00:22:34,276
to the basement of the science
center, kiosks and all of that,

543
00:22:34,396 --> 00:22:36,186
frankly it will be a
few more years, I think,

544
00:22:36,186 --> 00:22:38,336
before the university
converges on just one account.

545
00:22:38,586 --> 00:22:40,326
But for now you have these
accounts, though they're

546
00:22:40,326 --> 00:22:41,536
of decreasing utility.

547
00:22:41,926 --> 00:22:44,256
But for this course
at least at the start

548
00:22:44,256 --> 00:22:48,446
of the semester we do acquaint
you with nice.fas.harvard.edu

549
00:22:48,446 --> 00:22:49,316
because what it means is

550
00:22:49,316 --> 00:22:52,846
that after the semester you'll
actually still have a Linux

551
00:22:52,846 --> 00:22:56,276
server environment on which
you can run and write code,

552
00:22:56,276 --> 00:22:58,896
whether it's for an
economic class in the further

553
00:22:58,896 --> 00:23:00,646
or a chemistry project
or anything

554
00:23:00,646 --> 00:23:03,326
where you need the ability to
write programs and you're not

555
00:23:03,376 --> 00:23:05,636
in CS-50 anymore, you'll
at least have this resource

556
00:23:05,636 --> 00:23:08,046
for some number for a
good amount of time.

557
00:23:08,416 --> 00:23:10,656
What we will start using
mid-semester, though,

558
00:23:10,826 --> 00:23:12,896
and this is the extent
of my Photoshop ability,

559
00:23:13,316 --> 00:23:15,596
we will use the so-called Cloud.

560
00:23:15,686 --> 00:23:17,786
So this is another one of
these buzzwords these days,

561
00:23:17,836 --> 00:23:20,316
Cloud computing refers
to the ability

562
00:23:20,316 --> 00:23:23,636
to rent server space somewhere
out there on the Internet,

563
00:23:23,636 --> 00:23:27,236
you don't know or care who
or where the servers are

564
00:23:27,236 --> 00:23:28,676
or who is running these servers.

565
00:23:28,956 --> 00:23:31,176
So we're essentially going
to implement this same idea.

566
00:23:31,176 --> 00:23:33,356
CS-50 has its own
cluster of servers now

567
00:23:33,356 --> 00:23:35,876
in the computer science
building called Maxwell Dworkin

568
00:23:35,876 --> 00:23:36,096
[sounds like].

569
00:23:36,096 --> 00:23:38,426
We've got a couple of
really big Dell servers

570
00:23:38,426 --> 00:23:41,596
that have a whole lot of CPUs,
32 gigabytes of RAM inside

571
00:23:41,596 --> 00:23:43,446
of them, 6 terabytes
of storage space.

572
00:23:43,866 --> 00:23:46,346
So we can hold our
own against HCS here.

573
00:23:46,516 --> 00:23:47,466
We have a couple of those.

574
00:23:47,466 --> 00:23:49,866
We have an Apple X Serve
and long story short all

575
00:23:49,866 --> 00:23:52,336
of this stuff is wired
together in such a way

576
00:23:52,336 --> 00:23:54,666
that soon this semester you
will have your own accounts

577
00:23:54,976 --> 00:23:59,976
on cloud.cs50.net, and what this
will mean is you'll start using

578
00:23:59,976 --> 00:24:02,596
at some point this
cluster instead of nice.fas

579
00:24:03,166 --> 00:24:04,376
but it's really the same thing.

580
00:24:04,376 --> 00:24:05,586
Now what do I mean by cluster?

581
00:24:05,936 --> 00:24:08,356
In the basement of the science
center, and also in the basement

582
00:24:08,356 --> 00:24:10,396
of Maxwell Dworkin, there's
a bunch of computers.

583
00:24:10,466 --> 00:24:12,766
They're all networked
together using Ethernet cables

584
00:24:12,766 --> 00:24:13,816
into a thing called a switch.

585
00:24:14,106 --> 00:24:18,866
They are exposed publicly on the
Internet by way of IP address

586
00:24:18,866 --> 00:24:21,596
or these names,
nice.fas.harvard.edu,

587
00:24:22,056 --> 00:24:25,316
and there is this illusion
that when you connect to one

588
00:24:25,316 --> 00:24:27,466
of these servers you're
connecting to just one,

589
00:24:27,656 --> 00:24:29,886
but in fact there's
like six computers

590
00:24:29,916 --> 00:24:32,896
in this nice.fas cluster,
but they've set them up in

591
00:24:32,896 --> 00:24:34,856
such a way that you the
user don't know or care

592
00:24:34,856 --> 00:24:35,946
which particular one you're

593
00:24:35,946 --> 00:24:39,646
on because your files are
all visible on any one

594
00:24:39,646 --> 00:24:41,006
of those machines and all

595
00:24:41,006 --> 00:24:43,046
of the commands are
identical across the same.

596
00:24:43,326 --> 00:24:46,746
So when it comes time to tackle
PSET 1, what you're going to do

597
00:24:46,746 --> 00:24:48,736
if you're a Mac person
is something like this,

598
00:24:48,946 --> 00:24:50,516
I just ran the terminal program,

599
00:24:50,936 --> 00:24:52,576
I'm going to type a
command called SSH.

600
00:24:52,666 --> 00:24:54,786
This stands for Secure Shell,

601
00:24:54,786 --> 00:24:57,226
and this just means make
an encrypted connection

602
00:24:57,226 --> 00:25:00,646
between my laptop, wherever I am
in the world, and this server.

603
00:25:00,836 --> 00:25:04,546
And so what I'm going to do is
type SSH.nice let's do Malan

604
00:25:04,646 --> 00:25:07,426
at nice.fas.harvard.edu.

605
00:25:07,426 --> 00:25:10,966
I'm going to get a prompt then,
looks like I am indeed connected

606
00:25:10,966 --> 00:25:12,266
to Harvard University server.

607
00:25:12,306 --> 00:25:13,646
It's asking me for a password.

608
00:25:13,816 --> 00:25:16,436
Fortunately it won't show you on
the screen what the password is

609
00:25:16,496 --> 00:25:18,576
but now I'm at the
so-called blinking prompt

610
00:25:18,846 --> 00:25:20,056
that these days no longer blinks

611
00:25:20,276 --> 00:25:21,906
but a blinking prompt
it once was.

612
00:25:22,246 --> 00:25:24,096
So I apparently have
three unread messages,

613
00:25:24,196 --> 00:25:26,496
which has been some time
since I never check this email

614
00:25:26,536 --> 00:25:27,296
account anymore.

615
00:25:27,556 --> 00:25:29,316
So Malan at nice and
then there's this thing

616
00:25:29,316 --> 00:25:30,066
in parentheses.

617
00:25:30,206 --> 00:25:32,426
Well this is the command
line, so to speak,

618
00:25:32,426 --> 00:25:34,216
where you can type
anything you want.

619
00:25:34,536 --> 00:25:36,956
Well, that command was found.

620
00:25:37,566 --> 00:25:40,046
And this is where initially
we're going to be writing things

621
00:25:40,086 --> 00:25:42,406
like C source code
and compiling it.

622
00:25:42,806 --> 00:25:44,496
So what does that mean exactly?

623
00:25:44,496 --> 00:25:46,536
Well I'm going to do
the following just

624
00:25:46,536 --> 00:25:48,396
to give you a sense
of some basics and all

625
00:25:48,396 --> 00:25:52,666
of this realize will be repeated
in problem set 1 own PDF.

626
00:25:52,826 --> 00:25:55,266
So your hand will be held
certainly early in the semester

627
00:25:55,376 --> 00:25:56,956
so you needn't try to
memorize or scribble

628
00:25:56,956 --> 00:25:58,836
down all these commands but
just to give you a sense

629
00:25:58,836 --> 00:26:01,326
of what we're doing here,
if I type the command LS,

630
00:26:01,576 --> 00:26:04,816
this is list, this means list
the files in this directory,

631
00:26:04,816 --> 00:26:06,816
this folder, well
there's nothing there.

632
00:26:07,096 --> 00:26:09,756
So when I'm at this prompt
it's equivalent conceptually

633
00:26:09,756 --> 00:26:12,816
to being inside of a folder on
your own Mac or your own PC,

634
00:26:12,936 --> 00:26:15,266
it's just graphically
very underwhelming.

635
00:26:15,266 --> 00:26:16,866
And it's more mechanical
at the keyboard

636
00:26:16,866 --> 00:26:18,046
than it is mouse oriented.

637
00:26:18,286 --> 00:26:21,246
If I want to make a directory
called CS-50 for the duration

638
00:26:21,246 --> 00:26:24,556
of the semester, MKDIR
is make directory.

639
00:26:24,836 --> 00:26:27,006
Back in the day people didn't
like to write long commands,

640
00:26:27,106 --> 00:26:30,306
pretty reasonable, so it's
just MKDIR CS-50 hit enter,

641
00:26:30,536 --> 00:26:34,686
nothing seems to happen but if I
type LS now for list notice aha,

642
00:26:34,686 --> 00:26:36,336
there's a directory as implied

643
00:26:36,336 --> 00:26:38,476
by the slash there,
that's called CS-50.

644
00:26:38,756 --> 00:26:41,176
Well, I can't, unfortunately,
it's a little blue

645
00:26:41,176 --> 00:26:44,576
on this screen here but that
indeed says CS-50 if I type LS.

646
00:26:45,186 --> 00:26:48,596
Notice that I can't, and this
is kind of a neophyte mistake,

647
00:26:48,596 --> 00:26:51,236
can't really double click and
nothing really happens there

648
00:26:51,396 --> 00:26:56,136
so you have to change directory
CD into CS-50 and then hit enter

649
00:26:56,376 --> 00:26:58,926
and now my parentheses, this
is just a special feature

650
00:26:58,926 --> 00:27:01,266
of the way we've configured
your accounts, you'll see.

651
00:27:01,536 --> 00:27:03,786
This just reminds you where
you are, which is nice

652
00:27:03,786 --> 00:27:06,116
when you start to have
lots of folders open.

653
00:27:06,456 --> 00:27:09,996
Now if I type in LS here there's
nothing here, so if I want

654
00:27:09,996 --> 00:27:13,106
to write my first program I
need to open a text editor

655
00:27:13,106 --> 00:27:16,266
and the text editor that
we'll encourage most students

656
00:27:16,266 --> 00:27:18,336
to use is this little
program called NANO.

657
00:27:18,706 --> 00:27:22,366
If you've ever used notepad
on PCs or text edit on Macs,

658
00:27:22,766 --> 00:27:25,586
this is like the most user
friendly text editor you can

659
00:27:25,586 --> 00:27:27,996
find but as such it's
also lacking a number

660
00:27:27,996 --> 00:27:29,856
of useful features
as some of you

661
00:27:29,856 --> 00:27:31,116
with more experience will see.

662
00:27:31,376 --> 00:27:34,456
So let me go ahead and
write hello.c. Notice

663
00:27:34,526 --> 00:27:36,866
that I have just
a simple window.

664
00:27:37,056 --> 00:27:38,466
It's a little cryptic
at the bottom,

665
00:27:38,466 --> 00:27:40,166
but again this is
just stupid stuff

666
00:27:40,166 --> 00:27:41,946
that once you know it
you kind of never forget.

667
00:27:42,146 --> 00:27:44,356
The carrot symbol means
hold the control key,

668
00:27:44,626 --> 00:27:45,856
so we'll get to that
in a second.

669
00:27:45,856 --> 00:27:48,246
But let me go each and recreate
really quickly what I did

670
00:27:48,246 --> 00:27:48,816
last week.

671
00:27:49,096 --> 00:27:51,266
So I type this cryptic
sequence of characters,

672
00:27:51,266 --> 00:27:55,706
include STDIO.h in
angle brackets.

673
00:27:56,056 --> 00:28:00,936
I then set into main
ent arg C char star,

674
00:28:02,156 --> 00:28:07,196
delete key is not working,
we've got to fix that.

675
00:28:07,716 --> 00:28:09,626
Well, that's interesting.

676
00:28:11,786 --> 00:28:14,326
Wow. All right, I'm
going to do this again

677
00:28:14,326 --> 00:28:15,736
without making any mistakes.

678
00:28:16,496 --> 00:28:23,556
Standardio.h ent main ent arg
C char star, here it comes,

679
00:28:23,606 --> 00:28:28,916
arg v bracket parentheses
open curly brace 1234 Print F

680
00:28:28,916 --> 00:28:31,486
and I'm just going
to say hello, world.

681
00:28:31,486 --> 00:28:34,386
I'm going to make it look pretty
at the end with the backslash n,

682
00:28:34,536 --> 00:28:37,126
closed parentheses,
semicolon, close.

683
00:28:37,126 --> 00:28:39,996
Okay. we're not going to
indent properly here we go,

684
00:28:41,086 --> 00:28:41,526
closed brace.

685
00:28:41,656 --> 00:28:43,076
So that's my first C program,

686
00:28:43,076 --> 00:28:45,246
and this is the thing we
completely underwhelmed you

687
00:28:45,246 --> 00:28:45,956
with last week.

688
00:28:45,956 --> 00:28:47,396
But what does this mean
I'm going to do now?

689
00:28:47,396 --> 00:28:49,106
I'm going to hit control X

690
00:28:49,176 --> 00:28:51,686
because down here it
said carrot X for exit

691
00:28:52,016 --> 00:28:53,966
so now it's saying
save modified buffer.

692
00:28:54,146 --> 00:28:55,626
This is the un-user friendly way

693
00:28:55,626 --> 00:28:57,356
of saying do you want
to save your changes?

694
00:28:57,576 --> 00:28:58,636
I hit Y for yes.

695
00:28:58,746 --> 00:29:01,326
It then checks what name do
you want to give this file?

696
00:29:01,326 --> 00:29:03,086
Again it's just reminding
me what it's already called

697
00:29:03,086 --> 00:29:03,986
in case I want to change it.

698
00:29:03,986 --> 00:29:06,596
I hit enter and voila,
I'm back at the prompt.

699
00:29:06,806 --> 00:29:09,446
But if I now type LS
what should I see?

700
00:29:10,196 --> 00:29:12,656
Hopefully hello.c and
this time it's all white

701
00:29:12,726 --> 00:29:15,346
because it's not a directory so
now I need to run this thing,

702
00:29:15,566 --> 00:29:19,056
so I could try doing hello.c
or again the newbie mistake

703
00:29:19,056 --> 00:29:21,296
of really that's not
going to do anything,

704
00:29:21,616 --> 00:29:23,726
hit enter, permission denied.

705
00:29:23,936 --> 00:29:26,316
But that makes sense because
what's inside this file?

706
00:29:27,086 --> 00:29:30,396
This is not 0s and 1s clearly,
this is just source code.

707
00:29:30,396 --> 00:29:33,136
So source code is the human
readable form of this stuff

708
00:29:33,286 --> 00:29:36,816
but I remember from last week
GCC hello.c. Let me go ahead

709
00:29:36,816 --> 00:29:40,666
and hit enter now, nothing
seems to happen, but wait, LS.

710
00:29:40,956 --> 00:29:41,486
Interesting.

711
00:29:41,916 --> 00:29:45,446
Again foolishly named default
name for a program anything

712
00:29:45,446 --> 00:29:48,026
like hello.exe would seem
a little more reasonable,

713
00:29:48,256 --> 00:29:50,586
but a.out is sort of the
standard but we'll be able

714
00:29:50,586 --> 00:29:51,756
to override that soon.

715
00:29:52,056 --> 00:29:55,836
So a.out has an asterisks which
just means I am executable.

716
00:29:55,836 --> 00:29:57,436
I have 0s and 1s inside of me.

717
00:29:57,436 --> 00:30:01,076
In fact if I get a little
adventurous and now open a.out

718
00:30:01,076 --> 00:30:03,596
with nano, nano being
a text editor,

719
00:30:03,596 --> 00:30:07,106
a.out being a binary file,
binary meaning just 0s

720
00:30:07,106 --> 00:30:10,526
and 1s inside, so that's
the program I wrote.

721
00:30:10,656 --> 00:30:13,676
Now here's too where you can
do very bad things very easily.

722
00:30:13,676 --> 00:30:16,556
If you so much as hit the
space bar accidentally

723
00:30:16,556 --> 00:30:18,496
as you're flailing
about in the basement

724
00:30:18,496 --> 00:30:19,236
of the science center trying

725
00:30:19,236 --> 00:30:20,946
to fix whatever problem
you've just created,

726
00:30:21,186 --> 00:30:23,936
you just inserted
aski characters,

727
00:30:23,936 --> 00:30:26,556
random text into what's
otherwise a binary file.

728
00:30:26,806 --> 00:30:28,466
Why does this look so cryptic?

729
00:30:28,746 --> 00:30:30,406
Well, nano is again
a text editor.

730
00:30:30,406 --> 00:30:34,466
It doesn't know -- it's not
supposed to show you 0s and 1s.

731
00:30:34,696 --> 00:30:37,646
It's supposed to show you
the letters and the numbers

732
00:30:37,646 --> 00:30:40,296
and the punctuation that
those 0s and 1s represent.

733
00:30:40,566 --> 00:30:42,916
So when you see all this
crazy stuff on the screen,

734
00:30:43,136 --> 00:30:45,276
that's just nano's
misinterpretation

735
00:30:45,476 --> 00:30:48,796
of what's really 0s and 1s,
not necessarily aski letters.

736
00:30:49,006 --> 00:30:51,086
Fortunately control
X will save me here,

737
00:30:51,086 --> 00:30:53,096
save modified buffer,
no this time.

738
00:30:53,356 --> 00:30:55,236
So I hit N, I go
back to the prompt

739
00:30:55,416 --> 00:30:58,776
and now I can run a.out just
by typing this command here

740
00:30:58,776 --> 00:31:02,446
and in fact that's my
first repeated C program.

741
00:31:02,806 --> 00:31:03,356
So that's it.

742
00:31:03,356 --> 00:31:05,606
that's the basic building
block of writing a C program.

743
00:31:05,666 --> 00:31:08,706
Hopefully we can do much more
interesting things than that

744
00:31:09,146 --> 00:31:10,826
but that covers the
basic workflow.

745
00:31:10,826 --> 00:31:12,766
Now just to give you a
sense that what we're doing

746
00:31:12,766 --> 00:31:16,336
in this course is ultimately
pretty much operating system

747
00:31:16,336 --> 00:31:20,246
agnostic, here we have Mac,
here we have PC, running Mac OS,

748
00:31:20,246 --> 00:31:23,246
running Windows, on the Windows
world it's not quite as easy

749
00:31:23,246 --> 00:31:25,886
as going to a terminal window
because Windows does not ship

750
00:31:25,966 --> 00:31:28,616
with that little
program called SSH

751
00:31:28,616 --> 00:31:31,366
with which I created the secure
connection, but there's plenty

752
00:31:31,366 --> 00:31:32,776
of free tools that do that.

753
00:31:33,166 --> 00:31:33,866
One thing you'll see

754
00:31:33,866 --> 00:31:35,876
on the course's website
is program called putty.

755
00:31:35,966 --> 00:31:38,666
So if you're a PC
user you run a program

756
00:31:38,666 --> 00:31:41,306
like this it's a little more
user friendly if overwhelming

757
00:31:41,306 --> 00:31:42,296
with all of its options.

758
00:31:42,546 --> 00:31:44,846
What I'm simply going to do
here is say, you know what,

759
00:31:44,846 --> 00:31:48,196
I want to connect to
nice.Fas.harvard.edu.

760
00:31:48,466 --> 00:31:50,666
so if I were doing this for the
first time I would simply do

761
00:31:50,666 --> 00:31:54,066
something like that, typing in
its host name or its IP address.

762
00:31:54,066 --> 00:31:55,696
Now I'm going to go
ahead and click --

763
00:31:55,866 --> 00:31:57,666
actually I'm going to
load something there

764
00:31:57,716 --> 00:31:58,806
so I get the bigger font size.

765
00:31:58,806 --> 00:32:02,296
I'm going to click open and now
I get this black window again.

766
00:32:02,296 --> 00:32:03,566
I'm going to log in as Malan.

767
00:32:03,696 --> 00:32:07,526
I get a similar prompt, enter,
notice I'm back in here.

768
00:32:07,526 --> 00:32:10,606
if I type LS there's
CS-50, CD CS-50, aha,

769
00:32:10,606 --> 00:32:11,996
there's my a.out program.

770
00:32:12,536 --> 00:32:13,386
Same exact thing.

771
00:32:13,586 --> 00:32:14,906
So this is actually powerful.

772
00:32:14,906 --> 00:32:17,096
Even though I'm on a PC
here, a Mac over there,

773
00:32:17,216 --> 00:32:20,466
I've accessed the same account
so that's one of the upsides

774
00:32:20,466 --> 00:32:21,746
of having this central server.

775
00:32:21,746 --> 00:32:23,856
You'll be able to access
your code anywhere.

776
00:32:23,856 --> 00:32:26,116
If you go home on break, if you
get sick, if you're just hanging

777
00:32:26,116 --> 00:32:28,566
out a friend's you can access
your code in this course

778
00:32:28,896 --> 00:32:31,106
from any computer on the
Internet and you'll find

779
00:32:31,266 --> 00:32:33,676
that for better or for
worse to be at least useful.

780
00:32:33,856 --> 00:32:35,876
So just to give you a sense
of what else you can do

781
00:32:36,126 --> 00:32:40,136
on a typical Linux server, so
the machine I connected to,

782
00:32:40,166 --> 00:32:42,016
nice.fas, it's running at Linux,

783
00:32:42,336 --> 00:32:45,306
which is again an operating
system that's larger text-based

784
00:32:45,716 --> 00:32:49,246
but there are goes,
graphical user interfaces,

785
00:32:49,426 --> 00:32:51,006
and in fact if you
headed to the basement

786
00:32:51,006 --> 00:32:53,206
of the science center someday
as you might for office hours,

787
00:32:53,466 --> 00:32:56,196
they've got like 33
percent Linux computers

788
00:32:56,196 --> 00:32:56,976
in the basement there.

789
00:32:57,256 --> 00:32:59,326
they do have Windows,
they do have menus,

790
00:32:59,376 --> 00:33:01,306
things you can click,
because the world has come

791
00:33:01,306 --> 00:33:03,596
up with more user-friendly
interfaces to Linux.

792
00:33:03,826 --> 00:33:05,786
But you'll get your hands
directly this semester with some

793
00:33:05,786 --> 00:33:08,636
of the building blocks because
frankly when you get comfortable

794
00:33:08,636 --> 00:33:12,026
with this stuff you can do so
much more, so much more easily

795
00:33:12,026 --> 00:33:13,756
and so much more
efficiently just

796
00:33:13,756 --> 00:33:16,216
by telling the computer
precisely what you want.

797
00:33:16,216 --> 00:33:19,226
So CD we saw, CP, any guess?

798
00:33:19,836 --> 00:33:23,416
CP, copy, kind of the
shorthand version of copy,

799
00:33:23,416 --> 00:33:26,276
and in fact it is, CP we'll
use to copy code this semester.

800
00:33:26,276 --> 00:33:32,186
LS is list, MK-Dir is
make directory, MV, move,

801
00:33:32,186 --> 00:33:35,826
so that's synonymous,
we'll see renaming a file,

802
00:33:35,826 --> 00:33:39,846
so to rename a file you'll say
MV space the original name space

803
00:33:39,936 --> 00:33:43,206
the new name, not terribly hard.

804
00:33:43,896 --> 00:33:45,856
PWD. Yeah, so print
working directory.

805
00:33:45,856 --> 00:33:48,726
What this means here is I'm
back on the nice.fas server.

806
00:33:48,906 --> 00:33:50,946
If I kind of get a
little confused sometime

807
00:33:50,946 --> 00:33:55,596
and I type PWD it will tell
me the full path to where I am

808
00:33:55,596 --> 00:33:58,446
on the server, so right
now I'm logged in as Malan

809
00:33:58,486 --> 00:34:01,096
and Malan does not have access
to everything on the server

810
00:34:01,096 --> 00:34:04,156
because there's like 20,000
other people with fas accounts.

811
00:34:04,486 --> 00:34:07,096
So in fact, fas took
the approach years ago,

812
00:34:07,096 --> 00:34:08,066
very intelligently,

813
00:34:08,256 --> 00:34:10,196
of organizing things
hierarchically.

814
00:34:10,456 --> 00:34:11,936
So notice there's a directory,

815
00:34:11,976 --> 00:34:14,716
the root of the hard drive
apparently has a directory

816
00:34:14,716 --> 00:34:17,616
in it called NFS,
network file system, home,

817
00:34:17,616 --> 00:34:19,826
which is where everyone's
home directories live.

818
00:34:19,826 --> 00:34:22,066
And a home directory is just
your personal storage space.

819
00:34:22,586 --> 00:34:25,756
M/A just looks like there's a
whole hierarchy of names there

820
00:34:25,756 --> 00:34:28,536
to keep things clean, slash
CS-50, this is the full path

821
00:34:28,866 --> 00:34:32,516
to where I am, and realize
that I can definitely go poking

822
00:34:32,516 --> 00:34:33,616
around if I get curious.

823
00:34:33,616 --> 00:34:34,716
This is not a bad thing.

824
00:34:34,716 --> 00:34:39,216
If I type CD DOT DOT it doesn't
go this way it goes backwards

825
00:34:39,216 --> 00:34:41,846
so to speak or higher up in this
tree, so now notice I'm back

826
00:34:41,956 --> 00:34:43,326
in my so-called home directory.

827
00:34:43,676 --> 00:34:46,126
Where am I going to be
if I do it once more?

828
00:34:47,496 --> 00:34:49,316
Yeah, the directory
called A. So with all

829
00:34:49,316 --> 00:34:52,976
of the people whose
usernames start with MA,

830
00:34:52,976 --> 00:34:56,616
which if I do CD it's
getting a little curious here.

831
00:34:56,866 --> 00:34:59,186
Maybe will this backfire?

832
00:34:59,456 --> 00:35:01,546
Maybe if we could put the camera
down here for just a moment

833
00:35:01,546 --> 00:35:03,306
so that we're not podcasting
this to the Internet,

834
00:35:03,556 --> 00:35:07,056
these are all of the people on
fas.harvard.edu whose names,

835
00:35:07,146 --> 00:35:10,206
sorry folks at home can't
see this, that begin with MA.

836
00:35:10,406 --> 00:35:14,066
If we go back one more directory
keep the camera underwhelming

837
00:35:14,216 --> 00:35:15,796
down here, there's
the whole alphabet.

838
00:35:16,036 --> 00:35:17,376
We can see people that start

839
00:35:17,376 --> 00:35:22,326
with probably no MZs I'm
guessing, CDZ, LS, oh there are.

840
00:35:22,326 --> 00:35:25,026
So it's not a full
name, it's first letter

841
00:35:25,136 --> 00:35:26,826
of the first name, last name.

842
00:35:26,826 --> 00:35:29,646
So again you can poke around all
you want and see a whole bunch

843
00:35:29,646 --> 00:35:31,366
of stuff on this
computer, but Linux,

844
00:35:31,366 --> 00:35:33,626
like most operating systems,
has various permissions

845
00:35:33,626 --> 00:35:37,086
and fancy control features so
you really can't do bad things.

846
00:35:37,086 --> 00:35:39,726
you can't access ,for
instance, if I get nosy,

847
00:35:39,726 --> 00:35:44,496
let's say home/a/y idida

848
00:35:44,496 --> 00:35:46,006
for Jansue's [sounds
like] home directory.

849
00:35:46,816 --> 00:35:55,346
Let me, I'm forgetting
the day, so I'm in it.

850
00:35:55,796 --> 00:35:58,086
Thankfully, permission denied.

851
00:35:58,086 --> 00:36:00,076
So things are, in fact,
configured correctly.

852
00:36:00,306 --> 00:36:01,826
So again just random
little trivia

853
00:36:01,826 --> 00:36:03,246
that ultimately gets useful

854
00:36:03,246 --> 00:36:05,276
because just understanding
the world

855
00:36:05,276 --> 00:36:07,366
in which you're playing I
think lets you get yourself

856
00:36:07,366 --> 00:36:09,786
out of problems and actually
solve more interesting ones.

857
00:36:10,026 --> 00:36:11,746
So there are some other
commands that we'll see,

858
00:36:11,746 --> 00:36:14,176
and again every piece
that's pdf will walk you

859
00:36:14,236 --> 00:36:15,366
through any of this minutiae.

860
00:36:15,576 --> 00:36:17,066
You're not going to have
to figure it out or figure

861
00:36:17,066 --> 00:36:20,446
out from week 1 what the heck
was the command for this.

862
00:36:20,446 --> 00:36:23,846
So we're going to see
things like GCC, GDB,

863
00:36:24,046 --> 00:36:26,996
commands like more and
then someone got clever.

864
00:36:26,996 --> 00:36:28,076
More is just a program

865
00:36:28,076 --> 00:36:29,686
that shows you the
contents of a file.

866
00:36:29,906 --> 00:36:31,606
Well, someone wanted to
be clever and kind of one

867
00:36:31,606 --> 00:36:33,306
up that person so they'd
called their program less

868
00:36:33,626 --> 00:36:35,366
but it does really
the same thing.

869
00:36:35,686 --> 00:36:39,056
Man for manual, which is
something you'll get familiar

870
00:36:39,056 --> 00:36:39,936
with as well.

871
00:36:40,176 --> 00:36:42,576
So what does it take to write
a program, so it's three steps

872
00:36:42,576 --> 00:36:43,346
and we've done it twice.

873
00:36:43,346 --> 00:36:46,546
Last week and this, one, open
up a text editor like nano

874
00:36:46,676 --> 00:36:49,046
in a Linux environment and
call the file whatever,

875
00:36:49,146 --> 00:36:53,866
hi1.c or hello1.c, using the
dot C convention for namesake.

876
00:36:54,236 --> 00:36:56,806
GCC the name of the file
and then you run the thing

877
00:36:56,886 --> 00:36:59,516
by simply running
the default name.

878
00:36:59,906 --> 00:37:02,046
Well, what if we want to do
something more interesting,

879
00:37:02,046 --> 00:37:04,866
give it a non-stupid
name like a.out?

880
00:37:05,226 --> 00:37:07,636
So there are again these things
called command line arguments

881
00:37:07,676 --> 00:37:10,736
or switches with which you
can control the behavior

882
00:37:10,736 --> 00:37:12,586
of code more interestingly.

883
00:37:12,586 --> 00:37:15,736
So if I go back over here
into my CS-50 account,

884
00:37:16,166 --> 00:37:19,526
my CS-50 directory, here's
hello.c. my goal now is

885
00:37:19,526 --> 00:37:22,416
to create a more
interesting name than a.out,

886
00:37:22,776 --> 00:37:25,856
so per that previous slide
I say dash 0 for output.

887
00:37:26,156 --> 00:37:27,816
Let's call the program hello

888
00:37:28,026 --> 00:37:31,286
and then let's say compile
this particular program, LS.

889
00:37:31,286 --> 00:37:35,346
Now I have a binary, 0s
and 1s called hello up.

890
00:37:35,726 --> 00:37:39,796
So you'll notice little
keystrokes that save you time.

891
00:37:39,796 --> 00:37:42,526
If you hit tab it will do auto
complete which saves you time.

892
00:37:42,776 --> 00:37:44,766
Unfortunately it also
reveals all the other commands

893
00:37:44,766 --> 00:37:47,926
on the server which start with
he, in this case head, hello,

894
00:37:47,926 --> 00:37:50,316
help desk, help desk server,
help z tags and [inaudible],

895
00:37:50,546 --> 00:37:54,636
none of which I care
about so hello, enter,

896
00:37:54,766 --> 00:37:56,996
same exact program even
though it's called something

897
00:37:56,996 --> 00:37:57,606
different still.

898
00:37:57,606 --> 00:38:00,366
So more interesting but
again just a fundamentally

899
00:38:00,366 --> 00:38:01,586
uninteresting building block.

900
00:38:01,736 --> 00:38:04,056
But your programs are going to
get more complicated over time.

901
00:38:04,296 --> 00:38:06,846
Most of your programs by
semester's end are not going

902
00:38:06,846 --> 00:38:09,456
to be this big in
a simple text file,

903
00:38:09,736 --> 00:38:11,196
they're going to
be multiple files.

904
00:38:11,196 --> 00:38:12,896
And they're not going to
be terribly long each,

905
00:38:12,896 --> 00:38:14,936
but we'll see one of the
principals we'll preach

906
00:38:14,936 --> 00:38:17,316
in the course is
design, and good design,

907
00:38:17,546 --> 00:38:20,116
and as some of you may have
experienced in scratch already,

908
00:38:20,746 --> 00:38:23,266
if you're implementing a
script that's kind of this big

909
00:38:23,266 --> 00:38:25,106
and then you add some more
features and then it's this big

910
00:38:25,106 --> 00:38:27,566
and then this big and this
big and God forbid this wide

911
00:38:27,566 --> 00:38:30,646
and wide and wide because you
keep adding ifs and its and ifs

912
00:38:30,646 --> 00:38:33,706
and its you're probably crossing
the boundary at some point

913
00:38:33,706 --> 00:38:35,596
of what we'd call good design.

914
00:38:35,596 --> 00:38:37,486
If you're scrolling up,
down, left right just

915
00:38:37,486 --> 00:38:40,416
to see your own program there's
probably a better approach.

916
00:38:40,416 --> 00:38:43,256
Now for PSET 0, they don't need
to worry so much about this

917
00:38:43,256 --> 00:38:45,336
because we haven't really
told you what good design is,

918
00:38:45,636 --> 00:38:48,596
but among the features will
be chopping up your code

919
00:38:48,596 --> 00:38:50,936
into much more manageable
bite-sized pieces,

920
00:38:50,986 --> 00:38:53,146
putting it in separate
files, putting it in scratch

921
00:38:53,146 --> 00:38:55,436
in separate scripts, and
you'll see that a command

922
00:38:55,436 --> 00:38:57,626
that will facilitate
this is called make.

923
00:38:57,886 --> 00:39:00,316
So you'll see both of
these approaches in PSET 1,

924
00:39:00,566 --> 00:39:03,766
simply typing Make and then the
name of the program you want

925
00:39:03,826 --> 00:39:05,876
to make, in this case, hello.

926
00:39:05,876 --> 00:39:08,966
Notice I'm not typing DOT
C, I'm saying make hello.

927
00:39:09,236 --> 00:39:11,636
Make is a program that's
going to infer, well,

928
00:39:11,636 --> 00:39:13,806
if this guy wants to make
a program called make it's

929
00:39:13,856 --> 00:39:16,216
probably based on a
file called make.c

930
00:39:16,476 --> 00:39:20,566
so if I hit enter it tried to
recompile it but it realized --

931
00:39:20,566 --> 00:39:22,646
and this is what's cool about
make -- it realized, oh,

932
00:39:22,686 --> 00:39:23,776
you haven't changed the program,

933
00:39:23,776 --> 00:39:27,316
why should I waste my time
regenerated identical 0s and 1s.

934
00:39:27,626 --> 00:39:29,046
So in fact it didn't bother.

935
00:39:29,226 --> 00:39:33,016
But if I do LS to see my files
how do I remove hello and a.out?

936
00:39:34,146 --> 00:39:37,916
Yeah, rm, hello, enter, it's
going to ask for a sanity check.

937
00:39:37,916 --> 00:39:39,786
Remove regular file
hello question mark?

938
00:39:39,786 --> 00:39:42,826
Yes. RM a.out, yes for that too.

939
00:39:42,826 --> 00:39:46,426
So now I just have my source
code, make hello enter,

940
00:39:46,646 --> 00:39:49,496
so the command's going to look a
little more cryptic because we,

941
00:39:49,496 --> 00:39:51,206
just to get you started
at term start,

942
00:39:51,296 --> 00:39:53,086
are configuring these
accounts in a way

943
00:39:53,086 --> 00:39:54,666
that just makes some
things easier

944
00:39:54,666 --> 00:39:56,666
and we'll take those
training wheels off over time,

945
00:39:56,936 --> 00:39:58,806
but it achieves the same
results because I now

946
00:39:58,806 --> 00:40:02,296
in fact have a program called
hello in that directory as well.

947
00:40:02,616 --> 00:40:04,606
So what are these
programs you can use?

948
00:40:04,606 --> 00:40:07,686
So nano is by far the
simplest editor you can use

949
00:40:07,686 --> 00:40:08,786
at least at term start.

950
00:40:09,026 --> 00:40:11,596
On my computer it doesn't
support the delete feature

951
00:40:11,596 --> 00:40:14,016
but we will fix that
so that you don't need

952
00:40:14,016 --> 00:40:17,486
to fail the very first time
you make a program like I did,

953
00:40:17,746 --> 00:40:19,116
but there are other
tools out there.

954
00:40:19,116 --> 00:40:21,646
Those of you might know programs
like VIM and in fact most

955
00:40:21,646 --> 00:40:24,446
of the time I'll use a
program called VIM just

956
00:40:24,446 --> 00:40:26,516
because that's what I was
taught in CS-50 years ago.

957
00:40:26,516 --> 00:40:27,556
I kind of grew up on it.

958
00:40:27,766 --> 00:40:30,816
EMAX is another popular one,
but more on this in the future.

959
00:40:30,816 --> 00:40:33,446
I mention it now only so
that you don't get confused

960
00:40:33,446 --> 00:40:35,776
or puzzled when you see
this TF used this program,

961
00:40:35,936 --> 00:40:36,736
this TF used another.

962
00:40:37,016 --> 00:40:40,126
It's just like using pages
versus Microsoft Word

963
00:40:40,126 --> 00:40:42,936
versus Word Pad, it's all
the same kind of stuff

964
00:40:42,936 --> 00:40:43,946
in this world as well.

965
00:40:44,256 --> 00:40:45,366
So in a moment we're going

966
00:40:45,436 --> 00:40:47,236
to actually tease
apart what it means

967
00:40:47,236 --> 00:40:49,626
to write a more interesting
program than hello world.

968
00:40:49,626 --> 00:40:52,756
The basic building block for
almost any C program is going

969
00:40:52,756 --> 00:40:55,766
to start with this construct
here, but why don't we go ahead

970
00:40:55,766 --> 00:40:56,896
since this is a lot to digest,

971
00:40:57,096 --> 00:40:58,326
and take a five minute
break first.

972
00:41:05,486 --> 00:41:06,456
All right.

973
00:41:06,876 --> 00:41:08,826
We are back, so Yuki
[assumed spelling]

974
00:41:08,826 --> 00:41:09,966
and Jantsu [assumed
spelling] informed me

975
00:41:09,966 --> 00:41:12,186
that sectioning will
indeed debut this evening,

976
00:41:12,186 --> 00:41:13,846
go to CS-50.net, take a look

977
00:41:13,846 --> 00:41:15,966
at any instructions you'll
find on the home page.

978
00:41:15,966 --> 00:41:18,076
Check around midnight
or certainly by morning,

979
00:41:18,416 --> 00:41:21,026
and note this, so per
last week's announcements

980
00:41:21,026 --> 00:41:23,826
in the syllabus we do offer
three tracks of sections,

981
00:41:23,826 --> 00:41:27,146
just like First Nights does
for our students who are more

982
00:41:27,146 --> 00:41:29,566
and less comfortable with
music, so we do the same.

983
00:41:29,566 --> 00:41:32,136
It's just the idea
of being in CS-50,

984
00:41:32,386 --> 00:41:34,106
so there's no steadfast rule

985
00:41:34,106 --> 00:41:36,056
as to what makes you
less comfortable,

986
00:41:36,056 --> 00:41:37,316
what makes you more comfortable

987
00:41:37,316 --> 00:41:38,746
and what makes you
somewhere in between.

988
00:41:39,036 --> 00:41:41,036
To be honest, it's a term
you can probably just slap

989
00:41:41,036 --> 00:41:43,606
on yourself based on just how
comfortable you actually are

990
00:41:43,826 --> 00:41:46,396
having returned this
week to CS-50.

991
00:41:46,396 --> 00:41:48,616
You've seen what the percentage
break-downs are so we get plenty

992
00:41:48,616 --> 00:41:49,776
of folks from each
of the buckets,

993
00:41:50,036 --> 00:41:51,946
so when you use fas's
sectioning tool,

994
00:41:52,226 --> 00:41:54,346
realize that because we have
these three different types

995
00:41:54,346 --> 00:41:56,206
of sections you're
going to see a long list

996
00:41:56,206 --> 00:41:57,616
of about 30 sections,
since we have

997
00:41:57,616 --> 00:41:59,756
about 30 teaching fellows
and thus 30 sections.

998
00:41:59,976 --> 00:42:01,336
and in the comments fields

999
00:42:01,336 --> 00:42:03,116
of the sectioning
program you'll see

1000
00:42:03,116 --> 00:42:05,086
if that section slot
has been designated

1001
00:42:05,086 --> 00:42:06,946
for those more comfortable,
those less

1002
00:42:07,246 --> 00:42:08,586
or those somewhere in between.

1003
00:42:08,846 --> 00:42:09,776
If you're on the fence as to

1004
00:42:09,776 --> 00:42:12,476
where you belong you
cannot go wrong being

1005
00:42:12,476 --> 00:42:14,016
in the somewhere
in between section.

1006
00:42:14,016 --> 00:42:16,146
That's really for folks that
just don't feel themselves

1007
00:42:16,146 --> 00:42:19,256
at either extreme, but certainly
feel free to chat with me or any

1008
00:42:19,256 --> 00:42:20,576
of the TFs if you're curious

1009
00:42:20,576 --> 00:42:23,336
as to what this all means
or where you belong.

1010
00:42:23,336 --> 00:42:25,206
Now what does this mean in
real terms for sections?

1011
00:42:25,896 --> 00:42:27,486
Frankly it just means
that especially

1012
00:42:27,486 --> 00:42:29,906
for those less comfortable, it
just means you're going to be

1013
00:42:29,906 --> 00:42:33,406
in a room full of people
who feel as apprehensive

1014
00:42:33,406 --> 00:42:36,656
or as inexperienced as you
and it's sort of a safer space

1015
00:42:36,656 --> 00:42:37,826
in which to put up your hand

1016
00:42:37,826 --> 00:42:40,416
and ask a question you think is
the proverbial dumb question.

1017
00:42:40,716 --> 00:42:43,106
The sections for those
more comfortable are meant

1018
00:42:43,106 --> 00:42:45,956
to digress intentionally from
say the course's syllabus,

1019
00:42:46,266 --> 00:42:48,196
from the week's problem
set and really go off

1020
00:42:48,196 --> 00:42:50,336
in any interesting
technical direction that some

1021
00:42:50,336 --> 00:42:51,716
of the savvier students might

1022
00:42:51,716 --> 00:42:54,186
like to explore even though it's
not going to help them per se

1023
00:42:54,356 --> 00:42:55,896
on that particular
week's problem set.

1024
00:42:56,036 --> 00:42:58,296
But again if you don't feel
yourself needing that sort

1025
00:42:58,296 --> 00:43:00,866
of comfort zone here or
you don't really think

1026
00:43:00,916 --> 00:43:02,786
that you'd want to try
holding your own with some

1027
00:43:02,786 --> 00:43:04,836
of those more comfortable
then the sweet spot,

1028
00:43:04,836 --> 00:43:06,776
just like the standard
editions for most students is

1029
00:43:06,776 --> 00:43:08,886
that for those somewhere
in between.

1030
00:43:09,276 --> 00:43:12,246
All right, so let's do something
a little more interesting now.

1031
00:43:12,246 --> 00:43:15,206
I've gone ahead and per the
handout you have I have a whole

1032
00:43:15,206 --> 00:43:17,456
bunch of code already
in this account.

1033
00:43:17,456 --> 00:43:19,246
So I'm going to go
back to my screen here

1034
00:43:19,476 --> 00:43:21,486
and you'll see again
commands lli this on PSETs.

1035
00:43:21,486 --> 00:43:26,066
What I'm going to do for a
moment is copy, tilda, CS-50,

1036
00:43:26,276 --> 00:43:27,956
so henceforth the
Tilda character

1037
00:43:27,956 --> 00:43:29,306
which on most U.S. keyboards is

1038
00:43:29,306 --> 00:43:33,336
on the top left near the escape
key, Tilda CS-50 means go

1039
00:43:33,336 --> 00:43:35,946
into CS-50's home directory.

1040
00:43:36,116 --> 00:43:37,996
Now I'm in Malan's home
directory at the moment,

1041
00:43:37,996 --> 00:43:41,916
so go into CS-50 is just like I
tried to idida's home directory,

1042
00:43:41,916 --> 00:43:43,976
so go into CS-50's
and then I know

1043
00:43:43,976 --> 00:43:46,226
because I'm reading this
off of a pdf or whatever

1044
00:43:46,426 --> 00:43:49,526
that there's a pub directory,
that there's a source, src,

1045
00:43:49,526 --> 00:43:51,996
again, nicknames in
Linux, source directory,

1046
00:43:51,996 --> 00:43:54,536
and then I know there's a
lectures directory in there

1047
00:43:54,536 --> 00:43:55,706
and then we're in week 1.

1048
00:43:55,706 --> 00:43:56,966
So it turns out there
is, in fact,

1049
00:43:56,966 --> 00:44:00,626
a directory called 1 inside
of lectures, inside of source,

1050
00:44:00,626 --> 00:44:04,086
inside of pub, inside of CS-50's
home directory and my goal is

1051
00:44:04,086 --> 00:44:08,346
to copy it from there to
here because I, now a student

1052
00:44:08,346 --> 00:44:11,146
in the class, decide that I want
to push ahead, I want to do this

1053
00:44:11,146 --> 00:44:12,406
after class and play around.

1054
00:44:12,406 --> 00:44:14,106
I want to copy David's
source code

1055
00:44:14,106 --> 00:44:16,176
into my own account
just so I can play.

1056
00:44:16,176 --> 00:44:20,516
Perfectly fine, notice that DOT
represents this directory just

1057
00:44:20,516 --> 00:44:22,946
as DOT DOT represents
the parent directory.

1058
00:44:23,156 --> 00:44:24,966
So if hit enter this
is actually flawed,

1059
00:44:25,246 --> 00:44:27,536
notice that something
bad happened and then

1060
00:44:27,536 --> 00:44:29,726
if I type LS I didn't
get any files.

1061
00:44:29,966 --> 00:44:33,286
So realize, too, sort of
FAQ early on is if you want

1062
00:44:33,286 --> 00:44:37,396
to copy a directory with CP
you need to do it recursively,

1063
00:44:37,436 --> 00:44:39,656
which is a keyword that
we'll revisit in the term,

1064
00:44:39,656 --> 00:44:41,596
it means to do it not
only to that directory

1065
00:44:41,846 --> 00:44:45,166
but all the files inside and all
the subdirectories inside again

1066
00:44:45,166 --> 00:44:46,856
and again and again
until you bottom out.

1067
00:44:46,856 --> 00:44:49,846
So CP-R means copy
the file recursively

1068
00:44:50,146 --> 00:44:52,836
into my current directory,
enter, took a second

1069
00:44:52,836 --> 00:44:54,256
because there are, in
fact, a bunch of files.

1070
00:44:54,546 --> 00:44:57,706
In fact if I go into my one
directory now here are some

1071
00:44:57,706 --> 00:45:00,056
examples that I've pre-prepared
so that we can go off

1072
00:45:00,056 --> 00:45:01,686
in a known direction here.

1073
00:45:01,926 --> 00:45:06,976
So one direction we already went
in was hi-1.c. but your version,

1074
00:45:06,976 --> 00:45:08,826
on paper looks a little
different from the quick

1075
00:45:08,826 --> 00:45:10,566
and dirty thing I've
been whipping up.

1076
00:45:10,676 --> 00:45:14,086
there's a lot more purple on the
screen here, so all the stuff

1077
00:45:14,086 --> 00:45:17,796
at top is fundamentally
irrelevant to the computer,

1078
00:45:17,916 --> 00:45:20,606
but just like scratch does
if you've discovered it yet,

1079
00:45:20,846 --> 00:45:22,466
lets you put little
comments here and there,

1080
00:45:22,466 --> 00:45:24,376
sort of sticky notes, so
to speak, post-it notes,

1081
00:45:24,646 --> 00:45:26,936
so does C in most any
high-level language.

1082
00:45:27,156 --> 00:45:29,086
and by high-level language
henceforth I mean something

1083
00:45:29,086 --> 00:45:30,626
that's not 0s and 1s,

1084
00:45:30,626 --> 00:45:32,066
not something called
assembly language.

1085
00:45:32,386 --> 00:45:33,846
It means ignore all

1086
00:45:33,846 --> 00:45:36,246
of the following this is
useful only to me the human.

1087
00:45:36,606 --> 00:45:39,466
So in C the means by which
you write a comment is

1088
00:45:39,496 --> 00:45:43,926
to do a forward slash star and
that means anything after that

1089
00:45:43,926 --> 00:45:47,556
on this line or below will be
ignored by the compiler, GCC,

1090
00:45:47,726 --> 00:45:50,976
and the comment stops
when you hit the opposite

1091
00:45:50,976 --> 00:45:52,286
of that, star slash.

1092
00:45:52,916 --> 00:45:56,196
So the fact that I actually have
all of these stars and I've kind

1093
00:45:56,196 --> 00:45:59,206
of boxed things in is really
just some OCD on my part.

1094
00:45:59,206 --> 00:46:02,036
It's not technical I just wanted
to make a nice pretty purple box

1095
00:46:02,246 --> 00:46:03,466
to box in all of my code.

1096
00:46:03,676 --> 00:46:07,416
But one key takeaway is that
I made sure that this line

1097
00:46:07,566 --> 00:46:10,666
of stars and the slashes
does not exceed the width

1098
00:46:10,666 --> 00:46:11,346
of my window.

1099
00:46:11,566 --> 00:46:13,866
So another lesson that's
important early on is

1100
00:46:13,866 --> 00:46:15,876
to make sure your -- the
code you start writing is

1101
00:46:16,186 --> 00:46:17,296
pretty printed.

1102
00:46:17,516 --> 00:46:19,746
And this just means it
actually displays nicely

1103
00:46:19,746 --> 00:46:20,906
on a typical window.

1104
00:46:21,126 --> 00:46:24,136
In real terms this means
you should never at least

1105
00:46:24,186 --> 00:46:27,546
for the C part of this
course, have a line of text

1106
00:46:27,606 --> 00:46:29,556
that is longer than
80 characters long.

1107
00:46:29,776 --> 00:46:32,676
And even that's pushing it,
76 is kind of the geek's rule

1108
00:46:32,676 --> 00:46:34,346
of thumb as to how
long a line should be.

1109
00:46:34,646 --> 00:46:37,316
How do you know what's 76
or what's 80 characters?

1110
00:46:37,486 --> 00:46:41,446
Well, right now my window has
essentially been expanded to be

1111
00:46:41,636 --> 00:46:45,506
as long as I wanted it to be,
but you get into the habit

1112
00:46:45,666 --> 00:46:48,276
of knowing when to hit enter to
move things onto the next line

1113
00:46:48,476 --> 00:46:52,356
and nano and programs like it
will actually force your lines

1114
00:46:52,356 --> 00:46:54,276
to wrap if they start
to get long.

1115
00:46:54,596 --> 00:46:56,036
And this is just a
matter of good style,

1116
00:46:56,066 --> 00:46:58,956
because as you could imagine,
if I wrote my whole program

1117
00:46:58,956 --> 00:47:00,946
on just one line it
would still work,

1118
00:47:00,946 --> 00:47:04,546
the computer would not care, but
you the humans probably would.

1119
00:47:04,546 --> 00:47:07,426
In fact, this is
hi1.c, let me go ahead

1120
00:47:07,426 --> 00:47:09,036
and delete all that stuff.

1121
00:47:09,086 --> 00:47:11,016
I'm going to go ahead and
do this, you know, this,

1122
00:47:11,136 --> 00:47:13,096
I really don't need that there.

1123
00:47:13,096 --> 00:47:15,016
Let me go ahead and
put that there.

1124
00:47:15,016 --> 00:47:16,946
Right, if I'd given you a
printout of this, I mean,

1125
00:47:16,946 --> 00:47:20,666
rightly so would things start
to look pretty damn complicated.

1126
00:47:20,666 --> 00:47:22,216
Now some of these
spaces are important

1127
00:47:22,216 --> 00:47:23,646
so that you can distinguish
what's ints

1128
00:47:23,646 --> 00:47:26,206
and what's [inaudible], but
if I go ahead and save this

1129
00:47:26,206 --> 00:47:31,796
with control X, hit Y and now
go ahead and run GCC of hi1.c,

1130
00:47:32,016 --> 00:47:33,796
the program thankfully
still compiles.

1131
00:47:33,796 --> 00:47:36,636
I still have eight odd out, so
it still works, this version,

1132
00:47:36,906 --> 00:47:39,256
but it's certainly not
very pretty printed

1133
00:47:39,256 --> 00:47:40,396
and not very legible.

1134
00:47:40,396 --> 00:47:42,016
So what else is inside
of this file?

1135
00:47:42,256 --> 00:47:44,676
Well, hmm, I didn't
really want to do that.

1136
00:47:44,676 --> 00:47:48,886
Let me go ahead and go back into
CS-50, Pub source lectures 1,

1137
00:47:48,886 --> 00:47:50,196
let me just get a
new copy of this.

1138
00:47:50,546 --> 00:47:53,856
So now it's going to prompt me,
wait a minute, overwrite hi1.c,

1139
00:47:54,016 --> 00:47:55,366
little sanity check, say yes.

1140
00:47:55,466 --> 00:47:56,856
So now I have the
prettier version back.

1141
00:47:56,856 --> 00:47:59,776
I'm going to start using a
program called VI or VIM just

1142
00:47:59,776 --> 00:48:01,936
because frankly I'm more
comfortable with it than nano,

1143
00:48:01,936 --> 00:48:05,266
even though nano is simpler,
do as I say not as I do.

1144
00:48:05,666 --> 00:48:09,526
Hi1.c, still kind of
pretty prints everything

1145
00:48:09,526 --> 00:48:10,356
and color codes it.

1146
00:48:10,576 --> 00:48:13,136
And the color is actually a
feature that you don't get

1147
00:48:13,136 --> 00:48:16,596
if you trying writing your
first programs with notebad.exe

1148
00:48:16,846 --> 00:48:19,236
or other similar
client side programs.

1149
00:48:19,476 --> 00:48:22,266
It's just making some inferences
about the different roles

1150
00:48:22,266 --> 00:48:23,646
that some of these
keywords play,

1151
00:48:23,646 --> 00:48:25,936
so I didn't write the
word int in green.

1152
00:48:26,196 --> 00:48:29,096
I didn't write the word
char in green, it just knows

1153
00:48:29,136 --> 00:48:31,076
that that represents
the type of a variable.

1154
00:48:31,076 --> 00:48:33,016
Let me draw David's
attention to it

1155
00:48:33,056 --> 00:48:34,656
with this default color scheme.

1156
00:48:34,986 --> 00:48:36,316
So what's interesting here?

1157
00:48:36,396 --> 00:48:38,916
Well, this stuff at the
top is all comments,

1158
00:48:38,916 --> 00:48:40,946
and actually now we get
to use a fun little toy.

1159
00:48:40,946 --> 00:48:43,106
this stuff up here
is all comments.

1160
00:48:43,236 --> 00:48:43,696
All right.

1161
00:48:43,696 --> 00:48:45,846
So it's fundamentally
irrelevant but useful

1162
00:48:45,846 --> 00:48:47,196
for documenting your code.

1163
00:48:47,496 --> 00:48:48,896
This things here I'm going

1164
00:48:48,896 --> 00:48:52,016
to briefly call today a
pre-processor directive,

1165
00:48:52,366 --> 00:48:55,416
which is just a fancy way
of saying tell the compiler

1166
00:48:55,616 --> 00:48:57,586
to go get the contents
of some other file

1167
00:48:57,706 --> 00:48:59,246
that often someone else wrote.

1168
00:48:59,576 --> 00:49:02,966
So what we don't
have here is any kind

1169
00:49:02,966 --> 00:49:05,186
of specification of
what Print F does.

1170
00:49:05,246 --> 00:49:06,646
Like who wrote Print F?

1171
00:49:06,746 --> 00:49:10,096
I wrote this program called
main, but who wrote Print F?

1172
00:49:10,096 --> 00:49:11,716
Right, I'm kind of
cutting corners here.

1173
00:49:11,986 --> 00:49:15,566
Well, it turns out that when
Linux shipped with the compiler,

1174
00:49:15,806 --> 00:49:18,086
someone else, some really
smart people spent a lot

1175
00:49:18,086 --> 00:49:21,706
of time getting this very basic
important function implemented

1176
00:49:21,706 --> 00:49:23,206
in their own source code.

1177
00:49:23,446 --> 00:49:29,156
So by saying sharp include open
angle bracket standard io.h

1178
00:49:29,246 --> 00:49:32,056
closed bracket, that's just
telling GCC, you know what,

1179
00:49:32,186 --> 00:49:34,876
the following code has some
functions, some programs

1180
00:49:35,106 --> 00:49:36,066
that other people wrote.

1181
00:49:36,296 --> 00:49:37,976
Those programs happen
to be inside

1182
00:49:37,976 --> 00:49:41,666
of a file called standardio.h.
please go grab the contents,

1183
00:49:41,926 --> 00:49:44,676
paste them in here so that I
don't have to do it myself.

1184
00:49:44,976 --> 00:49:47,136
So I'm kind of oversimplifying
there

1185
00:49:47,136 --> 00:49:50,356
because in real terms what
we're using here is a library.

1186
00:49:50,426 --> 00:49:53,886
So a library of code henceforth
is just a bunch of code

1187
00:49:53,886 --> 00:49:56,976
that someone else wrote or maybe
you wrote but that you factored

1188
00:49:56,976 --> 00:49:59,336
out into separate files so
that you don't have to copy

1189
00:49:59,336 --> 00:50:01,426
and paste it into every
future program you write.

1190
00:50:01,836 --> 00:50:04,566
To technically my white
lie I just told is

1191
00:50:04,566 --> 00:50:06,926
that these program
Print F these functions,

1192
00:50:06,926 --> 00:50:09,706
actually as they should really
be called, are not actually

1193
00:50:09,736 --> 00:50:11,746
in that file, standardio.h,

1194
00:50:12,406 --> 00:50:15,646
inside of that file will
eventually see is just a minimal

1195
00:50:15,646 --> 00:50:18,026
amount of information,
essentially a summary of all

1196
00:50:18,026 --> 00:50:20,636
of the functions in that file
because as you might imagine,

1197
00:50:20,636 --> 00:50:23,806
where is the actual
implementation, the actual lines

1198
00:50:23,806 --> 00:50:25,346
of code that implement Print F?

1199
00:50:25,386 --> 00:50:28,726
What would the file
name probably be called?

1200
00:50:29,496 --> 00:50:33,336
Standardio.c, so in fact, yeah,
there is a file out somewhere

1201
00:50:33,336 --> 00:50:35,486
on the system called
standardio.c

1202
00:50:35,486 --> 00:50:38,206
that someone else implemented
but there's a lot of lines

1203
00:50:38,206 --> 00:50:40,256
of code in there, a normal human
doesn't want to read through it

1204
00:50:40,256 --> 00:50:43,016
so we have these header files
which are called .h files

1205
00:50:43,266 --> 00:50:45,446
and then c files
which have actual code

1206
00:50:45,636 --> 00:50:48,196
and initially I'm not making
any header files myself,

1207
00:50:48,196 --> 00:50:50,126
we'll get to that
eventually but just realize,

1208
00:50:50,126 --> 00:50:52,466
some of these minor tripping
points early on are just

1209
00:50:52,466 --> 00:50:54,826
about keeping programs
relatively clean, so this,

1210
00:50:54,876 --> 00:50:58,416
to be short, simply says, go
get the contents of this file

1211
00:50:58,466 --> 00:51:00,646
so that I can use
functions like Print F

1212
00:51:01,216 --> 00:51:02,476
and henceforth let me clarify.

1213
00:51:02,816 --> 00:51:06,256
So I am writing a program, this
program is arbitrarily going

1214
00:51:06,256 --> 00:51:09,896
to be called hello, but I can
override that as we've seen

1215
00:51:09,896 --> 00:51:11,866
with the -o flag from GCC

1216
00:51:12,266 --> 00:51:15,256
so programs are made
up of functions.

1217
00:51:15,256 --> 00:51:17,676
one or more functions
and these are kind

1218
00:51:17,676 --> 00:51:19,626
of like miniature programs just

1219
00:51:19,626 --> 00:51:21,856
like your scratch
projects are probably made

1220
00:51:21,856 --> 00:51:23,316
up of multiple scripts.

1221
00:51:23,616 --> 00:51:27,036
A script is a function so a
function is just a piece of code

1222
00:51:27,036 --> 00:51:29,456
that does something and as
you can have multiple scripts

1223
00:51:29,456 --> 00:51:32,506
in scratch you can have
multiple functions in c and just

1224
00:51:32,506 --> 00:51:35,866
as the default block or
script and scratch is

1225
00:51:35,866 --> 00:51:37,636
that thing called
wingreenflag clicked,

1226
00:51:38,066 --> 00:51:40,516
so in c is the default script,

1227
00:51:40,516 --> 00:51:42,456
the default function
called main.

1228
00:51:42,886 --> 00:51:46,006
Now why, if its just called main
do I need to write something

1229
00:51:46,006 --> 00:51:49,906
like int and then ints and then
RGC, char or this asterisks,

1230
00:51:49,906 --> 00:51:52,836
this RGV this bracket notation,
there's just a lot of crap

1231
00:51:52,836 --> 00:51:55,156
at the beginning of a program
when you're writing it in c,

1232
00:51:55,316 --> 00:51:57,156
those of you who took
app computer science know

1233
00:51:57,156 --> 00:52:00,546
that there's classfoo public
static, voidmain, its even worse

1234
00:52:00,596 --> 00:52:02,406
in Java and frankly this is just

1235
00:52:02,406 --> 00:52:06,456
because these languages were
not designed with week 0

1236
00:52:06,456 --> 00:52:07,916
of a programming class in mind.

1237
00:52:08,056 --> 00:52:10,366
So again, its stupid stuff,
you get past it quickly.

1238
00:52:10,546 --> 00:52:13,006
For today we're going to wave
our hands at details like that

1239
00:52:13,006 --> 00:52:15,136
but to say for now it's
a copy paste thing.

1240
00:52:15,136 --> 00:52:18,166
To start writing a function
you literally write int main

1241
00:52:18,166 --> 00:52:20,926
and all the rest of that
stuff but then the fun part

1242
00:52:20,926 --> 00:52:23,996
or at least the intellectually
interesting part happens

1243
00:52:23,996 --> 00:52:25,536
in between the curly
braces for now.

1244
00:52:25,996 --> 00:52:28,406
So that's how I would go about
printing this simple program.

1245
00:52:28,406 --> 00:52:31,036
Let me actually take a look
at a more interesting version,

1246
00:52:31,036 --> 00:52:33,656
thought only slightly
called high2.c

1247
00:52:33,656 --> 00:52:35,586
and you have a printout of this.

1248
00:52:36,166 --> 00:52:39,006
So what's the fundamental
difference between version one

1249
00:52:39,116 --> 00:52:41,356
and this one version two?

1250
00:52:41,956 --> 00:52:44,926
Hopefully it jumps out at you.

1251
00:52:44,986 --> 00:52:47,686
yeah, so if bit egotistically
this jumps out at you

1252
00:52:47,686 --> 00:52:50,656
that this program has my
name immortalized in it,

1253
00:52:50,916 --> 00:52:51,946
but how did I do this?

1254
00:52:51,946 --> 00:52:53,816
Well it looks like I've
got this additional line

1255
00:52:53,816 --> 00:52:56,396
and so David is mentioned
but what's going on?

1256
00:52:56,396 --> 00:53:00,026
well even just using the jargon
I started using earlier, string,

1257
00:53:00,246 --> 00:53:01,586
because it comes before the name

1258
00:53:01,586 --> 00:53:03,606
of a variable, means
what precisely?

1259
00:53:03,606 --> 00:53:06,706
Yeah, it's the type of variable.

1260
00:53:06,706 --> 00:53:09,566
So the variable is called
main, what's going to be inside

1261
00:53:09,566 --> 00:53:11,626
of this variable, well its
not going to be an int,

1262
00:53:11,626 --> 00:53:14,346
because David is not an int,
it's a string, it's a word,

1263
00:53:14,346 --> 00:53:16,546
it's a sentence so I'm going
to specify that the type

1264
00:53:16,546 --> 00:53:18,046
of this variable is string,

1265
00:53:18,316 --> 00:53:21,176
now the equal sign again
is the assignment operator,

1266
00:53:21,176 --> 00:53:23,446
it means put the value
of the thing on the right

1267
00:53:23,596 --> 00:53:26,096
on the inside the thing
on the left so the value

1268
00:53:26,096 --> 00:53:30,136
on the right is itself a string
and in c as in many languages

1269
00:53:30,136 --> 00:53:32,776
if you want to have
a literal sequence

1270
00:53:32,776 --> 00:53:34,586
of characters be
treated as one unit,

1271
00:53:34,836 --> 00:53:37,266
you put them in all
double quotes.

1272
00:53:37,546 --> 00:53:40,906
You can use single quotes but
only around single characters,

1273
00:53:40,906 --> 00:53:42,646
so those of you who
know a little

1274
00:53:42,646 --> 00:53:44,476
of html probably know it
doesn't matter if you use single

1275
00:53:44,476 --> 00:53:45,976
or double, it does matter.

1276
00:53:45,976 --> 00:53:48,186
In many programming
languages including c,

1277
00:53:48,396 --> 00:53:50,176
so strings are double quotes.

1278
00:53:50,436 --> 00:53:52,406
All right, then what do
I do on the second line?

1279
00:53:52,646 --> 00:53:54,766
Well, we've not quite
seen this syntax

1280
00:53:55,466 --> 00:53:58,366
but in laymen's term can someone
explain what the percent s

1281
00:53:58,486 --> 00:53:59,996
there represents?

1282
00:54:00,776 --> 00:54:02,956
String. Right, really
not that hard.

1283
00:54:02,956 --> 00:54:06,716
Percent d happened to represent
int, percent s represents string

1284
00:54:06,946 --> 00:54:09,536
and so the thing after the
comma means what do you want

1285
00:54:09,536 --> 00:54:12,686
to substitute in for that
formatting code, the percent s,

1286
00:54:13,176 --> 00:54:14,506
put the name of the variable.

1287
00:54:14,556 --> 00:54:20,206
All right, so more
interesting, so GCC hi2.c enter,

1288
00:54:20,446 --> 00:54:23,706
now I'm going to
run a.out, ohi David

1289
00:54:23,786 --> 00:54:26,906
but unfortunately its
really not that interesting

1290
00:54:26,946 --> 00:54:30,276
if your program is completely
deterministic and always spits

1291
00:54:30,276 --> 00:54:32,706
out the same result
because really, I mean,

1292
00:54:32,706 --> 00:54:35,066
there are so many easier ways
to just say something like this

1293
00:54:35,066 --> 00:54:38,366
than this set up so let's take
a look at the third version

1294
00:54:38,366 --> 00:54:40,246
of this which is a
little more interesting

1295
00:54:40,246 --> 00:54:42,746
and finally we get a
compelling use for a keyboard,

1296
00:54:43,046 --> 00:54:46,056
so this program differs
for the comments up top

1297
00:54:46,206 --> 00:54:48,296
in that there's a
third line of code here

1298
00:54:48,586 --> 00:54:51,396
that uses a new function,
so we've been using Print F

1299
00:54:51,546 --> 00:54:53,856
to send output, it turns
out that you can also,

1300
00:54:54,176 --> 00:54:56,056
nicely enough, get
input from the user.

1301
00:54:56,296 --> 00:54:58,896
So not by way of the mouse, at
least in this context but by way

1302
00:54:58,896 --> 00:55:00,906
of the keyboard and
what's the new function,

1303
00:55:01,246 --> 00:55:04,216
just to take an obvious
guess here, get string.

1304
00:55:04,576 --> 00:55:07,696
As the name implies, the
function get strings purpose

1305
00:55:07,696 --> 00:55:10,586
in life is to get a string
from the user and do what

1306
00:55:10,586 --> 00:55:12,856
with the result apparently?

1307
00:55:13,026 --> 00:55:16,256
Store it. so functions
we now see right away,

1308
00:55:16,256 --> 00:55:19,536
have this ability not only to
do something aesthetically,

1309
00:55:19,586 --> 00:55:23,246
Print F has this so called side
effect of displaying something

1310
00:55:23,246 --> 00:55:26,516
on the screen but functions
can also return a value

1311
00:55:26,516 --> 00:55:29,236
so you can think of a
function kind of as a black box

1312
00:55:29,456 --> 00:55:32,316
that does some work and then
can maybe hand you some answers

1313
00:55:32,496 --> 00:55:33,926
and its up to you,
the programmer,

1314
00:55:33,926 --> 00:55:36,006
to decide if you're going to
do anything with that answer.

1315
00:55:36,226 --> 00:55:37,446
How do you do something with it?

1316
00:55:37,446 --> 00:55:39,976
well one of the things you could
do is keep the answer around.

1317
00:55:40,556 --> 00:55:42,886
So by having string
name on the left

1318
00:55:42,886 --> 00:55:45,596
that says give me a variable
called name, it's a type sting,

1319
00:55:45,596 --> 00:55:47,586
what are you going
to put inside of it?

1320
00:55:47,746 --> 00:55:49,396
well, whatever the result is.

1321
00:55:49,476 --> 00:55:52,066
The return value so to speak
of the thing on the right

1322
00:55:52,166 --> 00:55:54,246
so get string returns
now a value.

1323
00:55:54,546 --> 00:55:56,336
How does this all
behave in reality?

1324
00:55:56,336 --> 00:56:00,086
So GCC hi3.c, I'm getting
a little tired of a.out,

1325
00:56:00,086 --> 00:56:01,286
how do I give this
a better name?

1326
00:56:02,656 --> 00:56:07,276
yes, so -o for output, and then
let's call it hi3 though I could

1327
00:56:07,276 --> 00:56:13,726
call it anything I want, oh, oh,
did I create a problem there?

1328
00:56:13,846 --> 00:56:20,286
its kind of a coincidence, so
still the same error, yeah,

1329
00:56:20,286 --> 00:56:24,006
so correct, fancy correct
answer so there's a problem here

1330
00:56:24,136 --> 00:56:25,976
in that get string
is a function,

1331
00:56:25,976 --> 00:56:28,526
yes but it wasn't written
by the original authors

1332
00:56:28,526 --> 00:56:31,976
of the c programming language,
in fact just getting input

1333
00:56:31,976 --> 00:56:34,146
from the user in c
is really a pain.

1334
00:56:34,146 --> 00:56:36,596
You have to use a function
among others called scanf

1335
00:56:36,596 --> 00:56:39,006
and we'll use this eventually,
there's other tricks too,

1336
00:56:39,126 --> 00:56:41,196
but long story short,
in week one of a course,

1337
00:56:41,196 --> 00:56:44,566
especially this course, to
complete a bunch of hurdles,

1338
00:56:44,566 --> 00:56:47,076
you have to jump through as
simple as get a character

1339
00:56:47,076 --> 00:56:49,006
from the keyboard, even in
Java, those of you familiar

1340
00:56:49,006 --> 00:56:51,046
with the scanner class,
even that is a pain

1341
00:56:51,046 --> 00:56:53,766
to actually use I'd say in
the first week of a class

1342
00:56:54,256 --> 00:56:56,096
so notice what did
I do in this file?

1343
00:56:56,316 --> 00:56:58,946
I did one other thing
besides mention get string.

1344
00:56:59,226 --> 00:57:01,696
There's one other key difference
outside of the comments.

1345
00:57:02,096 --> 00:57:03,116
What else is new here?

1346
00:57:05,386 --> 00:57:08,246
yeah, so I'm not just using
the standard io library,

1347
00:57:08,276 --> 00:57:12,086
it turns out that the cs50
staff a few years ago wrote our

1348
00:57:12,086 --> 00:57:13,296
own library.

1349
00:57:13,466 --> 00:57:15,486
And we wrote what
that means, two files.

1350
00:57:15,486 --> 00:57:18,526
One called cs50.h
and one called cs50.c

1351
00:57:18,726 --> 00:57:20,506
and in the latter we
wrote a bunch of functions

1352
00:57:20,506 --> 00:57:22,156
that we thought would
be useful to students

1353
00:57:22,156 --> 00:57:26,746
in the semester namely get int
and get string and silly name,

1354
00:57:26,746 --> 00:57:29,296
but get long long but
other functions still

1355
00:57:29,476 --> 00:57:32,236
that really simplify the
process at least at the start

1356
00:57:32,236 --> 00:57:34,556
of the semester of getting
keyboard input from users

1357
00:57:34,826 --> 00:57:37,826
but unfortunately its not
sufficient just to tell GCC,

1358
00:57:37,886 --> 00:57:41,466
hey make sure you go get
a copy of the header file

1359
00:57:41,466 --> 00:57:43,936
for cs50's library so I know
that these functions exist,

1360
00:57:43,936 --> 00:57:44,906
because I've done that.

1361
00:57:44,906 --> 00:57:48,736
So one mistake might have been
to omit this line together,

1362
00:57:48,736 --> 00:57:49,636
that would in fact be bad.

1363
00:57:50,256 --> 00:57:51,836
But that's not the only
mistake that I made,

1364
00:57:51,836 --> 00:57:54,116
I now need to tell
cs50's library

1365
00:57:54,416 --> 00:57:57,516
when you compile this
you don't just get away

1366
00:57:57,516 --> 00:58:01,676
with turning one file,
hi3.c into 0's and 1's

1367
00:58:01,896 --> 00:58:05,106
because you need access to
one other file, called what?

1368
00:58:05,566 --> 00:58:08,626
Yes, so cs50.c so there's
a couple solutions here,

1369
00:58:08,626 --> 00:58:10,436
I could email the core
staff, and I could say,

1370
00:58:10,436 --> 00:58:14,126
hey could I have a copy of
cs50.c or they'll be happy

1371
00:58:14,126 --> 00:58:16,066
to give it because there's no
intellectual property there,

1372
00:58:16,066 --> 00:58:19,276
its just standard c stuff, but
that seems kind of wasteful

1373
00:58:19,276 --> 00:58:21,326
if every time in the semester
if you write a program you need

1374
00:58:21,326 --> 00:58:23,556
to copy the R file, put
it in your own directory,

1375
00:58:23,556 --> 00:58:24,706
kind of bloat your own code,

1376
00:58:24,956 --> 00:58:26,446
well in fact there's
an easier way.

1377
00:58:26,446 --> 00:58:31,756
You can tell GCC to
link against our library

1378
00:58:31,756 --> 00:58:33,406
and long story short
is what this mean,

1379
00:58:33,406 --> 00:58:37,106
is when GCC is converting
hi3.c from source code

1380
00:58:37,106 --> 00:58:39,646
into object code, from
human readable to 0's

1381
00:58:39,676 --> 00:58:43,486
and 1's it means, oh by the
way, go get the library, -l,

1382
00:58:43,746 --> 00:58:47,546
called cs50 and merge the
two at the very last minute

1383
00:58:47,586 --> 00:58:51,886
so that the file you get out hi3
or a.out contains 0's and 1's

1384
00:58:51,886 --> 00:58:53,256
from both of those files.

1385
00:58:53,506 --> 00:58:56,556
So again, another silly detail
that's very easy to forget

1386
00:58:56,796 --> 00:58:58,626
but just to kind of
drill it into your minds

1387
00:58:58,626 --> 00:59:00,276
if you ever see this semester,

1388
00:59:00,496 --> 00:59:02,566
a message about undefined
references,

1389
00:59:03,046 --> 00:59:05,276
probably means you
forgot something simple

1390
00:59:05,276 --> 00:59:08,126
like that library that
you wanted to use, -lcs50.

1391
00:59:08,186 --> 00:59:11,216
there's others, -lm
gives you a math library.

1392
00:59:11,216 --> 00:59:14,536
There's -lcrypt which gives you
a cryptography library and stuff

1393
00:59:14,536 --> 00:59:16,236
like that, but it
did work this time.

1394
00:59:16,236 --> 00:59:19,776
when I said go ahead and link in
this file, I did in fact get hi3

1395
00:59:19,886 --> 00:59:25,616
so if I run hi3 state your
name, we'll say Jensue,

1396
00:59:25,886 --> 00:59:28,096
well hi Jensue, well
what about Yuki?

1397
00:59:28,936 --> 00:59:31,926
Okay so now I have, still
kind of overwhelming right?

1398
00:59:31,926 --> 00:59:33,556
not anything to write home about

1399
00:59:33,746 --> 00:59:36,026
but at least it's a dynamic
program that's starting

1400
00:59:36,026 --> 00:59:39,096
to do a little bit more so
just to give you a sense now

1401
00:59:39,096 --> 00:59:42,626
of some formalities, one
I've used here, Print F,

1402
00:59:42,706 --> 00:59:45,646
a couple of times, I've used
it pretty simply to use things

1403
00:59:45,646 --> 00:59:48,936
like strings and ints but we can
do more powerful things with it

1404
00:59:48,936 --> 00:59:51,946
and in fact you'll have to
get used to sort of a new sort

1405
00:59:51,946 --> 00:59:54,266
of a world where people write
things somewhat cryptically

1406
00:59:54,266 --> 00:59:57,046
sometimes, this is an
excerpt from the man page,

1407
00:59:57,046 --> 00:59:59,556
the manual page for
Print F and the way you,

1408
00:59:59,556 --> 01:00:00,646
the programmer know, how

1409
01:00:00,646 --> 01:00:03,466
to use a function is
often frankly, by rtfm.

1410
01:00:03,746 --> 01:00:04,946
Just read the manual.

1411
01:00:05,266 --> 01:00:07,346
And actually look up what
the instructions are,

1412
01:00:07,436 --> 01:00:08,486
Google it later if you'd like.

1413
01:00:08,836 --> 01:00:11,766
For those less comfortable and
less familiar with the speak.

1414
01:00:12,576 --> 01:00:13,736
So what you'll find

1415
01:00:13,736 --> 01:00:15,806
on the courses website
throughout the semester

1416
01:00:15,806 --> 01:00:17,356
under resources we have a bunch

1417
01:00:17,356 --> 01:00:19,676
of links we think you'll
find useful and one

1418
01:00:19,676 --> 01:00:22,716
of them is this wonderful
site called cppreference.com.

1419
01:00:22,716 --> 01:00:25,506
the only downside of it
means cpp means c plus plus

1420
01:00:25,886 --> 01:00:28,676
so the author of the site
actually made it fairly c plus

1421
01:00:28,756 --> 01:00:31,036
plus specific so under resources

1422
01:00:31,036 --> 01:00:34,276
on the courses web sit we have
our own copy of this website

1423
01:00:34,276 --> 01:00:37,036
because he open sourced it to
everyone so if I actually go

1424
01:00:37,036 --> 01:00:42,456
to cs50.net/resources
what you'll see up here

1425
01:00:42,456 --> 01:00:44,686
under the link to c
is a bunch of things,

1426
01:00:44,686 --> 01:00:50,226
most of which we won't need just
yet, but if I go to reference,

1427
01:00:50,606 --> 01:00:53,686
this is a wonderful useful sight
and this is one of the reasons

1428
01:00:53,686 --> 01:00:55,856
that you don't really need a
book for a course like this

1429
01:00:56,076 --> 01:00:58,566
because so much good stuff
is available on line.

1430
01:00:58,876 --> 01:01:01,006
If I know that I'm
doing something with io,

1431
01:01:01,006 --> 01:01:04,336
that sounds cryptic, but it just
means input output print get

1432
01:01:04,496 --> 01:01:06,696
stuff like that, notice
if I scroll down here,

1433
01:01:06,926 --> 01:01:08,966
clearly we're only
scratching the surface today

1434
01:01:08,966 --> 01:01:10,836
of what C can do
and I was interested

1435
01:01:10,836 --> 01:01:12,236
in what Print F can do

1436
01:01:12,486 --> 01:01:14,906
and here's just fairly
user friendly documentation

1437
01:01:14,906 --> 01:01:18,746
of the other codes you can use
so we saw already, let's see,

1438
01:01:18,746 --> 01:01:21,586
percent d and there it is at
the top for signed integers,

1439
01:01:21,686 --> 01:01:23,826
things that may or may not
have negative sign in front,

1440
01:01:24,026 --> 01:01:25,796
percents for a string
of characters

1441
01:01:26,016 --> 01:01:27,886
but there's some other
stuff here too that we may

1442
01:01:27,886 --> 01:01:30,086
or may not use in this
semester, but what's nice

1443
01:01:30,086 --> 01:01:32,126
about this site is it
give you sample code,

1444
01:01:32,306 --> 01:01:35,416
it tells you what library, what
header file you must include

1445
01:01:35,416 --> 01:01:36,606
at the top of your program

1446
01:01:36,866 --> 01:01:39,356
and then it tells you what
the functions signature is,

1447
01:01:39,586 --> 01:01:41,626
what is sort of the
canonical form of this thing

1448
01:01:41,626 --> 01:01:44,036
so you can begin to
understand how you use it

1449
01:01:44,036 --> 01:01:46,296
and again you'll get much
more acclimated to this

1450
01:01:46,626 --> 01:01:47,866
over the course of the semester

1451
01:01:47,866 --> 01:01:50,266
but we can do more
interesting things with this.

1452
01:01:50,556 --> 01:01:53,496
So just as there's
n for new line,

1453
01:01:53,796 --> 01:01:57,216
there's r for carriage
return, so if you think back

1454
01:01:57,216 --> 01:01:59,676
to retro typewriters
when you hit enter

1455
01:01:59,676 --> 01:02:01,056
on an old fashioned typewriter,

1456
01:02:01,286 --> 01:02:03,236
not only did the
paper move this way,

1457
01:02:03,236 --> 01:02:06,616
the little typing
head moved this way

1458
01:02:06,856 --> 01:02:08,906
so a carriage return moves the
curser all the curser all the

1459
01:02:08,906 --> 01:02:11,656
way back to the left, a new
line moves it down one line

1460
01:02:12,086 --> 01:02:14,526
so this is another
sort of stupid thing

1461
01:02:14,526 --> 01:02:17,836
from religious battles past
so in the Windows world,

1462
01:02:17,836 --> 01:02:21,196
most text files end with
what's called a carriage return

1463
01:02:21,356 --> 01:02:22,836
and a line feed.

1464
01:02:22,836 --> 01:02:25,346
Crlf, which means rn.

1465
01:02:26,326 --> 01:02:29,036
in the Linux world people
were much more conservative,

1466
01:02:29,036 --> 01:02:33,756
they just used new lines and
n in many systems, Macs and,

1467
01:02:34,486 --> 01:02:36,266
actually not so much
Mac sometimes,

1468
01:02:36,266 --> 01:02:39,546
so in Linux computers n
means yes go to the next line

1469
01:02:39,756 --> 01:02:42,556
and go all the way to the left
so I'm just kind of grimacing

1470
01:02:42,556 --> 01:02:44,426
at some of these details
because these things too,

1471
01:02:44,646 --> 01:02:46,206
are little things you
trip over early on.

1472
01:02:46,206 --> 01:02:48,136
they're fundamentally not that
interesting but you get used

1473
01:02:48,136 --> 01:02:50,006
to little trivia or
minutia like this.

1474
01:02:50,276 --> 01:02:53,936
But this does beg the
question, if n means new line,

1475
01:02:54,326 --> 01:02:56,866
r means carriage return,

1476
01:02:57,166 --> 01:03:01,306
how do you just write a literal
? Well the answer is there

1477
01:03:01,306 --> 01:03:03,806
but this is a common
pattern, what is it?

1478
01:03:04,776 --> 01:03:07,396
 so unfortunately a little
weird, you kind of have

1479
01:03:07,446 --> 01:03:09,476
to have a corner case like
that if you want a literal

1480
01:03:09,686 --> 01:03:12,536
and a string you escape
the but this notion

1481
01:03:12,536 --> 01:03:14,686
of escaping is something
we'll see quite a bit.

1482
01:03:15,036 --> 01:03:16,356
Well, it turns out
we can do stuff

1483
01:03:16,416 --> 01:03:19,776
with not just ints not just
strings, there are chars

1484
01:03:19,776 --> 01:03:21,026
for individual characters,

1485
01:03:21,026 --> 01:03:23,616
doubles for floating
point values and float

1486
01:03:23,616 --> 01:03:25,026
for floating point
values and all

1487
01:03:25,026 --> 01:03:27,556
of these have some different
meaning, in fact let me go ahead

1488
01:03:27,846 --> 01:03:29,986
to an example that's
not just text space now,

1489
01:03:30,226 --> 01:03:32,406
but just to demonstrate,
again, to underwhelm,

1490
01:03:32,406 --> 01:03:33,636
at first because we
can definitely one

1491
01:03:33,636 --> 01:03:37,036
up ourselves easily that
math is not hard and c.

1492
01:03:37,346 --> 01:03:40,066
so here's a program that
assigns a variable called x,

1493
01:03:40,426 --> 01:03:44,486
I'll type integer
the value of 1;

1494
01:03:44,556 --> 01:03:48,696
variable called y type is int,
assign it a value of 2 right,

1495
01:03:48,696 --> 01:03:50,256
new syntax, not interesting.

1496
01:03:50,346 --> 01:03:53,476
Finally I declare a third
variable called z its also going

1497
01:03:53,476 --> 01:03:56,516
to store an int, its going
to store the sum of x and y,

1498
01:03:57,186 --> 01:04:01,406
looks pretty good but let me
go ahead and let me run GCC

1499
01:04:01,576 --> 01:04:03,896
of math1.c okay, interesting.

1500
01:04:04,186 --> 01:04:07,036
So it turns out GCC, especially
the way we have it configured

1501
01:04:07,036 --> 01:04:11,826
on nice.faf can also be a bit
anal and a little cautious

1502
01:04:11,826 --> 01:04:13,466
on your behalf saying
you know what,

1503
01:04:13,896 --> 01:04:15,186
odds are you screwed up here.

1504
01:04:15,296 --> 01:04:16,526
right? its saying
it very nicely,

1505
01:04:16,576 --> 01:04:19,106
warning unused variable
z but that's telling you

1506
01:04:19,106 --> 01:04:21,236
that you did something,
you're not using it,

1507
01:04:21,486 --> 01:04:23,756
probably wasn't a very
productive program you just

1508
01:04:23,756 --> 01:04:24,896
wrote, but let's try it.

1509
01:04:24,936 --> 01:04:28,866
let's be stubborn and
let's run a.out enter, hmm.

1510
01:04:29,316 --> 01:04:32,876
Is it not working?

1511
01:04:34,226 --> 01:04:35,886
Its actually working perfectly.

1512
01:04:37,456 --> 01:04:40,256
Right, the math is
correct, what is in z

1513
01:04:40,256 --> 01:04:41,496
at this point, oh, missed it.

1514
01:04:41,656 --> 01:04:44,146
the problem is what?

1515
01:04:44,146 --> 01:04:44,746
Simple answer.

1516
01:04:46,796 --> 01:04:49,736
I'm not doing anything with
it, yes I created 3 variables,

1517
01:04:49,736 --> 01:04:52,406
all of them at some point in
time, do in fact have numbers

1518
01:04:52,406 --> 01:04:55,036
like 1 2 and hopefully
3 inside of them

1519
01:04:55,236 --> 01:04:56,896
but if you don't do
anything with it,

1520
01:04:57,166 --> 01:04:58,986
sort of the tree falling
in the woods right?

1521
01:04:58,986 --> 01:05:01,876
no one knows that the answer you
just computed was three unless

1522
01:05:01,876 --> 01:05:05,046
you print it, save it,
do something with it,

1523
01:05:05,266 --> 01:05:07,096
so hopefully there's
a solution here.

1524
01:05:07,096 --> 01:05:09,386
so let's take a look at the
second, slightly fixed version

1525
01:05:09,386 --> 01:05:11,246
of this, so how do I
print out the result?

1526
01:05:11,246 --> 01:05:13,316
Well again, let's just go into
our little tool kit that's going

1527
01:05:13,316 --> 01:05:15,006
to start being filled
with more function still,

1528
01:05:15,286 --> 01:05:18,086
Print F quote unquotes
the quotes are important,

1529
01:05:18,266 --> 01:05:21,416
the percent means display this
as an integer and then I pass

1530
01:05:21,416 --> 01:05:24,556
in z and so, okay, so now not
only have I written a correct

1531
01:05:24,556 --> 01:05:28,456
program, I'm actually doing
something productive with it,

1532
01:05:28,516 --> 01:05:32,876
a.out, now gives -- hmm, still a
little weird, its not incorrect

1533
01:05:32,876 --> 01:05:34,906
but what's that explanation
for this weirdness.

1534
01:05:36,366 --> 01:05:38,696
Right, so there's just no new
line, so let me go in here

1535
01:05:38,946 --> 01:05:42,056
and I'm going to move over
here, add between the quotes

1536
01:05:42,056 --> 01:05:45,686
but after the percentn
let me recompile,

1537
01:05:45,686 --> 01:05:47,706
another little Linux
trick, if you get tired

1538
01:05:47,706 --> 01:05:49,516
of typing the same commands
over and over again,

1539
01:05:49,786 --> 01:05:52,006
you can either hit up
and that will scroll

1540
01:05:52,036 --> 01:05:53,846
through the commands
you previously typed

1541
01:05:53,976 --> 01:05:55,576
or you can hit !

1542
01:05:55,746 --> 01:05:59,746
aka bang and type the first
letter or so of the last command

1543
01:05:59,746 --> 01:06:01,696
that starts with that
sequence and hit enter

1544
01:06:01,696 --> 01:06:04,016
and if I haven't forgotten
some other g command I wrote,

1545
01:06:04,306 --> 01:06:07,086
it should remember,
aha, it was gccmath2.c,

1546
01:06:07,086 --> 01:06:09,136
again little things
you pick up over time.

1547
01:06:09,806 --> 01:06:11,706
and if I run a.out,

1548
01:06:11,706 --> 01:06:14,106
okay slightly prettier,
slightly prettier.

1549
01:06:14,326 --> 01:06:16,946
So what else can we do
here, well there's going

1550
01:06:16,946 --> 01:06:20,436
to be other data types still but
there's also this format strings

1551
01:06:20,436 --> 01:06:23,206
so let's see if we
can't do a little bit

1552
01:06:23,206 --> 01:06:25,356
of something that's more
interesting underneath the hood

1553
01:06:25,356 --> 01:06:27,736
so it turns out we
have format strings,

1554
01:06:28,006 --> 01:06:29,586
but there's also
this approach here,

1555
01:06:29,586 --> 01:06:30,736
let me go back to the code.

1556
01:06:30,736 --> 01:06:32,666
This is a program, again,
that you have a print out of,

1557
01:06:33,016 --> 01:06:36,046
called size of, and here's where
we finally scratch the surface

1558
01:06:36,046 --> 01:06:37,786
of actual design issues.

1559
01:06:38,186 --> 01:06:40,586
Actual questions that you
are interested to answer

1560
01:06:40,736 --> 01:06:43,436
because they really will affect
the correctness of your program

1561
01:06:43,436 --> 01:06:44,726
and what you can do with it.

1562
01:06:44,986 --> 01:06:47,516
so this program, its not very
long, its about half comments

1563
01:06:47,516 --> 01:06:48,546
and half actual code,

1564
01:06:48,806 --> 01:06:51,306
notice that at the top here
I've declared some variables

1565
01:06:51,306 --> 01:06:53,656
in advance, so this is
kind of interesting.

1566
01:06:54,036 --> 01:06:56,486
Even though thus far
I've very efficiently,

1567
01:06:56,486 --> 01:06:58,706
very analy been declaring
a variable on the left

1568
01:06:58,706 --> 01:07:01,436
and then doing something with it
on the right, all in one line,

1569
01:07:01,836 --> 01:07:02,916
you don't need to do that.

1570
01:07:02,916 --> 01:07:04,686
If you know you're going
to need a variable,

1571
01:07:04,886 --> 01:07:07,946
you can declare the variable
earlier in your program

1572
01:07:07,996 --> 01:07:11,286
and then use it later and you'll
learn over time when this is

1573
01:07:11,286 --> 01:07:14,486
and isn't appropriate, but
here I've declared three types

1574
01:07:14,486 --> 01:07:15,046
of values.

1575
01:07:15,046 --> 01:07:17,466
A char, a double,
a float and an int.

1576
01:07:17,976 --> 01:07:20,356
a float is a floating
point value a real number,

1577
01:07:20,356 --> 01:07:24,276
something with a decimal
point, what's a double perhaps?

1578
01:07:25,156 --> 01:07:26,086
Any ideas?

1579
01:07:27,156 --> 01:07:28,496
So its twice something,

1580
01:07:28,726 --> 01:07:31,786
its actually a floating
point value that's twice

1581
01:07:31,786 --> 01:07:32,916
as big potentially.

1582
01:07:33,176 --> 01:07:35,916
So it turns out that in this
language and in some others,

1583
01:07:36,126 --> 01:07:37,766
there are actually
pre defined meanings

1584
01:07:37,766 --> 01:07:40,486
for how many bits are used
to represent a data type,

1585
01:07:40,796 --> 01:07:44,816
so we have chars or 8 bits
on a typical modern computer

1586
01:07:44,856 --> 01:07:48,396
with intel inside, a double is,
and let's come back to that.

1587
01:07:48,396 --> 01:07:52,036
A float is 32 bits,
now that's pretty big,

1588
01:07:52,036 --> 01:07:55,396
because if an int is also 32
bits, what did we say last week,

1589
01:07:55,396 --> 01:07:56,646
the biggest number
you can represent

1590
01:07:56,646 --> 01:07:59,726
with 32 bits is, like 4 billion.

1591
01:07:59,726 --> 01:08:01,866
Now wait a minute, if you're
omitting negative numbers

1592
01:08:01,866 --> 01:08:03,606
so if its negative
numbers also then its

1593
01:08:03,606 --> 01:08:06,746
like negative two billion,
to positive 2 billion roughly

1594
01:08:06,746 --> 01:08:08,056
but a total of 4 billion

1595
01:08:08,336 --> 01:08:12,106
so floating point values also
use 32 bits but they use them

1596
01:08:12,106 --> 01:08:15,286
to have some numbers
before the decimal point,

1597
01:08:15,596 --> 01:08:18,006
some after so you
can't, it's a trade off

1598
01:08:18,006 --> 01:08:19,686
between how big a
number you can represent

1599
01:08:19,686 --> 01:08:23,066
as a floating point value versus
how many points you can have

1600
01:08:23,066 --> 01:08:26,476
after the decimal point but
a double now is not 32 bits,

1601
01:08:26,566 --> 01:08:29,246
right, I'm kind of beyond
the scope of my arms here,

1602
01:08:29,496 --> 01:08:31,656
its 64 bits and that's
pretty damn big

1603
01:08:31,826 --> 01:08:34,296
so you'll have some discretion
there so what does it mean

1604
01:08:34,296 --> 01:08:38,746
when I run this down here, Print
F all right, so print an int

1605
01:08:38,746 --> 01:08:40,226
and a new line, but what
do you want to print?

1606
01:08:40,556 --> 01:08:43,286
Well there's this operator in c,
that's only occasionally useful,

1607
01:08:43,356 --> 01:08:46,596
today its marginally
enlightening, tell me the size

1608
01:08:46,596 --> 01:08:50,046
of the variable c. tell me
the size of the variable d, f,

1609
01:08:50,046 --> 01:08:53,886
and i so let me go ahead and run
a GCC on sizeof.c now I'm going

1610
01:08:54,136 --> 01:08:58,176
to go ahead and run a.out,
wait a minute, those are not

1611
01:08:58,176 --> 01:09:00,366
in fact values like 32 and 64.

1612
01:09:00,366 --> 01:09:01,296
what am I seeing?

1613
01:09:02,816 --> 01:09:03,746
Yeah, I'm seeing bytes.

1614
01:09:03,936 --> 01:09:06,196
That's kind of interesting,
okay, so that's fine,

1615
01:09:06,446 --> 01:09:07,736
let me take a look
at one other thing.

1616
01:09:07,736 --> 01:09:10,686
I saw on the screen a moment
ago that there's this mention

1617
01:09:10,686 --> 01:09:15,106
of longs so let me say long
is an l and let me print

1618
01:09:15,106 --> 01:09:20,586
out its length here, so long
and I think its l, hopefully, %,

1619
01:09:21,336 --> 01:09:28,166
let's see, GCC sizeofc, oops
not l, dammit, what is it?

1620
01:09:28,806 --> 01:09:29,716
is it also d?

1621
01:09:30,796 --> 01:09:31,586
yes all right.

1622
01:09:31,916 --> 01:09:35,576
if I run this now what do you
think a long is going to be?

1623
01:09:35,576 --> 01:09:42,246
So int was 4, also 4, so again,
sort of a historical artifact,

1624
01:09:42,386 --> 01:09:44,206
when I mentioned
long longs before,

1625
01:09:44,536 --> 01:09:46,796
so if you want an int
that's bigger than 32 bits,

1626
01:09:46,796 --> 01:09:49,446
you want a 64 bit int, its not a
long, because a long is the same

1627
01:09:49,446 --> 01:09:51,646
as an int, it's a long
long that you want.

1628
01:09:51,646 --> 01:09:54,536
So again, sort of
random c trivia.

1629
01:09:54,886 --> 01:09:55,796
But I thought we'd
end with this.

1630
01:09:56,206 --> 01:09:59,256
Here's a crazy looking program,
I loaded this off the internet,

1631
01:09:59,546 --> 01:10:02,196
written by a really, either
a really brilliant person

1632
01:10:02,196 --> 01:10:04,486
or a person with really
a lot of free time.

1633
01:10:04,836 --> 01:10:07,836
so this is an obfuscated
C program

1634
01:10:08,156 --> 01:10:10,756
and you are entering a community
that you don't have to stay

1635
01:10:10,756 --> 01:10:14,246
in in perpetuity of people who
like writing programs like this,

1636
01:10:14,246 --> 01:10:16,196
or have the ability to do
it and there are contests

1637
01:10:16,336 --> 01:10:20,146
that people run for obfuscated
c contests whereby you challenge

1638
01:10:20,216 --> 01:10:23,176
people to write the most
confusing looking program you

1639
01:10:23,176 --> 01:10:25,876
could possibly come up with
that no human should be able

1640
01:10:25,876 --> 01:10:28,076
to understand and then you
challenge the audience,

1641
01:10:28,116 --> 01:10:31,116
the other geeks in the room,
what that program actually does,

1642
01:10:31,686 --> 01:10:34,816
do you have any idea
what this is going to do?

1643
01:10:34,816 --> 01:10:37,006
We will not be writing
things like this.

1644
01:10:37,216 --> 01:10:39,606
This is an exercise in the
opposite of good design,

1645
01:10:39,886 --> 01:10:41,986
but its kind of a
powerful thing,

1646
01:10:42,196 --> 01:10:44,146
that if we run the compiler
here, the warnings I'm going

1647
01:10:44,146 --> 01:10:45,276
to ignore for now, that's fine.

1648
01:10:45,566 --> 01:10:47,026
A.out, I'll give you the results

1649
01:10:47,026 --> 01:10:50,236
of your first obfuscated
c program which is,

1650
01:10:51,546 --> 01:10:52,866
that's what that does.

1651
01:10:53,036 --> 01:10:53,796
We'll see you on Friday.