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>> [MUSIC PLAYING]

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>> DAVID MALAN: All right this is CS50
and this is the end of week one.

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So I'd like to introduce my former
advisee, Andel Duff, who not only makes

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iPhone 5 stands, as well as this
lectern, he also makes cars.

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ANSEL DUFF: How's it going, everybody?

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I just want to tell you a little
bit about Formula SAE, of which I

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am the captain.

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It's an MIT team.

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I just want to tell you
guys a little bit about it

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and hopefully generate some interest.

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>> So let me just run through a
couple things about our team.

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So we're completely
student run, wherein all

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of the members design, simulate,
test, and manufacturer a subsystem

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of the car.

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We have our own machine shop
and garages, and N52-- or sorry,

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N51-- which is the same building
as the MIT Science Museum.

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And we are the people
that drive the car.

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There's a national competition
in the beginning of the summer,

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and then there are smaller shootout
competitions throughout the year.

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And then we do driver training
days to get new members acquainted.

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>> Our build days are on
Saturdays from 11 AM to 8 PM.

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Lunch and dinner are taking care
of, from Bertucci's and Beantown

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respectively.

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We have a weekly meeting and
seminar, which is for course credit.

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Which is new as of this semester, and
it will also be for next semester,

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so you can cross-reg for that.

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And then we have designing, shop,
and CAD days during the week,

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whereas members will independently
CAD parts, order stock et cetera.

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>> So for electrical
engineering at the team

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we have our tractive,
high voltage system,

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which runs at a nominal
300 volts, which includes

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motors, motor controllers,
and battery packs.

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Then we have a low
voltage system, which runs

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at 12 volts, which includes
controls, CAN nodes, the brain

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box, and the dashboard.

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As far CS goes, we do
have website maintenance

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that we need done for events, membership
changes, new pictures, and PR material.

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And then we have code that controls the
car for our traction control, launch

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control, dash control, torque vectoring,
and regenerative braking systems, which

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run typically in MATLAB,
Simulink, and LabView.

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>> Tell you a little bit about the car.

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So it's about 500 pounds.

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It's rules limited to 85 kilowatts.

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It has 2 EMRAX 207 motors in the
back, one for each rear wheel.

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It has custom-built battery packs
from Prismatic A123 pouch cells,

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totaling 300 volts and
5.6 kilowatt hours.

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And it's direct drive with
a 2.64 to 1 drive ratio.

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Here are some pictures we
took of it at competition.

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Here's the back, nose cone,
and here are the motors,

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those big black disks
underneath the et mens manus.

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Here's a slide that I pulled
out of our competition scheme.

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And this is just comparing our car to a
couple of commercially available cars,

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like the KTM XBOW, Ariel
Atom, and Caterha M 7-280.

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If you're familiar with price, power
pull weight, dollars per horsepower,

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dollars per torque, and our car
blows them all out of the water.

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If you have any questions,
jot down this email address.

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Shoot us an email at fsae@mit.edu.

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>> We're happy to talk about
anything you'd like.

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Harvard students are kind
of the minority on the team.

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If you have any questions about
that please do shoot us an email,

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happy to talk, and I'm going
to head back over to David.

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Thanks.

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I appreciate it, guys.

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[APPLAUSE]

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>> DAVID MALAN: Ansel spent the summer
working with us here on campus in CS50

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this summer.

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And in addition to working on
things like the binary bulbs,

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he actually really opened our
eyes-- and mine in particular--

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to what really the intersection of
not of hardware and software can be.

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In fact a lot of the things he just
toured you through are ultimately

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created first in software,
and then ultimately fabricated

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in the real world in hardware.

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So if you're interested in
exploring that intersection,

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perhaps take up that group.

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>> Now meanwhile, those of you
who are fans of technology,

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and Apple products in
particular know that there

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were a couple of
announcements yesterday.

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But we thought we'd show
you one that came out

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a few days prior that
you might not have seen.

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>> JORGEN EGHAMMER: You know,
once in awhile something

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comes along that
changes the way we live.

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A device so simple and intuitive
using it feels almost familiar.

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Introducing 2015 IKEA catalogue.

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It's not a digital book or
an e-book, it's a book-book.

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>> The first thing to note is no
cables, not even a power cable.

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The 2015 IKEA catalogue
comes fully charged,

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and the battery life is eternal.

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The interface is 7.5 by 8 inches,
but can expand to 15 by 8 inches.

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The navigation is based on tactile touch
technology that you can actually feel.

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>> Content comes pre-installed
via 328 high-definition pages

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of inspiring home furnishing ideas.

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To start browsing,
simply touch and drag.

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Right to left to move forward,
left to right to move backwards.

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>> Notice something else?

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That's right.

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No lag.

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Each crystal clear page
loads instantaneously,

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no matter how fast you scroll.

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If you want to get a quick overview,
just hold it in the palm of your hand,

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and using just your thumb,
speed browse the content.

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>> If you find something you want to save
for later, you can simply bookmark it.

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And even if you close the application,
you can easily find the bookmark again.

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Amazing.

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>> [LAUGHTER]

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[APPLAUSE]

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So that brings us to
the end of week one.

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A few announcements now.

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So sectioning is now in progress.

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Go to this URL here sometime
between now and Friday at noon

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to actually specify whether you are
among those less comfortable, more

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comfortable, or somewhere in between.

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>> This is typically one of those
things that you just kind of know.

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And in fact those of you who are less
comfortable probably know as much,

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those of you who are more
comfortable know as much,

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and again if you aren't quite sure which
of those buckets you fall into you're

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probably somewhere in between.

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But rest assured you can swap
after a couple of weeks if need be.

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>> Meanwhile, for the coming week
we don't start sections per se,

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but super sections, which
are open to all students.

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This coming Sunday at a location to
be announced on the course website

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we will have one for
those less comfortable,

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as well as one for
those more comfortable.

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And those somewhere in between
can choose which of those two,

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or both if you'd like, to sit-in on.

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And the focus of those
will be ultimately on C,

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which we'll continue our
conversation of today.

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>> Problem set 0 meanwhile is
hopefully in your hands.

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Officially due tomorrow,
Thursday, at noon.

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Unless you cash in one
of your 5 late days,

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which would bring it
until Friday at noon.

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Problem set 1 meanwhile will be
posted on the course's website.

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And this will be our first
foray as a class into C.

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And that problem set spec will walk you
through a lot of the mechanics of what

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we started talking about on Monday,
and we'll continue talking about today.

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C, Linux, the CS50
appliance, all of that

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will await you in the specification.

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>> Office hours meanwhile are in progress,
in the undergraduate dining hall.

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Head to this URL here to see the
schedule for tonight and tomorrow,

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if you would like to partake.

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Questions meanwhile, so that you
have opportunities to ask questions

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asynchronously, 24/7, realize you're
not limited to sections and office

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hours, but rather this tool
here, CS50 Discuss at that URL

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will be the course's online
bulletin board software, via which

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you can ask questions of
classmates and also staff.

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So keep in mind that resource as well.

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Meanwhile a tradition
starts this Friday.

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As you may have glimpsed from
last Wednesday's teaser video,

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we have a tradition most
every Friday during term

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of taking some 50 students and
staff to a local restaurant called

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Fire and Ice, at which we're
typically joined by some alumni

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or friends from industry
to generally aspire

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to make a large class feel smaller.

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So it's a completely casual lunch.

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A bunch of us, the staff
especially, will play musical chairs

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and chat with students about
50, life outside of 50,

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life in the future, the real world,
I'm talking about job opportunities

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with friends in industry.

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>> So if you would like to join us
this Friday, head to this URL here.

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Space is limited, but we'll do this
recurringly throughout the term.

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Head there to cs50.harvard.edu/rsvp.

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And if you don't get in
this time, not to worry.

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We will do it again in
the coming weeks as well.

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>> So this is the picture
we painted last week,

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and you've probably been
dabbling in thus far.

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But let's try to connect this now
to what we're going to do today

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and onward.

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Could I have one volunteer?

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All right, or several.

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How about here on the
end, in the blue shirt.

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Come on up, what's your name?

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ALANA: Alana.

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DAVID MALAN: Alana.

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So Alana, for just a
moment-- nice to meet you--

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is going to play the role
of this function here.

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The purple block we've
called thus far a function.

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So what I'm going to go
ahead here and offer Alana

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is a little name tag, to make
clear what role you're playing.

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>> So you shall be the say block.

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If you want to go ahead and put that on.

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And we're going to
keep this super simple.

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If the goal at hand is simply
now to implement with Alana,

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this so-called say block,
here's how it's going to work.

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I have-- let's call it
an argument or parameter,

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it's really just a blank sheet of paper.

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And it's white to kind of be reminiscent
of the white box into which you

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can write words.

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>> Meanwhile I'm going to go here.

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I'm going to go ahead and
write H-E-L-L-O comma world.

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And now I, as say the programmer, don't
actually know how to say something.

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I can come up with the words,
but I don't necessarily

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know how to express myself
verbally or on the screen.

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And so what I'm going
to do is take this piece

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of functionality, whose input--
or argument, as we'll call it,

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I've written on this piece of paper--
and I'm going to outsource it.

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>> And indeed that's the role that
functions in a programming language

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can play.

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Something like the say
block can be outsourced

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to someone who really knows
what she or he is talking about.

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So if you'll step over
here for just a moment,

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I am going to pass this input to
Alana, and ask that you say this input.

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>> ALANA: Hello, world.

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DAVID MALAN: That's it.

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So that there is a function.

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Now we can take it a little more,
we can take a step forward here.

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Because that was clearly underwhelming.

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So let's now convert this to
something a little more sophisticated.

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>> So this of course is our C version.

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So now if you want to take that
one off, that name tag off,

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and let's have you put on a new name.

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But fundamentally you're
playing the same role,

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it just so happens to
be called printf now.

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>> The story is, of course,
going to be-- probably

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just as underwhelming-- is
going to be the following.

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You are now the function printf.

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I am the caller, or the
programmer, who wants

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to stand on the shoulders of
people in the past like you,

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who've already figured out how
to actually write something down.

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And so this time,
rather than say it let's

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actually use our screen over here.

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So if you'd like to step
over here, I'm going

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to pass again this as
input to my printf friend.

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If you could go ahead and
print that on the screen,

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simply by drawing with your
finger on the black screen.

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Excellent.

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All right, so lots of suspense for
how that was going to play out.

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>> So now let's take things up one
final notch, if we could, as follows.

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So this is a C program
as we said last time.

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And this just does what you did, it
prints out to the screen, hello, world.

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00:10:57,460 --> 00:11:00,290
Even though there's clearly a lot
of distracting stuff up there.

236
00:11:00,290 --> 00:11:03,630
But let's take things up a notch and
introduce this version, which recall

237
00:11:03,630 --> 00:11:06,160
was the third version that
we ended up with last time.

238
00:11:06,160 --> 00:11:09,120
>> And now there's clearly
two uses of printf.

239
00:11:09,120 --> 00:11:12,280
There's 2 calls to Alana
in this case for printf,

240
00:11:12,280 --> 00:11:14,740
but there's also a call
to another function.

241
00:11:14,740 --> 00:11:17,150
What's that function clearly?

242
00:11:17,150 --> 00:11:17,650
Getstring.

243
00:11:17,650 --> 00:11:18,980
>> Can we get one more volunteer?

244
00:11:18,980 --> 00:11:20,980
OK, come on up.

245
00:11:20,980 --> 00:11:21,805
What's your name?

246
00:11:21,805 --> 00:11:22,990
>> JAVIER: Javier.

247
00:11:22,990 --> 00:11:24,750
>> DAVID MALAN: Javier, come on up.

248
00:11:24,750 --> 00:11:27,270
So Javier's role is on
this piece of paper.

249
00:11:27,270 --> 00:11:28,860
Which I'm going to call generically s.

250
00:11:28,860 --> 00:11:32,250
Now this is a variable, it's
of type string, as implied

251
00:11:32,250 --> 00:11:34,290
by that second line in the middle there.

252
00:11:34,290 --> 00:11:36,540
And I'd like you to go get me a string.

253
00:11:36,540 --> 00:11:40,120
Specifically head down to the orchestra
section and get me the name of someone,

254
00:11:40,120 --> 00:11:43,142
and come back with a variable
containing that value.

255
00:11:43,142 --> 00:11:44,225
It can be anyone you want.

256
00:11:44,225 --> 00:11:47,590

257
00:11:47,590 --> 00:11:48,520
>> Here we go.

258
00:11:48,520 --> 00:11:53,620

259
00:11:53,620 --> 00:11:55,960
All right, Javier is
writing down the name.

260
00:11:55,960 --> 00:12:02,210

261
00:12:02,210 --> 00:12:03,280
All right.

262
00:12:03,280 --> 00:12:05,740
We can all probably guess how
this is going to play out.

263
00:12:05,740 --> 00:12:07,489
We're perhaps belaboring
the point of what

264
00:12:07,489 --> 00:12:10,320
a function is doing, but
nonetheless thank you very much.

265
00:12:10,320 --> 00:12:13,890
>> So Javier has returned this string
here, which, can't quite see,

266
00:12:13,890 --> 00:12:15,800
Jonathan is the string
that we've gotten.

267
00:12:15,800 --> 00:12:18,330
So now what I'm actually going
to provide to Alana though

268
00:12:18,330 --> 00:12:20,150
is something a little different.

269
00:12:20,150 --> 00:12:23,800
Because in this version of the
program, first there's state your name.

270
00:12:23,800 --> 00:12:26,160
So the very first argument
I need to provide Alana

271
00:12:26,160 --> 00:12:33,030
with is going to be
literally state your name.

272
00:12:33,030 --> 00:12:35,280
All right, so simple as that
I would hand this to you.

273
00:12:35,280 --> 00:12:36,730
And as I write up the
next argument, if you

274
00:12:36,730 --> 00:12:39,230
want to go ahead and write this
on the board, let's go ahead

275
00:12:39,230 --> 00:12:42,659
and do the final example here to make
clear what's ultimately got to happen.

276
00:12:42,659 --> 00:12:44,450
So state your name,
write it toward the top

277
00:12:44,450 --> 00:12:47,220
if you don't mind, just so that
we have room for one second line.

278
00:12:47,220 --> 00:12:52,890
Because the last thing I'm going
to now provide you with is not one,

279
00:12:52,890 --> 00:12:57,220
but two pieces of paper.

280
00:12:57,220 --> 00:12:58,074
>> All right.

281
00:12:58,074 --> 00:13:01,920
H-E-L-L-O comma and then a blank line.

282
00:13:01,920 --> 00:13:04,780
So what I'm about to hand Alana now
is something a little different.

283
00:13:04,780 --> 00:13:08,600
Not only is she getting this,
where fill in the blank represents

284
00:13:08,600 --> 00:13:12,050
that percent-- what was the
placeholder we've been using?

285
00:13:12,050 --> 00:13:13,890
Yeah, so percent s.

286
00:13:13,890 --> 00:13:16,530
Meanwhile I'm going to pass
her in a second argument, which

287
00:13:16,530 --> 00:13:19,290
is precisely what Javier
retrieved from Jonathan.

288
00:13:19,290 --> 00:13:20,534
>> So let me give you these two.

289
00:13:20,534 --> 00:13:22,575
If you'd like to, go ahead
and print that effect.

290
00:13:22,575 --> 00:13:25,660

291
00:13:25,660 --> 00:13:28,130
What's ultimately happening
now is, Alana again

292
00:13:28,130 --> 00:13:29,410
is playing the role of printf.

293
00:13:29,410 --> 00:13:32,160
Javier was playing
the role of GetString.

294
00:13:32,160 --> 00:13:34,700
So that will be your souvenir.

295
00:13:34,700 --> 00:13:39,910
And ultimately what's happening, despite
this hello-- yep go ahead and comma.

296
00:13:39,910 --> 00:13:41,410
ALANA: Did I receive this yet or no?

297
00:13:41,410 --> 00:13:43,451
DAVID MALAN: Yep, you
received them both at once.

298
00:13:43,451 --> 00:13:45,740
So the idea is that you
can now flip the page

299
00:13:45,740 --> 00:13:50,000
to handle the second
of those two inputs.

300
00:13:50,000 --> 00:13:53,280
So this is only to hammer home the
point that whereas Alana executed

301
00:13:53,280 --> 00:13:55,830
this green line here, and then
Javier did this one for us

302
00:13:55,830 --> 00:13:57,820
and return to me something
we'll call s, but it was really

303
00:13:57,820 --> 00:13:59,520
just that piece of paper of his own.

304
00:13:59,520 --> 00:14:02,320
And now Alana has done this third line.

305
00:14:02,320 --> 00:14:06,010
>> And even though all of this looks
so incredibly cryptic perhaps

306
00:14:06,010 --> 00:14:08,870
at first glance, it really
is as simple as that.

307
00:14:08,870 --> 00:14:11,390
Passing inputs around,
getting outputs, either

308
00:14:11,390 --> 00:14:14,560
being physically handed back something,
or seeing a side effect like this.

309
00:14:14,560 --> 00:14:17,170
Something visually
written on the screen.

310
00:14:17,170 --> 00:14:19,990
So let's go ahead and
thank Alana and Javier,

311
00:14:19,990 --> 00:14:25,012
who was down here, for
joining us up here.

312
00:14:25,012 --> 00:14:25,934
>> [APPLAUSE]

313
00:14:25,934 --> 00:14:29,360

314
00:14:29,360 --> 00:14:33,480
>> So now let's take for granted just how
straightforward all of that might be.

315
00:14:33,480 --> 00:14:36,775
And let's proceed to actually
do something in actual code.

316
00:14:36,775 --> 00:14:40,620
So up on the screen here is a screenshot
of the so-called CS50 appliance.

317
00:14:40,620 --> 00:14:43,980
And if you had to at, as a
2:00 PM on Wednesday afternoon,

318
00:14:43,980 --> 00:14:46,602
explain to some friend
what the CS50 appliance is,

319
00:14:46,602 --> 00:14:48,185
how would you define it in a sentence?

320
00:14:48,185 --> 00:14:52,160

321
00:14:52,160 --> 00:14:53,240
Any one sentence?

322
00:14:53,240 --> 00:14:54,876
Yeah?

323
00:14:54,876 --> 00:14:57,334
STUDENT: It's like a program
that makes everyone's computer

324
00:14:57,334 --> 00:14:58,460
run with the same system.

325
00:14:58,460 --> 00:14:59,210
DAVID MALAN: Good!

326
00:14:59,210 --> 00:15:02,810
It's a program that enables everyone's
computer to run with the same system.

327
00:15:02,810 --> 00:15:04,920
It's sort of your own
copy of an operating

328
00:15:04,920 --> 00:15:07,540
system that happens to
be called Ubuntu Linux.

329
00:15:07,540 --> 00:15:11,070
And it runs effectively inside of
a window on your own Mac or PC,

330
00:15:11,070 --> 00:15:15,080
so that this way right out of the gate
in week 1, everyone here in the class

331
00:15:15,080 --> 00:15:17,590
has access to the same tools,
the same configuration,

332
00:15:17,590 --> 00:15:21,250
and there's no learning curve specific
to a Mac or a PC in particular.

333
00:15:21,250 --> 00:15:25,620
>> Now we opened up, inside of the CS50
appliance on Monday, this program here.

334
00:15:25,620 --> 00:15:27,040
It happens to be called gedit.

335
00:15:27,040 --> 00:15:29,050
But that's really just
a graphical editor.

336
00:15:29,050 --> 00:15:33,170
It's something like Notepad or Text
Edit on Windows or Mac OS respectively.

337
00:15:33,170 --> 00:15:36,720
And I proposed that there were really
three important parts to this screen.

338
00:15:36,720 --> 00:15:39,560
The top side is where you
would write your code.

339
00:15:39,560 --> 00:15:44,080
And that's where we spent much of our
three examples in Monday's lecture.

340
00:15:44,080 --> 00:15:47,650
The bottom we called what,
this black window, yeah?

341
00:15:47,650 --> 00:15:48,640
>> STUDENT: Compiler?

342
00:15:48,640 --> 00:15:51,500
>> DAVID MALAN: Compiler we
accessed by way of that window.

343
00:15:51,500 --> 00:15:53,040
But more generally.

344
00:15:53,040 --> 00:15:54,540
Yeah, it was just a terminal window.

345
00:15:54,540 --> 00:15:57,780
This is an age old term that just
describes essentially a blinking

346
00:15:57,780 --> 00:16:01,910
prompt, that years ago used to be on
one of those big CRT style monitors.

347
00:16:01,910 --> 00:16:04,300
But nowadays it's sort
of virtual in software.

348
00:16:04,300 --> 00:16:06,710
And the terminal windows
let's us type commands.

349
00:16:06,710 --> 00:16:11,750
It sort of the simplified version
of the user interface or UI.

350
00:16:11,750 --> 00:16:14,060
It's not a graphical
user interface or GUI,

351
00:16:14,060 --> 00:16:17,250
it's a text-based interface
or a command line interface.

352
00:16:17,250 --> 00:16:20,770
>> And indeed that's where ultimately
I was able to write some code.

353
00:16:20,770 --> 00:16:24,200
Once I had written some code though,
recall that I used this command.

354
00:16:24,200 --> 00:16:26,210
And as it the name
kind of suggests, this

355
00:16:26,210 --> 00:16:29,860
allows me to make a
program called Hello.

356
00:16:29,860 --> 00:16:33,400
But what was it really doing?

357
00:16:33,400 --> 00:16:36,180
Well make was taking my
input, my source code,

358
00:16:36,180 --> 00:16:39,380
and converting it to what, ultimately?

359
00:16:39,380 --> 00:16:40,060
>> Object code.

360
00:16:40,060 --> 00:16:42,790
And object code is just a fancy
way of saying zeros and ones.

361
00:16:42,790 --> 00:16:46,420
And then once I wanted
to run that object code,

362
00:16:46,420 --> 00:16:50,940
once I wanted to pass as input those
zeros and ones into my computer's

363
00:16:50,940 --> 00:16:54,430
brain, the so-called CPU
or central processing unit,

364
00:16:54,430 --> 00:16:55,730
I had to run the program.

365
00:16:55,730 --> 00:16:59,042
But it wasn't quite a simple on Monday
as just double clicking some icon.

366
00:16:59,042 --> 00:17:01,375
What instead did I have to
do in order to run a program?

367
00:17:01,375 --> 00:17:04,130

368
00:17:04,130 --> 00:17:07,890
>> What was that second command?

369
00:17:07,890 --> 00:17:09,619
I'm hearing a little slashes?

370
00:17:09,619 --> 00:17:10,544
Yes, yeah?

371
00:17:10,544 --> 00:17:12,474
>> STUDENT: Dot slash and
the program's name.

372
00:17:12,474 --> 00:17:13,349
DAVID MALAN: Exactly.

373
00:17:13,349 --> 00:17:15,030
Dot slash and the program's name.

374
00:17:15,030 --> 00:17:16,210
Now what did this mean?

375
00:17:16,210 --> 00:17:19,470
Well dot is just an arcane way
of saying current directory.

376
00:17:19,470 --> 00:17:23,440
Whatever folder you are in is
referenced as, is represented as dot.

377
00:17:23,440 --> 00:17:26,680
The slash is just what you've seen
in Mac OS and Windows for years,

378
00:17:26,680 --> 00:17:30,329
it's a separator between a directory
or folder and the rest of some name.

379
00:17:30,329 --> 00:17:32,620
In Windows it happens to go
backwards, in Linux and Mac

380
00:17:32,620 --> 00:17:34,210
OS it happens to go forwards.

381
00:17:34,210 --> 00:17:37,560
But it's just the separator, so it's
just an uninteresting syntactic detail.

382
00:17:37,560 --> 00:17:40,440
>> The juicy part of course is
the name of the program, Hello.

383
00:17:40,440 --> 00:17:42,650
And that's what make created for us.

384
00:17:42,650 --> 00:17:44,910
It outputted that file for us.

385
00:17:44,910 --> 00:17:46,700
But how did we get to that point?

386
00:17:46,700 --> 00:17:48,870
Let's now ask the
question, what was really

387
00:17:48,870 --> 00:17:52,470
going on here, at least with
regard to some of this syntax?

388
00:17:52,470 --> 00:17:57,000
>> So in an sentence or so,
how did we explain away

389
00:17:57,000 --> 00:17:59,020
this first line that's
highlighted in green?

390
00:17:59,020 --> 00:18:02,998
What was that first line doing,
with respect to my program?

391
00:18:02,998 --> 00:18:03,974
Yeah?

392
00:18:03,974 --> 00:18:05,067
>> STUDENT: [INAUDIBLE].

393
00:18:05,067 --> 00:18:06,150
DAVID MALAN: Say it again?

394
00:18:06,150 --> 00:18:08,734
STUDENT: Including and downloading
past functions [INAUDIBLE].

395
00:18:08,734 --> 00:18:09,483
DAVID MALAN: Good.

396
00:18:09,483 --> 00:18:11,900
Including, I won't say
downloading, but let's say

397
00:18:11,900 --> 00:18:15,500
including functions that people
have written in the past.

398
00:18:15,500 --> 00:18:18,260
And those functions are implemented
somewhere in my computer.

399
00:18:18,260 --> 00:18:20,470
Someone wrote a file
years ago, and that file

400
00:18:20,470 --> 00:18:23,490
is somewhere inside of the CS50
appliance, or on my hard drive

401
00:18:23,490 --> 00:18:24,480
more generally.

402
00:18:24,480 --> 00:18:27,810
And so this line is essentially saying,
go find that file, standard I/O dot

403
00:18:27,810 --> 00:18:31,740
h, and copy and paste its contents
right here on the top of my file

404
00:18:31,740 --> 00:18:33,860
so that I don't have to
do that manually myself.

405
00:18:33,860 --> 00:18:40,970
>> And among the juiciest pieces inside of
that file we claimed was what function?

406
00:18:40,970 --> 00:18:45,740
What function did we say was declared
or mentioned in standard I/O dot h

407
00:18:45,740 --> 00:18:47,120
most likely?

408
00:18:47,120 --> 00:18:47,900
Printf, right?

409
00:18:47,900 --> 00:18:50,797
We did not write printf on
Monday, it just existed.

410
00:18:50,797 --> 00:18:52,880
Much like Alana just came
up on stage and she just

411
00:18:52,880 --> 00:18:55,540
existed and knew how to draw
something on the screen,

412
00:18:55,540 --> 00:18:58,660
so does printf exist for many years.

413
00:18:58,660 --> 00:19:00,240
And so this is how we access it.

414
00:19:00,240 --> 00:19:03,280
Now if we move on here,
main was analogous we

415
00:19:03,280 --> 00:19:06,620
said to the puzzle piece
called when green flag clicked

416
00:19:06,620 --> 00:19:07,620
in the world of Scratch.

417
00:19:07,620 --> 00:19:10,130
It's just sort of the
default name that humans

418
00:19:10,130 --> 00:19:13,340
decided would represent the
default entry point to a program.

419
00:19:13,340 --> 00:19:16,540
The chunk of programming code
that gets executed first.

420
00:19:16,540 --> 00:19:19,700
>> Meanwhile these curly braces are
kind of like the curved shape

421
00:19:19,700 --> 00:19:21,410
of a lot of those yellow scratch pieces.

422
00:19:21,410 --> 00:19:24,860
They kind of encapsulate a whole
bunch of instructions together.

423
00:19:24,860 --> 00:19:28,350
So it just kind of binds together
some related functionality.

424
00:19:28,350 --> 00:19:33,750
printf of course is a function that
prints the screen, as we just did.

425
00:19:33,750 --> 00:19:35,190
>> What about these parentheses?

426
00:19:35,190 --> 00:19:38,770
How would you define-- even if you've
never programmed before, but just based

427
00:19:38,770 --> 00:19:41,320
now on an increasing
hopefully intuition, what

428
00:19:41,320 --> 00:19:43,290
are the parentheses doing for us here?

429
00:19:43,290 --> 00:19:45,450
Or what are they surrounding?

430
00:19:45,450 --> 00:19:46,360
Yeah?

431
00:19:46,360 --> 00:19:49,360
>> STUDENT: They're saying what
you're going to print with printf.

432
00:19:49,360 --> 00:19:52,401
>> DAVID MALAN: Yeah, they're saying what
you're going to print with printf.

433
00:19:52,401 --> 00:19:56,260
Or more generally, they are
surrounding the inputs to the function.

434
00:19:56,260 --> 00:19:58,530
So you can almost think
of those parentheses

435
00:19:58,530 --> 00:20:01,620
as being like the metal
clip on this clipboard.

436
00:20:01,620 --> 00:20:05,940
It's what's holding, it's what's
going to be used to provide arguments

437
00:20:05,940 --> 00:20:08,500
into the function, which in
this case is called printf.

438
00:20:08,500 --> 00:20:12,390
>> So in general almost any
time we call a function,

439
00:20:12,390 --> 00:20:14,280
we are going to see a pair parentheses.

440
00:20:14,280 --> 00:20:17,000
Maybe with something in it,
like now, maybe nothing in it.

441
00:20:17,000 --> 00:20:19,770
But that's where you would put
the inputs or so-called arguments

442
00:20:19,770 --> 00:20:20,680
to a function.

443
00:20:20,680 --> 00:20:22,470
>> Here's one such argument.

444
00:20:22,470 --> 00:20:24,467
I've highlighted everything
between the quotes,

445
00:20:24,467 --> 00:20:26,300
because it turns out
in this language called

446
00:20:26,300 --> 00:20:29,950
C, when you want to represent a
string-- that is a word or a phrase

447
00:20:29,950 --> 00:20:33,470
or even a paragraph-- you have to
surround it with double quotes.

448
00:20:33,470 --> 00:20:35,420
Not single quotes, double quotes.

449
00:20:35,420 --> 00:20:37,460
And that's exactly what I've done here.

450
00:20:37,460 --> 00:20:39,900
>> Meanwhile there's a funky
symbol toward the end there.

451
00:20:39,900 --> 00:20:41,600
The backslash n.

452
00:20:41,600 --> 00:20:43,460
What did we say that represented?

453
00:20:43,460 --> 00:20:44,093
Yeah?

454
00:20:44,093 --> 00:20:44,980
>> STUDENT: A new line.

455
00:20:44,980 --> 00:20:45,896
>> DAVID MALAN: New line.

456
00:20:45,896 --> 00:20:47,910
Right, you don't
generally hit Enter when

457
00:20:47,910 --> 00:20:50,430
you want the program
to spit out a new line.

458
00:20:50,430 --> 00:20:54,210
Rather you tell it explicitly
with this admittedly arcane piece

459
00:20:54,210 --> 00:20:58,770
of syntax, backslash n, that
you want a new line to go there.

460
00:20:58,770 --> 00:20:59,810
Finally the semicolon.

461
00:20:59,810 --> 00:21:01,560
We didn't really talk
too much about this,

462
00:21:01,560 --> 00:21:04,893
and by far this will be the bane of some
of your existence for the first problem

463
00:21:04,893 --> 00:21:07,990
set, when you just failed to realize
you've forgotten something as stupid

464
00:21:07,990 --> 00:21:08,614
as a semicolon.

465
00:21:08,614 --> 00:21:11,600
And generally putting it
there will fix that problem.

466
00:21:11,600 --> 00:21:13,511
But what's it doing
for us, would you say?

467
00:21:13,511 --> 00:21:15,670
>> STUDENT: [INAUDIBLE].

468
00:21:15,670 --> 00:21:17,484
>> DAVID MALAN: Sorry?

469
00:21:17,484 --> 00:21:18,532
>> STUDENT: [INAUDIBLE].

470
00:21:18,532 --> 00:21:19,990
DAVID MALAN: Finishing a statement.

471
00:21:19,990 --> 00:21:22,770
It's sort of like the period at
the end of an English sentence,

472
00:21:22,770 --> 00:21:25,790
whereas in this language
C it's ending a statement.

473
00:21:25,790 --> 00:21:29,250
An instruction of programming
code that you just want to say,

474
00:21:29,250 --> 00:21:30,500
I'm done with this.

475
00:21:30,500 --> 00:21:32,240
Now notice that's the only one here.

476
00:21:32,240 --> 00:21:34,614
So you don't want to get in
a habit of putting semicolons

477
00:21:34,614 --> 00:21:36,190
after every closed parenthesis.

478
00:21:36,190 --> 00:21:38,610
For instance there's none
next to void, and we'll

479
00:21:38,610 --> 00:21:40,430
come back in the future
to what void means.

480
00:21:40,430 --> 00:21:44,080
But in this case your printf is a
function, or a statement being used,

481
00:21:44,080 --> 00:21:47,420
and so we want to have that
terminus at the very end of it.

482
00:21:47,420 --> 00:21:50,980
>> And I'm intentionally picking on C. And
frankly a lot of languages like C, C++,

483
00:21:50,980 --> 00:21:55,000
Java, JavaScript, and any number of
other languages have a lot of these

484
00:21:55,000 --> 00:21:56,730
sort of syntactic details.

485
00:21:56,730 --> 00:22:00,120
That certainly if you're new to
programming can genuinely trip you up.

486
00:22:00,120 --> 00:22:02,307
And you will, mark my
words, some of you will

487
00:22:02,307 --> 00:22:04,140
have this experience
in office hours or late

488
00:22:04,140 --> 00:22:07,554
at night working on some p-set, where
your damn program just doesn't compile,

489
00:22:07,554 --> 00:22:09,470
it doesn't run, you have
no idea what's wrong,

490
00:22:09,470 --> 00:22:11,513
it seems completely
logically correct to you.

491
00:22:11,513 --> 00:22:13,596
And it's because you forgot
something like hitting

492
00:22:13,596 --> 00:22:15,250
a semicolon at the end of some line.

493
00:22:15,250 --> 00:22:17,590
>> But these are the kinds of things you're
going to immediately learn to see.

494
00:22:17,590 --> 00:22:20,048
And indeed these are the things
that the CA's and TF's have

495
00:22:20,048 --> 00:22:23,210
been sort of trained to see
much more quickly than you.

496
00:22:23,210 --> 00:22:26,610
And so this is only to say, as you
dive into problem set 1 especially,

497
00:22:26,610 --> 00:22:28,217
don't get frustrated by this stuff.

498
00:22:28,217 --> 00:22:30,050
Once you sort of acclimate
to the world, you

499
00:22:30,050 --> 00:22:35,130
begin to see things that you might not
see at first glance this first week.

500
00:22:35,130 --> 00:22:37,440
>> So source code is something like that.

501
00:22:37,440 --> 00:22:40,370
We want to pass it into
the so-called compiler.

502
00:22:40,370 --> 00:22:42,850
And that compiler as
we said provides output

503
00:22:42,850 --> 00:22:45,660
known as object code, the
so-called zeros and ones.

504
00:22:45,660 --> 00:22:47,747
But from there, what does that give us?

505
00:22:47,747 --> 00:22:49,080
Well it gives us these patterns.

506
00:22:49,080 --> 00:22:52,020
And again your computer,
your Intel inside,

507
00:22:52,020 --> 00:22:54,350
understands these patterns
of zeros and ones.

508
00:22:54,350 --> 00:22:57,300
And sometimes the pattern
represents an actual decimal number

509
00:22:57,300 --> 00:22:59,770
as we saw last week, sometimes
it represents a letter

510
00:22:59,770 --> 00:23:02,730
like we saw last week, sometimes
it represents an instruction,

511
00:23:02,730 --> 00:23:05,460
like printing something to the screen.

512
00:23:05,460 --> 00:23:08,782
>> So printf for instance--
but rather, let me rewind.

513
00:23:08,782 --> 00:23:11,240
We've been taking for granted
that those zeros and ones are

514
00:23:11,240 --> 00:23:14,200
produced by this command make.

515
00:23:14,200 --> 00:23:16,050
But make is not a compiler.

516
00:23:16,050 --> 00:23:19,370
Make is not the thing in the middle
that's producing the zeros and ones.

517
00:23:19,370 --> 00:23:23,780
Rather make is just a very conveniently
named program whose purpose in life

518
00:23:23,780 --> 00:23:27,267
is essentially to figure out
how to compile your program.

519
00:23:27,267 --> 00:23:29,350
And we use it in these
first weeks because it just

520
00:23:29,350 --> 00:23:30,910
saves us a lot of trouble.

521
00:23:30,910 --> 00:23:34,080
>> But what make is really
doing when you compile hello,

522
00:23:34,080 --> 00:23:36,690
is, as we said last time, it's
looking on your hard drive

523
00:23:36,690 --> 00:23:40,620
or in the current folder, for
file called what, apparently?

524
00:23:40,620 --> 00:23:41,815
Hello.c, right?

525
00:23:41,815 --> 00:23:43,190
That's just kind of a convention.

526
00:23:43,190 --> 00:23:45,620
It's arbitrary, but
that's the way things are.

527
00:23:45,620 --> 00:23:48,620
Make, if you just specify the name
of a program that doesn't yet exist,

528
00:23:48,620 --> 00:23:52,200
it's going to look for the source code
in a file, by default means hello.c.

529
00:23:52,200 --> 00:23:53,530
And if it finds it, great.

530
00:23:53,530 --> 00:23:56,650
It's going to convert that source
code into object code for you.

531
00:23:56,650 --> 00:23:59,670
>> But every time I've hit Enter
after running make hello,

532
00:23:59,670 --> 00:24:03,560
do you recall seeing last time a
fairly long and cryptic sequence

533
00:24:03,560 --> 00:24:06,430
of other white letters and
characters on the screen?

534
00:24:06,430 --> 00:24:11,220
Well that was the actual command, the
actual compiler, that was running.

535
00:24:11,220 --> 00:24:15,500
So the actual compiler we'll use most of
the semester is something called clang.

536
00:24:15,500 --> 00:24:19,280
Some of you might have used
Visual Studio before, or GCC,

537
00:24:19,280 --> 00:24:21,340
or any number of other compilers.

538
00:24:21,340 --> 00:24:22,620
We'll use clang.

539
00:24:22,620 --> 00:24:26,950
>> And clang allows us to actually convert
that source code to object code.

540
00:24:26,950 --> 00:24:29,430
What does this actually
mean in practice?

541
00:24:29,430 --> 00:24:33,390
Well let me go into one of
my folders from last time.

542
00:24:33,390 --> 00:24:35,760
Inside of the CS50 appliance.

543
00:24:35,760 --> 00:24:42,080
And let me go ahead and create this
same file, include standard I/O dot h.

544
00:24:42,080 --> 00:24:42,875
Int main void.

545
00:24:42,875 --> 00:24:46,000
And we'll come back in the future as
to what int means and what void means.

546
00:24:46,000 --> 00:24:49,130
But for now let's do
hello world backslash n,

547
00:24:49,130 --> 00:24:50,500
just like we did on the board.

548
00:24:50,500 --> 00:24:53,930
>> Let me save this file called hello.c.

549
00:24:53,930 --> 00:24:58,280
And now if I want to compile this
I could-- let me zoom in-- run

550
00:24:58,280 --> 00:25:00,450
make hello enter.

551
00:25:00,450 --> 00:25:02,645
And this again was
that very cryptic line.

552
00:25:02,645 --> 00:25:05,120
But now at least one word
probably jumps out at you.

553
00:25:05,120 --> 00:25:07,087
Clang is that line we saw before.

554
00:25:07,087 --> 00:25:10,420
Now there's a lot of other stuff frankly
that we'll wave our hands at for today.

555
00:25:10,420 --> 00:25:14,020
But there's a few subsets,
a few characters in here

556
00:25:14,020 --> 00:25:14,907
that are of interest.

557
00:25:14,907 --> 00:25:16,490
But I'm going to go ahead and do this.

558
00:25:16,490 --> 00:25:18,281
I'm going to first run
hello, just to prove

559
00:25:18,281 --> 00:25:20,320
that this is working as it was Monday.

560
00:25:20,320 --> 00:25:22,330
But now I'm going to delete the program.

561
00:25:22,330 --> 00:25:25,870
And much like we had these
short names last time,

562
00:25:25,870 --> 00:25:29,787
the command for removing
something is rm, remove, enter.

563
00:25:29,787 --> 00:25:31,870
You're going to get some
fairly cryptic questions.

564
00:25:31,870 --> 00:25:35,210
Hard to believe that you can make
even this question hard to understand.

565
00:25:35,210 --> 00:25:38,100
But remove regular file hello just
means do you want to delete hello.

566
00:25:38,100 --> 00:25:42,650
>> And I can go ahead and type y for
yes, Enter, and now it's gone.

567
00:25:42,650 --> 00:25:45,130
What this means now,
if I clear my screen,

568
00:25:45,130 --> 00:25:47,301
is that instead of
doing make hello-- you

569
00:25:47,301 --> 00:25:48,800
know what, I'm getting good at this.

570
00:25:48,800 --> 00:25:50,690
I'm going to run the compiler myself.

571
00:25:50,690 --> 00:25:53,330
I don't need some program to
figure out what my file is called.

572
00:25:53,330 --> 00:25:55,370
I wrote hello.c, I
know what it's called,

573
00:25:55,370 --> 00:25:57,640
I'm clearly capable of typing that.

574
00:25:57,640 --> 00:26:02,620
So I'm going to go ahead
and do clang hello.c Enter.

575
00:26:02,620 --> 00:26:03,264
>> Seems to work.

576
00:26:03,264 --> 00:26:05,930
And there's no additional output,
because clang is the compiler.

577
00:26:05,930 --> 00:26:08,590
It's what's converting the
source code to zeros and ones.

578
00:26:08,590 --> 00:26:12,870
So if I now do dot
slash hello Enter, hm.

579
00:26:12,870 --> 00:26:14,630
New error message today.

580
00:26:14,630 --> 00:26:17,610
So bash is just the name of the
blinking prompt that we're using.

581
00:26:17,610 --> 00:26:20,360
That's a gross oversimplification,
but for now that's all that is,

582
00:26:20,360 --> 00:26:21,401
it's the blinking prompt.

583
00:26:21,401 --> 00:26:22,880
And that's why it's yelling at us.

584
00:26:22,880 --> 00:26:25,690
>> No such file or directory is
a little more straightforward.

585
00:26:25,690 --> 00:26:28,190
But surely I made that object code.

586
00:26:28,190 --> 00:26:30,930
But the catch is, that
if I type the command

587
00:26:30,930 --> 00:26:34,650
to list the contents of this
directory, which is what?

588
00:26:34,650 --> 00:26:36,210
ls for list.

589
00:26:36,210 --> 00:26:39,332
There's a bunch of stuff in here,
some of which came with the appliance,

590
00:26:39,332 --> 00:26:42,040
like my Desktop folder, Downloads
folder, Dropbox folder, and all

591
00:26:42,040 --> 00:26:42,850
that stuff.

592
00:26:42,850 --> 00:26:45,410
But what is interesting
is hello.c, which

593
00:26:45,410 --> 00:26:47,286
I created a moment ago with gedit.

594
00:26:47,286 --> 00:26:49,160
And what's the weirdest
looking name in here,

595
00:26:49,160 --> 00:26:53,110
perhaps that we didn't
see last time at all?

596
00:26:53,110 --> 00:26:54,710
>> a.out, right?

597
00:26:54,710 --> 00:26:56,920
So back in the day, it
was just decided that when

598
00:26:56,920 --> 00:26:58,760
you compile a program
for the first time,

599
00:26:58,760 --> 00:27:02,360
and you don't specify the name
of a file, let's call it a.

600
00:27:02,360 --> 00:27:05,800
And a.out just means this was
the output of the compiler.

601
00:27:05,800 --> 00:27:07,790
So it's a horrible name for a program.

602
00:27:07,790 --> 00:27:12,820
But by that logic, dot slash for
current directory, slash a.out,

603
00:27:12,820 --> 00:27:16,102
should mean there's in fact
my hello world program.

604
00:27:16,102 --> 00:27:17,560
Now a little warm up exercise, too.

605
00:27:17,560 --> 00:27:20,420
If I want to rename this
file, turns out that you

606
00:27:20,420 --> 00:27:23,000
move a file from one name to another.

607
00:27:23,000 --> 00:27:28,750
So mv a.out, and then I
can call it hello Enter.

608
00:27:28,750 --> 00:27:30,940
So this is a Linux command.

609
00:27:30,940 --> 00:27:34,370
DOS had this years ago,
Windows has a terminal window

610
00:27:34,370 --> 00:27:36,460
of some sort that looks
like this, Linux and Mac

611
00:27:36,460 --> 00:27:38,140
computers have this blinking prompt.

612
00:27:38,140 --> 00:27:41,490
Even though most of us probably
rarely, if ever, use it.

613
00:27:41,490 --> 00:27:44,590
>> But what I've done is run a
program, whose name is mv,

614
00:27:44,590 --> 00:27:47,900
and I've provided it with
two arguments, so to speak.

615
00:27:47,900 --> 00:27:48,630
Two inputs.

616
00:27:48,630 --> 00:27:50,260
a.out is the original name.

617
00:27:50,260 --> 00:27:51,630
Hello is the new name.

618
00:27:51,630 --> 00:27:55,670
So if I now do dot slash
hello, Enter, that exists.

619
00:27:55,670 --> 00:28:00,770
And if I do dot slash a.out,
what do you expect I should see?

620
00:28:00,770 --> 00:28:02,250
>> Bash, no such file or directory.

621
00:28:02,250 --> 00:28:03,350
Because I just renamed it.

622
00:28:03,350 --> 00:28:04,975
So this takes a bit of getting used to.

623
00:28:04,975 --> 00:28:06,977
But the problem set 1
specification will truly

624
00:28:06,977 --> 00:28:08,810
hold your hand through
some of this minutae.

625
00:28:08,810 --> 00:28:11,050
Because this is a complete
intellectual distraction

626
00:28:11,050 --> 00:28:12,966
from the more interesting
ideas at hand, which

627
00:28:12,966 --> 00:28:15,310
is actually creating
something out of code.

628
00:28:15,310 --> 00:28:18,830
But let's do one modification
now to this program.

629
00:28:18,830 --> 00:28:23,150
>> Recall that last time I did something
like this, printf state your name.

630
00:28:23,150 --> 00:28:25,790
And indeed we just did this
on the big screen over there.

631
00:28:25,790 --> 00:28:30,640
And then I did string s gets
GetString open paren, close paren.

632
00:28:30,640 --> 00:28:33,430
And paren just shorthand
notation for parenthesis.

633
00:28:33,430 --> 00:28:36,460
So does GetString take any arguments?

634
00:28:36,460 --> 00:28:38,440
No inputs no, but it
needs the parentheses,

635
00:28:38,440 --> 00:28:41,830
because that's what demarcates
the calling of a function.

636
00:28:41,830 --> 00:28:46,440
>> So I'm going to go ahead and try to
run the compiler on this program now.

637
00:28:46,440 --> 00:28:50,774
Clang hello.c, because I
didn't change its name for now.

638
00:28:50,774 --> 00:28:51,940
And I got a bunch of errors.

639
00:28:51,940 --> 00:28:53,570
Let's zoom out here.

640
00:28:53,570 --> 00:28:55,420
If I scroll up again,
like I said last time,

641
00:28:55,420 --> 00:28:58,420
to the first, same error
that I got last time.

642
00:28:58,420 --> 00:29:04,210
On line 6 of hello.c I had an
undeclared identifier string,

643
00:29:04,210 --> 00:29:05,590
did I mean standard in?

644
00:29:05,590 --> 00:29:06,750
I didn't.

645
00:29:06,750 --> 00:29:10,050
Because what mistake have I made in this
program, if you recall from last time?

646
00:29:10,050 --> 00:29:13,219

647
00:29:13,219 --> 00:29:15,010
Yeah, we need the
so-called training wheels

648
00:29:15,010 --> 00:29:16,510
that we'll just use for a few weeks.

649
00:29:16,510 --> 00:29:19,480
But I need to specify that also,
somewhere inside of the appliance,

650
00:29:19,480 --> 00:29:22,720
is a file that we wrote
just a few years ago.

651
00:29:22,720 --> 00:29:26,500
Inside of which are functions
like apparently GetString.

652
00:29:26,500 --> 00:29:35,160
So now if I go back down here,
zoom in, and re-run clang hello.c.

653
00:29:35,160 --> 00:29:36,500
Damn, another error.

654
00:29:36,500 --> 00:29:38,590
But we haven't seen this one before.

655
00:29:38,590 --> 00:29:41,900
>> This one's a little more
esoteric to figure out.

656
00:29:41,900 --> 00:29:46,910
But this is deliberately on the screen,
because we wanted to tell this story.

657
00:29:46,910 --> 00:29:51,280
When you compile hello.c, just as
the picture from before suggested,

658
00:29:51,280 --> 00:29:54,650
you're only converting that
source code to zeros and ones.

659
00:29:54,650 --> 00:30:00,340
Now CS50 staff a few years ago wrote
CS50.h, and a corresponding file,

660
00:30:00,340 --> 00:30:01,640
CS50.c.

661
00:30:01,640 --> 00:30:04,490
And we, a few years ago,
compiled those files

662
00:30:04,490 --> 00:30:07,290
into a file that happens
to be called CS50.o.

663
00:30:07,290 --> 00:30:09,180
>> Or it can be renamed a
few different things.

664
00:30:09,180 --> 00:30:11,054
But that's a simple way
of thinking about it.

665
00:30:11,054 --> 00:30:14,960
So we compiled CS50's
library into zeros and ones.

666
00:30:14,960 --> 00:30:20,340
But nowhere have I specified
that I want to combine my zeroes

667
00:30:20,340 --> 00:30:24,030
and ones for my hello world
program with the zeros and ones

668
00:30:24,030 --> 00:30:29,760
that CS50 staff created a few years
ago into one complete program.

669
00:30:29,760 --> 00:30:34,100
All I've specified by writing
clang hello.c is compile hello.c.

670
00:30:34,100 --> 00:30:38,380
>> I've not told clang to
link in the zeros and ones

671
00:30:38,380 --> 00:30:41,650
that CS50 staff created
for you some time ago.

672
00:30:41,650 --> 00:30:48,280
So it's an easy fix,
dash l for link, CS50.

673
00:30:48,280 --> 00:30:52,600
And we'll again see this
before long again and again.

674
00:30:52,600 --> 00:30:54,420
But notice now there was no complaints.

675
00:30:54,420 --> 00:30:59,770
So now if I run dot slash
hello, now it's working.

676
00:30:59,770 --> 00:31:03,840
Although that's a bit misleading
since I just recreated what file?

677
00:31:03,840 --> 00:31:05,880
a.out.

678
00:31:05,880 --> 00:31:07,930
>> So let me rewind for just a moment.

679
00:31:07,930 --> 00:31:10,900
The program we just compiled
has the three lines of code.

680
00:31:10,900 --> 00:31:14,710
When I ran hello world, hello a moment
ago, I didn't see state your name.

681
00:31:14,710 --> 00:31:17,080
And that's because I ran the
old version of the program.

682
00:31:17,080 --> 00:31:22,700
But if I do indeed run dot slash a.out
Enter, state your name, Rob, hello,

683
00:31:22,700 --> 00:31:23,370
world.

684
00:31:23,370 --> 00:31:23,870
Hm.

685
00:31:23,870 --> 00:31:26,110
That's kind of a bug.

686
00:31:26,110 --> 00:31:28,560
Probably meant to say Rob, right?

687
00:31:28,560 --> 00:31:30,997
So what's the fix here in code.

688
00:31:30,997 --> 00:31:31,872
STUDENT: [INAUDIBLE].

689
00:31:31,872 --> 00:31:38,415

690
00:31:38,415 --> 00:31:39,290
DAVID MALAN: Exactly.

691
00:31:39,290 --> 00:31:43,160
So I just need to make that final tweak
so that I'm passing in a placeholder,

692
00:31:43,160 --> 00:31:45,690
like a fill in the blank,
like we did a moment ago.

693
00:31:45,690 --> 00:31:47,870
But percent s is now
that fill in the blank.

694
00:31:47,870 --> 00:31:52,100
So lastly let's reveal one last
detail so that there's no magic.

695
00:31:52,100 --> 00:31:54,090
I'm going to go ahead
and get rid of hellow.

696
00:31:54,090 --> 00:31:56,070
I'm going to go ahead
and get rid of a.out.

697
00:31:56,070 --> 00:31:58,410
So now I have no programs,
just my source code.

698
00:31:58,410 --> 00:32:01,100
And I'm going to run a
slightly longer command, clang

699
00:32:01,100 --> 00:32:05,650
dash o hello hello.c dash LCS50.

700
00:32:05,650 --> 00:32:08,700
>> Now this is starting to sort of go
in one ear and out the other perhaps.

701
00:32:08,700 --> 00:32:10,366
But just tease apart what this is doing.

702
00:32:10,366 --> 00:32:14,910
Clang is the compiler, dash LCS50
deliberately at the very end

703
00:32:14,910 --> 00:32:18,330
does what in a sentence?

704
00:32:18,330 --> 00:32:19,607
Links to the CS50 library.

705
00:32:19,607 --> 00:32:21,440
Grabs the zeros and
ones that the staff made

706
00:32:21,440 --> 00:32:23,310
and crams them into my own program.

707
00:32:23,310 --> 00:32:27,040
So the last question at hand is,
what is dash o hello probably

708
00:32:27,040 --> 00:32:30,530
doing, even if you've never
seen this syntax before?

709
00:32:30,530 --> 00:32:34,920
>> Outputting a program that's
not named the default a.out.

710
00:32:34,920 --> 00:32:37,330
Rather it's named hello.

711
00:32:37,330 --> 00:32:39,460
So now there is no a.out.

712
00:32:39,460 --> 00:32:42,660
No such file or directory,
because I explicitly said

713
00:32:42,660 --> 00:32:46,720
call this program hello, so that
I can now type in a name like that

714
00:32:46,720 --> 00:32:47,790
and have it behave.

715
00:32:47,790 --> 00:32:50,180
>> Now frankly, boy is that
uninteresting to have

716
00:32:50,180 --> 00:32:52,170
to remember all of that
little minutia, right?

717
00:32:52,170 --> 00:32:54,180
So let's go ahead and
get rid of hello again.

718
00:32:54,180 --> 00:32:57,880
And let's now return to a world in
which it suffices to say make hello.

719
00:32:57,880 --> 00:33:00,410
But that's the magic that
make is doing for you,

720
00:33:00,410 --> 00:33:02,320
that's the tedium that
it's doing for you.

721
00:33:02,320 --> 00:33:04,630
And as our programs and
problems get more complex,

722
00:33:04,630 --> 00:33:06,142
make will truly be your friend.

723
00:33:06,142 --> 00:33:07,850
Because not too long
from now we're going

724
00:33:07,850 --> 00:33:11,150
to have programs that are written
not with one file, but several files,

725
00:33:11,150 --> 00:33:16,310
and make will automate the process of
compiling all of that for us together.

726
00:33:16,310 --> 00:33:20,620
>> So now print.

727
00:33:20,620 --> 00:33:24,960
printf is inside of this file, standard
I/O dot h, that we've seen before,

728
00:33:24,960 --> 00:33:26,980
but there's more to printf than that.

729
00:33:26,980 --> 00:33:30,482
There's backslash n, but there's also
a bunch of other escape sequences.

730
00:33:30,482 --> 00:33:32,940
And escape sequence is just a
fancy way of saying something

731
00:33:32,940 --> 00:33:35,990
that starts with a backslash,
not a forward slash, a backslash,

732
00:33:35,990 --> 00:33:37,920
and does some special thing.

733
00:33:37,920 --> 00:33:43,770
And we won't dwell on most of these,
but why is there backslash double quote,

734
00:33:43,770 --> 00:33:44,830
do you think?

735
00:33:44,830 --> 00:33:48,010
Why is there this weird sequence of
characters, backslash double quote,

736
00:33:48,010 --> 00:33:50,505
why might that be useful?

737
00:33:50,505 --> 00:33:51,380
STUDENT: [INAUDIBLE].

738
00:33:51,380 --> 00:33:56,594

739
00:33:56,594 --> 00:33:57,760
DAVID MALAN: Exactly, right?

740
00:33:57,760 --> 00:34:01,830
Think back to our hello world program
that we've seen quite a few times now,

741
00:34:01,830 --> 00:34:05,880
every time we've seen that hello world
program we've had double quotes inside

742
00:34:05,880 --> 00:34:08,210
of which is hello comma
world backslash n.

743
00:34:08,210 --> 00:34:10,070
But think to yourself,
what if you actually

744
00:34:10,070 --> 00:34:12,520
wanted to print out a quotation mark?

745
00:34:12,520 --> 00:34:16,100
Your first instinct might be, I don't
know why I might want to do-- here,

746
00:34:16,100 --> 00:34:18,780
we can be a little passive
aggressive, hello friend.

747
00:34:18,780 --> 00:34:21,350
We might want to do something like that.

748
00:34:21,350 --> 00:34:25,230
>> But why is this now problematic?

749
00:34:25,230 --> 00:34:28,730
Ignoring the curly quotes aside that
keynote made for us automatically.

750
00:34:28,730 --> 00:34:29,730
Why is this problematic?

751
00:34:29,730 --> 00:34:36,719

752
00:34:36,719 --> 00:34:37,270
>> Exactly.

753
00:34:37,270 --> 00:34:39,022
Because we have like 4 quotation marks.

754
00:34:39,022 --> 00:34:41,230
Well the first one probably
goes with the second one,

755
00:34:41,230 --> 00:34:43,271
and maybe the fourth one
goes with the third one,

756
00:34:43,271 --> 00:34:46,170
or, I mean I'm not even sure,
nor is the computer going to be.

757
00:34:46,170 --> 00:34:48,352
Computers again, compilers
again, are fairly dumb.

758
00:34:48,352 --> 00:34:50,060
They'll only do what
you tell them to do.

759
00:34:50,060 --> 00:34:54,270
And if you're not unambiguous they
are probably going to throw an error.

760
00:34:54,270 --> 00:34:57,700
>> If you're not unambiguous as to how to
make a peanut butter and jelly sandwich

761
00:34:57,700 --> 00:34:59,510
strange things might arrive.

762
00:34:59,510 --> 00:35:00,580
It's the same idea.

763
00:35:00,580 --> 00:35:03,100
Now suffice it to say there's
some other escape sequences,

764
00:35:03,100 --> 00:35:04,820
but we'll come to those before long.

765
00:35:04,820 --> 00:35:07,410
Essentially these escape
sequences represent something

766
00:35:07,410 --> 00:35:10,480
that you might not be able
too easily at the keyboard,

767
00:35:10,480 --> 00:35:12,010
without confusing the computer.

768
00:35:12,010 --> 00:35:15,733
>> Now meanwhile we have
placeholders in printf.

769
00:35:15,733 --> 00:35:19,630
Percent d, which can also be written
in almost all cases as percent i,

770
00:35:19,630 --> 00:35:21,610
is a placeholder for what type of data?

771
00:35:21,610 --> 00:35:23,920
What type of value?

772
00:35:23,920 --> 00:35:24,920
A decimal number.

773
00:35:24,920 --> 00:35:27,234
So actually we'll start
using percent i because it's

774
00:35:27,234 --> 00:35:28,650
a little simpler, like an integer.

775
00:35:28,650 --> 00:35:29,550
Percent i.

776
00:35:29,550 --> 00:35:31,320
An integer that happens to be decimal.

777
00:35:31,320 --> 00:35:35,690
>> Percent s we already said was
a placeholder for a string.

778
00:35:35,690 --> 00:35:38,630
Which is just a word, a phrase,
a paragraph, an essay, whatever.

779
00:35:38,630 --> 00:35:40,830
It's a sequence of
characters of some length.

780
00:35:40,830 --> 00:35:42,820
And we'll see before long
some of these others.

781
00:35:42,820 --> 00:35:45,950
But percent c is for a single character.

782
00:35:45,950 --> 00:35:48,500
Percent f is for a
floating point number,

783
00:35:48,500 --> 00:35:50,860
like a real number that has
a decimal point in it, which

784
00:35:50,860 --> 00:35:52,550
is of course not an integer.

785
00:35:52,550 --> 00:35:54,670
And there's a whole
bunch of others as well.

786
00:35:54,670 --> 00:35:56,990
>> Meanwhile in C, and
in a lot of languages,

787
00:35:56,990 --> 00:35:58,650
we have different data types.

788
00:35:58,650 --> 00:36:00,650
Different types of
glass bowls if you will.

789
00:36:00,650 --> 00:36:03,830
The bowl I used last time
was for ping pong balls,

790
00:36:03,830 --> 00:36:06,990
but we can also store different
values like chars and strings

791
00:36:06,990 --> 00:36:10,820
and integers in a container,
like a variable, and C has these.

792
00:36:10,820 --> 00:36:14,020
Char is the type of data,
the type of variable,

793
00:36:14,020 --> 00:36:15,700
in which you can store a character.

794
00:36:15,700 --> 00:36:19,560
Float is a type of variable in
which you can store a real number.

795
00:36:19,560 --> 00:36:21,220
>> Int is of course for an int.

796
00:36:21,220 --> 00:36:23,760
And rather ridiculously
named a long long

797
00:36:23,760 --> 00:36:26,830
is where you can store essentially
a really long number, with even

798
00:36:26,830 --> 00:36:29,930
more digits then a
typical int might store.

799
00:36:29,930 --> 00:36:32,420
But we'll come back to that before long.

800
00:36:32,420 --> 00:36:35,520
>> In CS50 dot h meanwhile
the other header file

801
00:36:35,520 --> 00:36:37,630
that we've seen a couple
times in use, there's

802
00:36:37,630 --> 00:36:41,350
two other data types that
don't exist in C typically.

803
00:36:41,350 --> 00:36:45,000
One is string, and we'll see in
a few weeks what strings really

804
00:36:45,000 --> 00:36:46,610
are underneath the hood.

805
00:36:46,610 --> 00:36:47,950
And one is bool.

806
00:36:47,950 --> 00:36:51,650
And a bool is a variable that
can only take on certain values.

807
00:36:51,650 --> 00:36:54,370
And just based on last week's
explanation of Scratch,

808
00:36:54,370 --> 00:36:56,680
and more recently C,
what would you guess

809
00:36:56,680 --> 00:37:01,770
are the two possible values for
some container that is of type bool?

810
00:37:01,770 --> 00:37:02,860
>> Yes and no.

811
00:37:02,860 --> 00:37:03,870
One and zero.

812
00:37:03,870 --> 00:37:04,502
True or false.

813
00:37:04,502 --> 00:37:07,460
And indeed it's the last that tends
to be conventional in a programming

814
00:37:07,460 --> 00:37:10,030
language, saying something
like true or false.

815
00:37:10,030 --> 00:37:13,160
>> Meanwhile the CS50 library comes
with a bunch of functionality

816
00:37:13,160 --> 00:37:16,640
that the staff wrote for you,
besides just getting a string.

817
00:37:16,640 --> 00:37:20,910
We wrote in advance functions that
can get an integer from the user,

818
00:37:20,910 --> 00:37:22,990
get a single character,
get a float, that

819
00:37:22,990 --> 00:37:25,160
is just a number with the decimal point.

820
00:37:25,160 --> 00:37:28,290
Get a long long, a really
big value from the user.

821
00:37:28,290 --> 00:37:30,930
So we've only seen this in
action in the form of GetString.

822
00:37:30,930 --> 00:37:33,230
But via these functions
will soon be able to write

823
00:37:33,230 --> 00:37:36,070
programs that get input from the user.

824
00:37:36,070 --> 00:37:39,230
>> Meanwhile consider this
program, and consider

825
00:37:39,230 --> 00:37:42,600
how we might now start to add
to it with new constructs.

826
00:37:42,600 --> 00:37:44,530
A quick whirlwind tour
and then we'll look

827
00:37:44,530 --> 00:37:47,340
at some writing some
programs manually ourselves.

828
00:37:47,340 --> 00:37:49,700
Henceforth if we want to
implement a condition,

829
00:37:49,700 --> 00:37:52,790
it's not going to look like a
nice little pretty puzzle piece.

830
00:37:52,790 --> 00:37:54,460
It's going to look a little more arcane.

831
00:37:54,460 --> 00:37:57,620
But this is the canonical
structure of a condition in C.

832
00:37:57,620 --> 00:38:00,380
>> The word if, two
parentheses, inside of which

833
00:38:00,380 --> 00:38:02,590
is going to be some kind
of Boolean expression.

834
00:38:02,590 --> 00:38:04,999
Now the slash slash
in the middle, this is

835
00:38:04,999 --> 00:38:07,290
at the moment sort of like
pseudocode code placeholder.

836
00:38:07,290 --> 00:38:11,576
But more properly, any line of code
in C that starts with slash slash

837
00:38:11,576 --> 00:38:12,850
is a comment.

838
00:38:12,850 --> 00:38:15,920
>> It's like a sticky note that you might
put on an essay, printed on paper.

839
00:38:15,920 --> 00:38:17,290
It's a note to yourself.

840
00:38:17,290 --> 00:38:20,100
It has no functional
impact on the program.

841
00:38:20,100 --> 00:38:23,580
It's sort of a reminder, it's your own
documentation as to what's going on.

842
00:38:23,580 --> 00:38:25,720
So do this at the moment
is sort of a placeholder

843
00:38:25,720 --> 00:38:27,560
for whatever's inside
of those curly braces

844
00:38:27,560 --> 00:38:29,810
eventually is going to do something.

845
00:38:29,810 --> 00:38:35,160
>> Meanwhile you have two forks in the
road, an if else construct like this.

846
00:38:35,160 --> 00:38:37,024
You can if, elseif, else.

847
00:38:37,024 --> 00:38:39,690
Now it's worth noting, especially
if you've been Googling around

848
00:38:39,690 --> 00:38:42,480
online for resources out of
curiosity, or if you pick up

849
00:38:42,480 --> 00:38:44,575
one of the course's
recommended books on C,

850
00:38:44,575 --> 00:38:48,430
you will see that humans have lots
of different preferences for how

851
00:38:48,430 --> 00:38:50,510
to express themselves in code.

852
00:38:50,510 --> 00:38:53,800
Some people like to put the
curly braces as I've done here.

853
00:38:53,800 --> 00:38:57,760
Some people like to put the curly
braces for instance up here.

854
00:38:57,760 --> 00:39:01,700
>> Some people like to put the elses
up here and then this over here.

855
00:39:01,700 --> 00:39:05,120
There's any number of ways to
write code, and none of them

856
00:39:05,120 --> 00:39:06,566
are right per se.

857
00:39:06,566 --> 00:39:08,440
Although there's a few
that are pretty wrong.

858
00:39:08,440 --> 00:39:11,315
But there are no-- if this is
ultimately a very religious debate.

859
00:39:11,315 --> 00:39:13,940
In fact, one of the most annoying
things about computer science

860
00:39:13,940 --> 00:39:16,310
is how opinionated
computer scientists can be.

861
00:39:16,310 --> 00:39:19,880
And you will find over time that
even you develop personal preferences

862
00:39:19,880 --> 00:39:22,130
as to how your code should look.

863
00:39:22,130 --> 00:39:27,162
>> Not just behave, not just create output,
but how it should look aesthetically.

864
00:39:27,162 --> 00:39:30,120
So what we will do is guide you along
the way toward, for now at least,

865
00:39:30,120 --> 00:39:34,825
a standard CS50 style that at least
keeps things uniform and easy to read.

866
00:39:34,825 --> 00:39:37,330
But more on those
distinctions before long.

867
00:39:37,330 --> 00:39:41,830
>> A Boolean expression in C, if you
wanted to say if this condition is true

868
00:39:41,830 --> 00:39:44,650
and this other condition is
true, you don't write the word

869
00:39:44,650 --> 00:39:46,610
and, as you would in English.

870
00:39:46,610 --> 00:39:50,580
You instead use ampersand ampersand,
for reasons we'll come back to.

871
00:39:50,580 --> 00:39:54,520
If you want to say or you use two
vertical bars, which on a US keyboard

872
00:39:54,520 --> 00:39:57,650
is generally above the Enter
key next to the backslash.

873
00:39:57,650 --> 00:40:03,080
And this just means if this condition
or this other condition is true do this.

874
00:40:03,080 --> 00:40:05,210
>> Meanwhile there's this
crazy thing, which

875
00:40:05,210 --> 00:40:07,760
you'll have occasion
to use once in awhile.

876
00:40:07,760 --> 00:40:10,390
But it's just an alternative
way to express yourself.

877
00:40:10,390 --> 00:40:12,400
This, for now, let's
stipulate, it's just

878
00:40:12,400 --> 00:40:16,880
another funky way of expressing
if, elseif, elseif, elseif, elseif.

879
00:40:16,880 --> 00:40:19,720
It's just formatted in terms
of cases, where you literally

880
00:40:19,720 --> 00:40:23,420
enumerate the values that
you might want to check for.

881
00:40:23,420 --> 00:40:24,410
>> Meanwhile loops.

882
00:40:24,410 --> 00:40:26,842
We saw a comparison of
Scratch with a loop.

883
00:40:26,842 --> 00:40:29,550
And I'm going to wave my hand at
what each of these things means.

884
00:40:29,550 --> 00:40:33,520
But notice that a loop will very
often start with the keyword for.

885
00:40:33,520 --> 00:40:36,350
Or it will start with the keyword while.

886
00:40:36,350 --> 00:40:41,360
Or it will start with the keyword
do, and end with the keyword while.

887
00:40:41,360 --> 00:40:43,920
>> But what we're about to see
is things more like this.

888
00:40:43,920 --> 00:40:47,950
Here is, in general in C, how you might
declare a variable and give it a value.

889
00:40:47,950 --> 00:40:50,610
In C it doesn't suffice,
like in Scratch, you just

890
00:40:50,610 --> 00:40:52,810
say give me a variable called something.

891
00:40:52,810 --> 00:40:55,960
You have to tell C, and tell
your compiler, more specifically,

892
00:40:55,960 --> 00:40:58,680
what type of value do you
want to put in the glass bowl.

893
00:40:58,680 --> 00:40:59,610
>> Is it an integer?

894
00:40:59,610 --> 00:41:02,210
If so, you have to
literally say int and then

895
00:41:02,210 --> 00:41:04,220
the name of the variable,
then a semicolon.

896
00:41:04,220 --> 00:41:08,050
If you instead wanted string and called
it s like I did my code here today,

897
00:41:08,050 --> 00:41:10,920
you would say string s semicolon.

898
00:41:10,920 --> 00:41:14,200
Once you want to give it a value
you can then use the equal sign,

899
00:41:14,200 --> 00:41:16,211
otherwise known as the
assignment operator.

900
00:41:16,211 --> 00:41:17,960
And notice how each
of these lines of code

901
00:41:17,960 --> 00:41:20,650
has the semicolon at the end of it.

902
00:41:20,650 --> 00:41:22,550
>> But frankly this just
looks a little ugly.

903
00:41:22,550 --> 00:41:26,580
Why can't I say to the computer, give
me a variable and initialize it to zero.

904
00:41:26,580 --> 00:41:28,380
That is, give me an empty glass bowl.

905
00:41:28,380 --> 00:41:28,980
While you can.

906
00:41:28,980 --> 00:41:32,250
And so one of the judgment calls you'll
have to make stylistically over time

907
00:41:32,250 --> 00:41:36,290
is, do you write your code like this, or
do you kind of clean it up and impress

908
00:41:36,290 --> 00:41:39,040
people by writing two lines as just one.

909
00:41:39,040 --> 00:41:41,830
And this would be considered
generally a more elegant way

910
00:41:41,830 --> 00:41:43,870
of writing your code stylistically.

911
00:41:43,870 --> 00:41:45,200
>> Functions we're of course seen.

912
00:41:45,200 --> 00:41:46,340
They're going to take this format.

913
00:41:46,340 --> 00:41:48,881
The name of a function, some
parentheses, and then maybe some

914
00:41:48,881 --> 00:41:49,910
stuff inside.

915
00:41:49,910 --> 00:41:52,540
Or you might see uses of
multiple functions side

916
00:41:52,540 --> 00:41:55,980
by side, like something like this.

917
00:41:55,980 --> 00:41:59,390
Now as a tangent, does
this joke now make sense?

918
00:41:59,390 --> 00:42:03,566

919
00:42:03,566 --> 00:42:04,960
>> [LAUGHTER]

920
00:42:04,960 --> 00:42:09,360
It doesn't mean it's funny, but it
does perhaps make a little more sense.

921
00:42:09,360 --> 00:42:12,140
So now let's go into the
CS50 appliance for a moment.

922
00:42:12,140 --> 00:42:14,640
And let me open up one teaser here.

923
00:42:14,640 --> 00:42:17,470
>> And indeed one of the reasons
for introducing IKEA catalog

924
00:42:17,470 --> 00:42:20,080
and flashing the picture
of the iPhone a bit ago

925
00:42:20,080 --> 00:42:24,700
was to actually tie in together the
fact that just a few years ago, in 2007,

926
00:42:24,700 --> 00:42:26,540
this piece of code was
released, which was

927
00:42:26,540 --> 00:42:29,820
one of the very first implementation
of a piece of software

928
00:42:29,820 --> 00:42:31,494
that does what's called jailbreaking.

929
00:42:31,494 --> 00:42:34,410
For those unfamiliar, jailbreaking
something like a phone or an iPhone

930
00:42:34,410 --> 00:42:37,680
means essentially figuring
out how to do things

931
00:42:37,680 --> 00:42:39,864
with it that the company
who made it didn't intend.

932
00:42:39,864 --> 00:42:42,780
Like you want to install certain
software that's not in the app store,

933
00:42:42,780 --> 00:42:45,405
jailbreaking your phone which
you do things like that and more.

934
00:42:45,405 --> 00:42:47,447
But what was fun in
2007-- and there's been

935
00:42:47,447 --> 00:42:49,780
dozens of different versions
of these things since then,

936
00:42:49,780 --> 00:42:53,540
in all sorts of languages-- is that
this program here, called iUnlock,

937
00:42:53,540 --> 00:42:57,036
was actually written in C. And so I
pulled up the source code for this,

938
00:42:57,036 --> 00:42:59,910
just to kind of show you some of
the similarities with the constructs

939
00:42:59,910 --> 00:43:01,450
we've been discussing thus far.

940
00:43:01,450 --> 00:43:05,150
Now there's way more complexity here
than we've seen already in class.

941
00:43:05,150 --> 00:43:07,160
Let me scroll up just a little bit here.

942
00:43:07,160 --> 00:43:11,520
>> But notice these-- well we notice a few
things-- but notice these lines here,

943
00:43:11,520 --> 00:43:14,090
including standard I/O dot h.

944
00:43:14,090 --> 00:43:17,660
And all-- line 24, please--
notice standard I/O

945
00:43:17,660 --> 00:43:21,380
dot h and a whole bunch of other
files that apparently other people

946
00:43:21,380 --> 00:43:22,160
have written.

947
00:43:22,160 --> 00:43:24,850
And if we scroll down through
this, little arbitrarily,

948
00:43:24,850 --> 00:43:27,910
notice I don't know how to write
all of this myself necessarily yet,

949
00:43:27,910 --> 00:43:28,880
but there's if.

950
00:43:28,880 --> 00:43:30,400
So apparently this has a branch.

951
00:43:30,400 --> 00:43:33,210
>> And if I scroll down a little
further there's another if.

952
00:43:33,210 --> 00:43:38,210
If I scroll up, I think
around line 100 I saw for.

953
00:43:38,210 --> 00:43:40,020
So I'm not quite sure
how this works yet,

954
00:43:40,020 --> 00:43:43,180
but this is a for loop that's going to
let me iterate some number of times.

955
00:43:43,180 --> 00:43:45,230
And if I scroll all
the way to the bottom

956
00:43:45,230 --> 00:43:51,020
I see that there's a
function down here called

957
00:43:51,020 --> 00:43:54,570
main, and some additional credits
for your reading pleasure.

958
00:43:54,570 --> 00:43:57,830
>> So this is only to say-- and we'll put
this online among the course's source

959
00:43:57,830 --> 00:44:01,480
code today-- that even some of these
devices that we take for granted,

960
00:44:01,480 --> 00:44:05,130
at the end of the day you're still
using these basic constructs that we've

961
00:44:05,130 --> 00:44:07,430
been introducing already thus far.

962
00:44:07,430 --> 00:44:08,620
That's enough of that one.

963
00:44:08,620 --> 00:44:11,480
All right, so now let's
actually build something.

964
00:44:11,480 --> 00:44:15,640
>> Let's go beyond something like
hello.c, and open up this,

965
00:44:15,640 --> 00:44:17,170
let's write this program here.

966
00:44:17,170 --> 00:44:19,850
I'm going to go ahead and
write a program called adder.c,

967
00:44:19,850 --> 00:44:23,310
whose purpose in life is going to be
like my first super simple calculator

968
00:44:23,310 --> 00:44:24,840
that just adds a couple of numbers.

969
00:44:24,840 --> 00:44:27,050
Not because that's a hard
problem, but because it

970
00:44:27,050 --> 00:44:29,390
allows me to now tie
together a few things.

971
00:44:29,390 --> 00:44:33,100
So I'm going to go ahead and include
standard I/O dot h and CS50 dot h.

972
00:44:33,100 --> 00:44:34,670
I'm going to say int main void.

973
00:44:34,670 --> 00:44:36,545
And again in the future
we'll come back to it

974
00:44:36,545 --> 00:44:38,560
into in this context and void means.

975
00:44:38,560 --> 00:44:43,340
>> And now I'm going to say something
like printf, give me an integer.

976
00:44:43,340 --> 00:44:47,610
And now on my second line,
how do I express myself such

977
00:44:47,610 --> 00:44:50,980
that I want to declare a variable
that's going to store an int?

978
00:44:50,980 --> 00:44:55,470
Literally what do I type if I want to
create a variable that stores an int

979
00:44:55,470 --> 00:44:58,270
would you say?

980
00:44:58,270 --> 00:44:59,650
int a equals getint.

981
00:44:59,650 --> 00:45:00,485
Sure, so that works.

982
00:45:00,485 --> 00:45:02,360
And just for consistency
with the code you'll

983
00:45:02,360 --> 00:45:04,485
see online, let me change
this to x, just because x

984
00:45:04,485 --> 00:45:06,830
is a common variable name
to use, at least in algebra.

985
00:45:06,830 --> 00:45:08,720
Now let me go ahead and do printf again.

986
00:45:08,720 --> 00:45:10,692
Give me another integer.

987
00:45:10,692 --> 00:45:12,900
And now I'm going to kind
of learn from that pattern,

988
00:45:12,900 --> 00:45:14,490
I'm going to say int y gets to GetInt.

989
00:45:14,490 --> 00:45:16,340
>> And we've not used
GetInt before, but just

990
00:45:16,340 --> 00:45:19,860
like GetString goes and gets a string,
like Javier did from the audience,

991
00:45:19,860 --> 00:45:22,780
similarly does GetInt
start blinking the prompt.

992
00:45:22,780 --> 00:45:26,750
And effectively waiting for the user
to give it an integer, by typing it in

993
00:45:26,750 --> 00:45:27,900
and hitting Enter.

994
00:45:27,900 --> 00:45:30,850
Now lastly I'm going to do
this, printf the sum of--

995
00:45:30,850 --> 00:45:36,080
and let me get fancy now--
percent i and percent i is present

996
00:45:36,080 --> 00:45:39,270
i exclamation point backslash n.

997
00:45:39,270 --> 00:45:42,750
And what do you think I'm going to
put in now as the additional inputs

998
00:45:42,750 --> 00:45:44,620
to printf?

999
00:45:44,620 --> 00:45:47,380
>> I don't want to say
literally quote unquote x.

1000
00:45:47,380 --> 00:45:52,210
I want to class in x and
then, and now let's get fancy

1001
00:45:52,210 --> 00:45:54,420
and let's just assume
we can do what we want.

1002
00:45:54,420 --> 00:45:56,330
What you want to say next?

1003
00:45:56,330 --> 00:45:57,070
x plus y.

1004
00:45:57,070 --> 00:45:58,520
And indeed that will work.

1005
00:45:58,520 --> 00:46:01,760
It's minimally programming languages
do understand basic arithmetic,

1006
00:46:01,760 --> 00:46:03,500
and so this should in fact work.

1007
00:46:03,500 --> 00:46:06,730
>> So let me go ahead and do make
adder in my black and white window

1008
00:46:06,730 --> 00:46:08,090
down here, Enter.

1009
00:46:08,090 --> 00:46:10,830
Cryptic line but no error
message, so that's good.

1010
00:46:10,830 --> 00:46:14,410
Dot slash adder, give me an integer, 1.

1011
00:46:14,410 --> 00:46:16,230
Give me another integer, 2.

1012
00:46:16,230 --> 00:46:18,770
The sum of 1 and 2 is 3.

1013
00:46:18,770 --> 00:46:21,730
>> All right, so a pretty trivial
program, but what's it done?

1014
00:46:21,730 --> 00:46:25,680
It's allowed me to use a function
call here, call another function here,

1015
00:46:25,680 --> 00:46:29,260
two calls to GetInt, a
third call to printf,

1016
00:46:29,260 --> 00:46:30,720
and then using these placeholders.

1017
00:46:30,720 --> 00:46:33,160
So it's a little more
sophisticated than last time.

1018
00:46:33,160 --> 00:46:35,260
But what if I want to
get a little fancier?

1019
00:46:35,260 --> 00:46:38,900
>> Let me go ahead and now do this.

1020
00:46:38,900 --> 00:46:42,890
Let me go ahead and create a
new file called conditions zero.

1021
00:46:42,890 --> 00:46:44,890
And I'm going to save
some time for classes sake

1022
00:46:44,890 --> 00:46:48,260
and just paste in that
code as a starting point.

1023
00:46:48,260 --> 00:46:51,510
And now I'm just going to put
something new here in the middle.

1024
00:46:51,510 --> 00:46:55,889
So printf I'd like an integer please.

1025
00:46:55,889 --> 00:46:58,180
And all this code is available
on the course's website,

1026
00:46:58,180 --> 00:47:00,556
you needn't type it all out
in class if you'd rather not.

1027
00:47:00,556 --> 00:47:02,638
int, and now I'm going to
use, instead of x and y,

1028
00:47:02,638 --> 00:47:05,270
I'm going to use n, which is
probably the most common name

1029
00:47:05,270 --> 00:47:09,220
for an integer in a computer program.

1030
00:47:09,220 --> 00:47:12,060
And now I want to do a
little judgment here.

1031
00:47:12,060 --> 00:47:15,460
I'm going to see if n
is greater than 0, then

1032
00:47:15,460 --> 00:47:23,670
I'm going to say printf-- whoops,
printf-- you picked a positive number,

1033
00:47:23,670 --> 00:47:25,370
exclamation point backslash n.

1034
00:47:25,370 --> 00:47:26,460
Close that.

1035
00:47:26,460 --> 00:47:33,150
Else I'm going to print out printf
you picked a negative number.

1036
00:47:33,150 --> 00:47:35,377
>> Now before you yell at me,
let's run this program.

1037
00:47:35,377 --> 00:47:36,210
Let me go down here.

1038
00:47:36,210 --> 00:47:40,730
And how do I compile a program
called conditions dash 0?

1039
00:47:40,730 --> 00:47:42,300
Make conditions dash 0.

1040
00:47:42,300 --> 00:47:45,560
Kind of a stupid name, but-- dammit.

1041
00:47:45,560 --> 00:47:47,560
OK, good teaching moment.

1042
00:47:47,560 --> 00:47:48,410
Why did that break?

1043
00:47:48,410 --> 00:47:51,360

1044
00:47:51,360 --> 00:47:54,710
>> What did I accidentally call this file?

1045
00:47:54,710 --> 00:47:56,876
You can kind of infer
from the tab up here.

1046
00:47:56,876 --> 00:47:58,750
So it's not bad that I
did this, because this

1047
00:47:58,750 --> 00:48:00,208
will happen perhaps to some of you.

1048
00:48:00,208 --> 00:48:03,880
So I accidentally named my
source code condition 0.

1049
00:48:03,880 --> 00:48:09,950
But my source code by convention should
be in a file called condition 0 dash c,

1050
00:48:09,950 --> 00:48:11,720
or whatever dot c.

1051
00:48:11,720 --> 00:48:12,610
>> So let me fix this.

1052
00:48:12,610 --> 00:48:16,880
Just like Microsoft Word I can go up
to Save As and do condition 0 dot c.

1053
00:48:16,880 --> 00:48:20,100
Now just to be tidy I'm
going to go ahead and remove

1054
00:48:20,100 --> 00:48:23,020
condition 0, which was my old version.

1055
00:48:23,020 --> 00:48:26,550
And now let's do make
condition 0, Enter.

1056
00:48:26,550 --> 00:48:27,870
OK, seems good.

1057
00:48:27,870 --> 00:48:29,510
>> Condition 0, Enter.

1058
00:48:29,510 --> 00:48:33,530
I'd like an integer please, 50,
you picked a positive number.

1059
00:48:33,530 --> 00:48:35,390
Now how about negative 50?

1060
00:48:35,390 --> 00:48:37,660
You picked a negative number.

1061
00:48:37,660 --> 00:48:39,605
How about now, 0.

1062
00:48:39,605 --> 00:48:42,250

1063
00:48:42,250 --> 00:48:45,350
Pretty sure 0's not negative.

1064
00:48:45,350 --> 00:48:47,780
>> So this is kind of a logical bug.

1065
00:48:47,780 --> 00:48:50,680
How do I fix this?

1066
00:48:50,680 --> 00:48:57,260
Else if n is less than 0, do this, else
we can have a three way branch here,

1067
00:48:57,260 --> 00:49:01,740
printf you picked 0.

1068
00:49:01,740 --> 00:49:06,150
So now if I rerun this
program, after compiling it

1069
00:49:06,150 --> 00:49:10,790
again with make, and now rerun
condition 0, what I'll see here

1070
00:49:10,790 --> 00:49:14,570
is 0 gives me that output.

1071
00:49:14,570 --> 00:49:16,280
So where are we going with this?

1072
00:49:16,280 --> 00:49:18,930
>> Well it's very easy to lose sight
of the forest for the trees.

1073
00:49:18,930 --> 00:49:23,590
And so allow us to conclude
with this 2 and 1/2 minute video

1074
00:49:23,590 --> 00:49:25,710
that ultimately is a bit
of an ad from Google,

1075
00:49:25,710 --> 00:49:28,850
but that really is testament to
what even using these basic building

1076
00:49:28,850 --> 00:49:33,020
blocks we can ultimately do once
we have a bit of programming

1077
00:49:33,020 --> 00:49:34,463
savvy under our belts.

1078
00:49:34,463 --> 00:49:35,150
>> [MUSIC PLAYING]

1079
00:49:35,150 --> 00:49:38,370

1080
00:49:38,370 --> 00:49:40,540
>> SAROO BRIERLEY (VOICEOVER):
It was 26 years ago,

1081
00:49:40,540 --> 00:49:42,470
and I was just about to turn 5.

1082
00:49:42,470 --> 00:49:45,465
We got to the train station and
we boarded our train together.

1083
00:49:45,465 --> 00:49:48,010
My brother just said I'll
stay here, and I'll come back.

1084
00:49:48,010 --> 00:49:51,140
And I just thought, well you know,
I might as well just go to sleep

1085
00:49:51,140 --> 00:49:52,690
and he'll just wake me up.

1086
00:49:52,690 --> 00:49:55,145
And when I woke up the next
day, the whole carriage

1087
00:49:55,145 --> 00:49:59,125
was empty on a runaway train, a ghost
train taking me I don't know where.

1088
00:49:59,125 --> 00:50:01,980

1089
00:50:01,980 --> 00:50:06,280
>> I was adopted out to Australia,
to an Australian family.

1090
00:50:06,280 --> 00:50:09,710
And mom had decorated my
room with the map of India,

1091
00:50:09,710 --> 00:50:11,950
which she put next to my bedside.

1092
00:50:11,950 --> 00:50:15,830
I woke up every morning
seeing that map, and hence

1093
00:50:15,830 --> 00:50:18,530
it sort of kept the memories alive.

1094
00:50:18,530 --> 00:50:20,530
People would say, you're
trying to find a needle

1095
00:50:20,530 --> 00:50:23,560
in a haystack, Saroo
you'll never find it.

1096
00:50:23,560 --> 00:50:28,540
>> I'd have flashed of the places that I
used to go, the flashes of my family

1097
00:50:28,540 --> 00:50:29,890
faces.

1098
00:50:29,890 --> 00:50:34,060
There was the image of my mother,
sitting down with her legs crossed,

1099
00:50:34,060 --> 00:50:35,740
just watching her cry.

1100
00:50:35,740 --> 00:50:38,160
Life is just so hard.

1101
00:50:38,160 --> 00:50:41,237
That was my treasure.

1102
00:50:41,237 --> 00:50:44,320
And I was looking at Google Map and
realized there's Google Earth as well.

1103
00:50:44,320 --> 00:50:48,140
In a world where you could zoom into,
I started to have all of these thoughts

1104
00:50:48,140 --> 00:50:51,500
and what possibilities
that this could do for me.

1105
00:50:51,500 --> 00:50:54,890
I said to myself, you know, you've
got all those photographic memories

1106
00:50:54,890 --> 00:50:58,590
and landmarks where you're from, and
you know what the town looks like.

1107
00:50:58,590 --> 00:51:03,060
This could be an application that
you can use to find your way back.

1108
00:51:03,060 --> 00:51:06,720
>> I thought, well I'll put a
dot on Calcutta train station,

1109
00:51:06,720 --> 00:51:10,890
and a radius line, you know, that you
should be searching around this area.

1110
00:51:10,890 --> 00:51:13,750
I sort of came across
these train tracks.

1111
00:51:13,750 --> 00:51:18,160
And I started following it, and
I came to a train station which

1112
00:51:18,160 --> 00:51:22,228
reflected the same image
that was in my memories.

1113
00:51:22,228 --> 00:51:23,707
Everything matched.

1114
00:51:23,707 --> 00:51:27,950
I just thought yep, I
know where I'm going.

1115
00:51:27,950 --> 00:51:30,116
I'm just going to let the
map that I have in my head

1116
00:51:30,116 --> 00:51:33,080
lead me and take me back to my hometown.

1117
00:51:33,080 --> 00:51:36,420
>> I came to the doorstep of
the house that I was born.

1118
00:51:36,420 --> 00:51:39,140
And walked around about 15
meters around the corner,

1119
00:51:39,140 --> 00:51:42,980
there was three ladies standing
outside, adjacent to each other.

1120
00:51:42,980 --> 00:51:47,830
And the middle one stepped forward, and
I just thought, this is your mother.

1121
00:51:47,830 --> 00:51:53,450
She came forward, she hugged me, and
we were there for about five minutes.

1122
00:51:53,450 --> 00:51:56,530

1123
00:51:56,530 --> 00:51:59,000
>> She grabbed my hand and
she took me to the house

1124
00:51:59,000 --> 00:52:02,450
and got on the phone, where she
rang my sister and my brother

1125
00:52:02,450 --> 00:52:05,600
to say that, you know,
your brother has just

1126
00:52:05,600 --> 00:52:09,340
all of sudden appeared like a ghost.

1127
00:52:09,340 --> 00:52:12,440
And then the family was reunited again.

1128
00:52:12,440 --> 00:52:15,110
Everything's all good,
I helped my mother out,

1129
00:52:15,110 --> 00:52:17,110
she doesn't have to be slaving away.

1130
00:52:17,110 --> 00:52:19,555
She can live the rest
of her life in peace.

1131
00:52:19,555 --> 00:52:23,460
>> It was a needle in a haystack,
but the needle was there.

1132
00:52:23,460 --> 00:52:24,400
Everything's there.

1133
00:52:24,400 --> 00:52:28,260
Everything we have in the world
is at the tap of a button.

1134
00:52:28,260 --> 00:52:32,010
But you've got to have the will and
the determination to wanting it.

1135
00:52:32,010 --> 00:52:32,950
>> [MUSIC PLAYING]

1136
00:52:32,950 --> 00:52:36,420

1137
00:52:36,420 --> 00:52:38,170
DAVID MALAN: Wonderfully
sweet testimonial

1138
00:52:38,170 --> 00:52:40,540
to just what you can
now do with technology.

1139
00:52:40,540 --> 00:52:43,390
We will see you next week.

1140
00:52:43,390 --> 00:52:44,374