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>> JASON HIRSCHHORN: Welcome to
CS50 section, everyone.

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My name is Jason Hirschhorn.

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I am a lot of yours TFs, but a lot of
people will hopefully be joining us

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online, extension students or students
trying to look through section if they

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missed their section that week or
just catch up if they wanted

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to review some material.

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>> So as per the syllabus, which all
of you have read, correct?

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I see a lot of head nods.

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Sections provide you with opportunities
to explore the course's

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material in a more intimate environment,
as well as dive into

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hands-on activities.

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>> So that's basically what we're
going to be doing every week.

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We're going to go through what we have
determined to be the most important

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topics covered in lecture, and the ones
probably also most useful and

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most likely to come up on problem sets
and in quizzes, and in your coding

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life outside of CS50.

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So we're going to go into those topics,
discuss them, go over them,

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answer any questions you guys have, and
then we're going to do a lot of

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practice problems in coding.

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>> You all brought laptops,
which is fantastic.

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This is a computer science class.

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However, a lot of the exercises we do,
you won't need your laptops for.

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You can do them on a sheet of paper.

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That's how you're going to
do them on the quizzes.

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And indeed, that's a great way to
really know that you how to do

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something if you don't have Check 50 to
rely on or Make to rely on, telling

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you made any errors.

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>> So you're welcome to code on the
appliance or run.cs50.net, which is

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another tool that allows you to code
online, but I also would recommend, if

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you haven't tried it, using a sheet
of paper and trying these practice

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problems on a sheet of paper.

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Because again, that's ultimately
what you're going to be

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asked to do on quizzes.

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>> So let's start with our
agenda for today.

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We're going to go over some norms for
our section, then we're going to dive

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into arrays, functions, and command line
arguments, which is what you guys

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have covered this past week in lecture
and then Monday, and what you'll cover

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more of tomorrow.

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And then finally, we're going
to talk about pset2.

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If I could get a show of hands if you
haven't looked at the problem set two

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specification.

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We'll say everyone.

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Fantastic.

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>> We'll talk about problem sets in a bit,
but if you read the problem set

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specification, that's
all you need to do.

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If you read that before you come to
class and section on Tuesday, you will

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be a great position.

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We can go through the problem set,
which is what we're going

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to do at the end.

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We're going to go through
the logic, make sure

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everybody's on the same page.

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>> I want you to be able to leave Tuesday
feeling very comfortable with what you

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need to do for this week's problem set,
and if you don't, I will happily

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stay after and work with you.

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But that is one thing that you have to
do but I think would be really smart,

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just to read the problem set
specification by class on Tuesday,

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which it sounds like a
lot of you guys did.

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>> I'll start with an agenda each class
so you know where we're going.

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This is also helpful for people watching
online because each section

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is color coded, so if you want to come
back and just watch 10 minutes rather

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than the whole 90-minute section, you
can just go to the color coded section

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and look at that part.

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So I've tried to break them up into nice
modules that will allow you to--

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if you want to come back and review,
or other students who are just

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watching a section to review--

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to find the specific thing they
want to pay attention to

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and learn more about.

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Sweet.

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>> Norms.

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I'm a FOP leader on campus.

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I don't know if anybody did FOP.

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Did anybody do FOP?

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One person did FOP.

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Two people did FOP.

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Awesome.

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So FOP is the First year Outdoor
Orientation Program.

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We take freshmen into the woods and make
the most magical experiences of

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their lives, correct?

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Yeah.

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Sounds about right.

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>> So I just wanted to start our very first
section by discussing some norms

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for the class and our relationship
because I think

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they're very important.

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First and foremost, as those of you who
are my section have received and

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has been discussed in that
email, I love this.

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This is my favorite part of the year.

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I've been so excited.

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I've been here for hours today.

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I haven't left this building except to
go eat because I love CS50 section, I

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love teaching, I love working
with you guys.

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>> One of my highest priorities, if not my
number one priority, not only for

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this class but including my other
classes and extracurriculars, is to

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support you guys, and help you guys, and
do my best to make sure that you

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guys have a wonderful, fun, productive,
engaging, exciting

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experience in this class.

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That's what I'm committed to.

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That's why I love to do this.

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That is my number one thing,
supporting you guys.

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If you have any questions about lecture,
the problem set, section,

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life, even if you want to talk about
life, please feel free to reach out.

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>> You have my cell phone number,
you have my email

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address, you have my GChat.

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You can contact me at any or all of
those whenever, but probably won't get

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a response whenever.

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I try and get back within 24 hours.

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Oftentimes it will be within
20 minutes or 24 minutes.

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I should have said that
to make it parallel.

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But you will get a response
pretty quickly.

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>> If you text me an hour before the
problem set's due, I will calm you

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down and do deep breathing exercises
with you, but if you haven't started,

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you're probably not going to finish, and
I can't really help you out there

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even though I'll try my best.

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So I will do everything
I can to support you.

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Please feel free to reach out whether
it's in section again, at Office Hours

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if you see me, or even outside.

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I'm happy to set up a time to meet
and work through problems

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with or go over material.

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So that is number one, support.

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I will do everything I
can to support you.

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>> Number two, meeting in the middle.

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As much as I would like to be, I'm
not omnipotent or all-powerful or

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all-knowing, so I don't know all the
time if what I'm doing is getting

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across, if the way I'm teaching the
topic helps you learn it, or if you

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are confused on a topic but don't
raise your hand or ask me

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or send me an email.

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Or even if you, again, are confused
about something on the problem set.

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If you don't reach out to me,
I can't know that for sure.

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I'm going to do my best to check in on
you guys, make sure everything is

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going well, try to teach things in a way
that will be conducive to most of

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your learning styles, but if there's
anything I can do, let me know.

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>> Along those lines, if you want some help
with the problem set, I'm more

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than happy to help you
with the problem set.

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We're going to do a lot of
that towards the second

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half of this section.

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But if you haven't read the problem sets
back and haven't watched the walk

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through and haven't watched the shorts
and expecting me to teach you

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everything that has been videotaped
online or that you can find at CS50

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study, that's not really fair to
me or to your other peers.

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>> So please, I'm more than happy to help
you, but you guys got to meet me in

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the middle.

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We have tons of resources for
you available on CS50.

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I mentioned some of them.

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Please take advantage of them.

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If you're confused, I'm more than happy
to help you explain them in

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another way, but it's not fair if
students show up at office hours,

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haven't started anything, and it's
Thursday night and they're expecting

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the TF to walk them through every single
thing for the problem set due

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at noon the next day.

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Make sense?

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Again, please meet us in the middle.

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>> And finally, high expectations.

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I'm going to be grading everybody who's
in my section, or scoring your

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problem sets.

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You haven't gone back problem
set zero or one yet.

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We're sorry about that.

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It took us awhile to finally
settle sections, and now

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we can start grading.

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>> I'll get them back to you
as soon as possible.

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Those are some of my
advisees out there.

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I'll get them back to you
as soon as possible.

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And for problem set two, we'll be on
schedule and we'll start getting them

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to you pretty quickly after
you turn them in.

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>> When you get your problem set back, you
will get scores, as you know from

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reading the syllabus-- correctness,
design, scope, and style scores.

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You'll also get comments.

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The comments are more important
than the scores.

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I think it's safe to say that.

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>> Scores are just a number
from one to five.

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It's hard to read into those.

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It's hard to improve from those.

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But the comments will point out specific
things you did well or areas

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you can improve on and give you some
ideas for the future or alternative

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ways of doing things.

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>> So please, please, please read
through the comments.

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I put a lot of time into
those comments.

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Please read through them and don't just
scroll down to the bottom of the

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PDF, look for your score, or go online
and find your score, and then ignore

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the comments.

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That's not how you're
going to get better.

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That's not how you're going to increase
your scores ultimately, just

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by looking at the scores.

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Please look at those comments.

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>> And along the line of high expectations,
again, like I said, you

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can be scored from one through five.

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Twos, threes, and fours are the
vast majority of scores

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you're going to be getting.

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Ones and fives are both really rare.

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There's always things
we can do better.

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Whenever I code a program, there's
always something I

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could have done better.

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>> So rather than saying you get a five
all the time on everything and did

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everything perfect, I don't think any
of us really believe that we-- or

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hopefully, we don't do everything
perfect all of the time.

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So don't be worried if you
didn't get a five.

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A five does not map onto an A, a four
does not map onto a B, a three does

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not map onto a C. That's not
how the scores work.

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>> Like David has said many times, at
semester's end all of your scores and

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input from me, your TF, come together
and determine a final grade for you.

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But again, it's not five,
A, four, B, et cetera.

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So I'm going to have high expectations
for you guys.

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I'm going to expect good problem
sets and I'm going to give

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you feedback as such.

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And it's not going to mean you're
going to get a worse grade.

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You are graded in terms of
your progress from week

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zero to course's end.

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>> And finally, in terms of high
expectations in class, I love when

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people participate.

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I understand some people feel more or
less comfortable participating.

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Other people are participating
not by raising their hand and

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talking all the time.

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But we in this class are going to be
doing a lot of small problems, a lot

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of mini questions, and then some
larger coding problems.

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So definitely work these exercises out
when we do them on your computer or on

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a sheet of paper.

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>> But I'll also give you some time before
anybody ever has to answer some

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of the questions I ask.

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So because I will give you some time to
think about it, because we're going

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to do things in partners sometimes,
I will expect everybody

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to be engaged somewhat.

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And for lack of a better term, there
will probably be cold calling, which

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is where everybody will have a chance
to think about the answer, and then

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I'll call on you, and then
you'll say the answer.

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>> Don't worry or be scared about that.

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I've done it in the past and we've found
that it's much more successful,

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also read educational research on it to
determine whether or not it was a

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good tactic.

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It is very helpful, especially if you
give students time to think about the

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answers and don't make them just come
out and say it right off the bat.

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But if you don't feel comfortable
answering, I'm also pretty good at

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sensing that out, and we're not going to
put you on the spot, especially on

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the camera, even though none of
you are on camera right now.

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00:10:28,240 --> 00:10:32,250
>> So those are the norms I have.

236
00:10:32,250 --> 00:10:34,670
One more thing along the lines
of meeting in the middle.

237
00:10:34,670 --> 00:10:36,490
I'm going to give you guys
a lot of feedback.

238
00:10:36,490 --> 00:10:38,310
Please give me feedback as well.

239
00:10:38,310 --> 00:10:46,190
There's a website, sayat.me/cs50.

240
00:10:46,190 --> 00:10:48,380
So it's an honest feedback.

241
00:10:48,380 --> 00:10:50,730
If you want to give it during section,
I get an email whenever it happens.

242
00:10:50,730 --> 00:10:53,820
We'll take a break in a bit, so I'll
look through the feedback then.

243
00:10:53,820 --> 00:10:55,970
If you ever have any feedback,
please give it to me there.

244
00:10:55,970 --> 00:11:01,380
>> If you give me feedback on the Q, that's
wonderful, but that's the guide

245
00:11:01,380 --> 00:11:02,570
that you do at the end.

246
00:11:02,570 --> 00:11:05,690
But that will be at semester's end so
I won't be able to take any of that

247
00:11:05,690 --> 00:11:08,620
feedback and turn it into teaching
you guys better.

248
00:11:08,620 --> 00:11:11,020
So please, if you have any
feedback, give it to me.

249
00:11:11,020 --> 00:11:12,010
I love feedback.

250
00:11:12,010 --> 00:11:14,490
Back to FOP, we get feedback
all the time.

251
00:11:14,490 --> 00:11:18,610
>> You can give me pluses, good things,
deltas, things I can improve upon.

252
00:11:18,610 --> 00:11:20,600
You can comment on my outfits.

253
00:11:20,600 --> 00:11:24,170
That won't be as helpful, but you're
welcome to comment on any and

254
00:11:24,170 --> 00:11:24,570
everything.

255
00:11:24,570 --> 00:11:26,280
Please, please give me feedback.

256
00:11:26,280 --> 00:11:28,740
I value it a lot and it
means a lot to me.

257
00:11:28,740 --> 00:11:31,760

258
00:11:31,760 --> 00:11:33,280
>> One more norm, fun.

259
00:11:33,280 --> 00:11:34,530
We're going to have a lot of fun.

260
00:11:34,530 --> 00:11:36,930
Hopefully in this course, you'll
have a lot of fun.

261
00:11:36,930 --> 00:11:38,780
We'll have candy at section.

262
00:11:38,780 --> 00:11:40,040
We'll have music.

263
00:11:40,040 --> 00:11:45,510
We will be hopefully getting to know one
another somewhat well, so I want

264
00:11:45,510 --> 00:11:47,690
this to be a fun, engaging experience.

265
00:11:47,690 --> 00:11:50,890
>> I know some of you have fallen asleep in
the lectures you had earlier today,

266
00:11:50,890 --> 00:11:53,750
so hopefully that won't
be the case now.

267
00:11:53,750 --> 00:11:55,960
And if it is the case now, just
don't snore because that might

268
00:11:55,960 --> 00:11:57,580
be caught on camera.

269
00:11:57,580 --> 00:11:58,770
Sweet.

270
00:11:58,770 --> 00:12:04,020
Any questions, comments, or concerns
before I move on about anything we've

271
00:12:04,020 --> 00:12:05,270
talked about?

272
00:12:05,270 --> 00:12:08,620

273
00:12:08,620 --> 00:12:11,350
Great.

274
00:12:11,350 --> 00:12:14,830
>> So we're going to go
into arrays first.

275
00:12:14,830 --> 00:12:17,140
This is how you make an array.

276
00:12:17,140 --> 00:12:18,410
We've seen this before.

277
00:12:18,410 --> 00:12:21,830
You have a data type, you have a
name, and then you have a size.

278
00:12:21,830 --> 00:12:24,430
Right here, I create two arrays.

279
00:12:24,430 --> 00:12:30,360
The first one is an array of characters,
and it has 26 spaces.

280
00:12:30,360 --> 00:12:33,790
The second one is an array of
scores, and it has three

281
00:12:33,790 --> 00:12:35,390
spaces for three integers.

282
00:12:35,390 --> 00:12:35,827
Yes?

283
00:12:35,827 --> 00:12:38,200
>> STUDENT: Are all of these slides
available somewhere for us?

284
00:12:38,200 --> 00:12:38,850
>> JASON HIRSCHHORN: Great question.

285
00:12:38,850 --> 00:12:40,650
The question was, are all of
these slides available?

286
00:12:40,650 --> 00:12:40,920
Yes.

287
00:12:40,920 --> 00:12:43,420
All of these slides will be available,
and in the future, I'll email them to

288
00:12:43,420 --> 00:12:44,640
you before section.

289
00:12:44,640 --> 00:12:47,340
But today, because this is the first
week, we had a late start.

290
00:12:47,340 --> 00:12:49,675
So usually, you'll get these slides, but
they will be available online and

291
00:12:49,675 --> 00:12:52,420
I'll email them to you
right after class.

292
00:12:52,420 --> 00:12:54,140
Any other questions?

293
00:12:54,140 --> 00:13:01,330
>> So again, arrays are boxes, for lack of
a better word, that hold multiple

294
00:13:01,330 --> 00:13:06,420
elements of one data type, and they're
next to one another in memory.

295
00:13:06,420 --> 00:13:11,370
And so again, this is how you can
make two types of arrays.

296
00:13:11,370 --> 00:13:15,090
>> So after you've made your array with
that first line-- int scores, bracket

297
00:13:15,090 --> 00:13:17,430
three, close bracket-- you
want to initialize it.

298
00:13:17,430 --> 00:13:19,050
You want to put some values in there.

299
00:13:19,050 --> 00:13:24,660
And this is how you put some values in
there, just choosing the indice and

300
00:13:24,660 --> 00:13:25,670
writing the number.

301
00:13:25,670 --> 00:13:27,130
Let's draw this on the board.

302
00:13:27,130 --> 00:13:28,270
Can everybody see?

303
00:13:28,270 --> 00:13:29,030
Everybody can see.

304
00:13:29,030 --> 00:13:30,790
Sweet.

305
00:13:30,790 --> 00:13:36,550
>> So that's going to be my
array called Scores.

306
00:13:36,550 --> 00:13:37,730
As you can see, it's size three.

307
00:13:37,730 --> 00:13:39,210
There are three boxes in there.

308
00:13:39,210 --> 00:13:43,120
Each one can fit what
type of data type?

309
00:13:43,120 --> 00:13:43,790
>> STUDENT: Integer.

310
00:13:43,790 --> 00:13:45,250
>> JASON HIRSCHHORN: An integer
in this one.

311
00:13:45,250 --> 00:13:48,790
We wrote int scores, and the first word
is the data type that's going to

312
00:13:48,790 --> 00:13:50,910
go in the boxes in that array.

313
00:13:50,910 --> 00:13:52,160
So I can fit three.

314
00:13:52,160 --> 00:13:54,880

315
00:13:54,880 --> 00:13:57,000
Why do I have zero up
there on the board?

316
00:13:57,000 --> 00:13:59,240
Why does it say score zero equals 1?

317
00:13:59,240 --> 00:14:02,090

318
00:14:02,090 --> 00:14:03,180
Remind me of your name.

319
00:14:03,180 --> 00:14:03,470
>> STUDENT: Kurt.

320
00:14:03,470 --> 00:14:04,070
>> JASON HIRSCHHORN: Kurt.

321
00:14:04,070 --> 00:14:04,480
>> STUDENT: Yes.

322
00:14:04,480 --> 00:14:04,920
>> JASON HIRSCHHORN: Why?

323
00:14:04,920 --> 00:14:07,230
>> STUDENT: Because the first index is
zero, we start counting at zero.

324
00:14:07,230 --> 00:14:08,480
>> JASON HIRSCHHORN: So we start
counting at zero.

325
00:14:08,480 --> 00:14:09,580
That's exactly right.

326
00:14:09,580 --> 00:14:12,340
This is the confusing part and will trip
you up, especially when you're

327
00:14:12,340 --> 00:14:14,900
trying to iterate through arrays,
which we'll do in a bit.

328
00:14:14,900 --> 00:14:15,980
We start counting at zero.

329
00:14:15,980 --> 00:14:19,170
So if I want to refer to the first
box, it's not box one.

330
00:14:19,170 --> 00:14:22,290
Unfortunately, it's box zero.

331
00:14:22,290 --> 00:14:26,460
The second one is box one, and in this
case, the third one is box two.

332
00:14:26,460 --> 00:14:28,700
>> What about box three?

333
00:14:28,700 --> 00:14:30,670
Where is box three?

334
00:14:30,670 --> 00:14:34,750
Or if I want to do scores, bracket
three, close bracket, what's that?

335
00:14:34,750 --> 00:14:37,650

336
00:14:37,650 --> 00:14:39,760
So you said the null character.

337
00:14:39,760 --> 00:14:41,600
Any other thoughts?

338
00:14:41,600 --> 00:14:44,590
What is score bracket three bracket?

339
00:14:44,590 --> 00:14:48,045
>> STUDENT: It doesn't really exist because
you only defined as scores,

340
00:14:48,045 --> 00:14:49,370
bracket three.

341
00:14:49,370 --> 00:14:51,400
>> JASON HIRSCHHORN: So you say
it doesn't really exist.

342
00:14:51,400 --> 00:14:54,230
In some cases, when we're dealing with
strings, generally we'll put a null

343
00:14:54,230 --> 00:14:54,790
character at the end.

344
00:14:54,790 --> 00:14:56,100
We'll get to that in a second.

345
00:14:56,100 --> 00:15:01,020
But in this case, there
is no third box here.

346
00:15:01,020 --> 00:15:06,390
So the last box in any array
is size minus 1.

347
00:15:06,390 --> 00:15:11,310
So if we have an array of size n, the
first box is at zero, the last one is

348
00:15:11,310 --> 00:15:12,700
at n minus 1.

349
00:15:12,700 --> 00:15:14,970
And in this case, it doesn't exist.

350
00:15:14,970 --> 00:15:17,970
There is something here in
memory but it's not ours.

351
00:15:17,970 --> 00:15:18,460
We don't own it.

352
00:15:18,460 --> 00:15:19,570
We didn't ask for it.

353
00:15:19,570 --> 00:15:22,606
So if we try to access it, which we'll
do in a bit, we're probably going to

354
00:15:22,606 --> 00:15:24,970
get into trouble.

355
00:15:24,970 --> 00:15:29,180
>> In this case, I'm initializing my
array with three variables.

356
00:15:29,180 --> 00:15:36,195
We're putting a one in box zero, a two
in box one, and a three in box two.

357
00:15:36,195 --> 00:15:39,280
Does anybody have any questions
about initializing an array?

358
00:15:39,280 --> 00:15:44,540

359
00:15:44,540 --> 00:15:46,970
>> So we went over this just now.

360
00:15:46,970 --> 00:15:52,725
Scores, bracket three, close bracket,
that doesn't exist.

361
00:15:52,725 --> 00:15:58,130
We're zero indexed in computer science,
so we start counting at zero.

362
00:15:58,130 --> 00:16:01,860
The last box available to us, it is
the third box, but it's called box

363
00:16:01,860 --> 00:16:03,110
number two.

364
00:16:03,110 --> 00:16:05,790

365
00:16:05,790 --> 00:16:09,710
>> Down below, this is an alternative
way of initializing arrays.

366
00:16:09,710 --> 00:16:13,410
So instead of creating the array on one
line, filling the first box, then

367
00:16:13,410 --> 00:16:17,070
the second box, then the third box,
you can do it all in one line.

368
00:16:17,070 --> 00:16:19,040
So in that line, both arrays.

369
00:16:19,040 --> 00:16:23,460
This creates the same exact array as
what we created right here, but it

370
00:16:23,460 --> 00:16:24,710
just does it in one line.

371
00:16:24,710 --> 00:16:29,740

372
00:16:29,740 --> 00:16:31,360
>> Now we have created our array.

373
00:16:31,360 --> 00:16:34,100
Usually once you create an array, you
want to do something with it, whether

374
00:16:34,100 --> 00:16:38,430
that's fill it or get things out of it
and print them out on the screen.

375
00:16:38,430 --> 00:16:42,270
In this case, we're going through our
scores array, which we initialized on

376
00:16:42,270 --> 00:16:44,740
the first line up here in gray.

377
00:16:44,740 --> 00:16:49,600
And then on the second line, we start
iterating through the array.

378
00:16:49,600 --> 00:16:52,640
Does anybody have any questions about
the structure of a for loop?

379
00:16:52,640 --> 00:16:55,250

380
00:16:55,250 --> 00:16:56,400
>> So this is the for loop.

381
00:16:56,400 --> 00:16:59,530
We have our initial condition, and then
our final condition, and then we

382
00:16:59,530 --> 00:17:01,160
iterate, i, each loop.

383
00:17:01,160 --> 00:17:05,579
And in the curly braces in this for
loop, what happens if I were to run

384
00:17:05,579 --> 00:17:06,290
this program?

385
00:17:06,290 --> 00:17:10,410
Can anybody tell me what we would get?

386
00:17:10,410 --> 00:17:11,589
[? Matu? ?]

387
00:17:11,589 --> 00:17:13,819
>> MATU: Print all the scores
on each line.

388
00:17:13,819 --> 00:17:14,900
It would print each score
on a separate line.

389
00:17:14,900 --> 00:17:16,300
>> JASON HIRSCHHORN: What exactly
would it print?

390
00:17:16,300 --> 00:17:19,790
>> STUDENT: It would print one,
then two, then three.

391
00:17:19,790 --> 00:17:20,235
>> JASON HIRSCHHORN: Exactly.

392
00:17:20,235 --> 00:17:23,490
It would print one, then two, then
three, each on separate lines, because

393
00:17:23,490 --> 00:17:27,810
we're going through the array and you'll
see that second, the printf

394
00:17:27,810 --> 00:17:29,275
statement, scores i.

395
00:17:29,275 --> 00:17:32,880
So on the first round through our for
loop, i equals zero, which means we're

396
00:17:32,880 --> 00:17:34,640
getting the value at score zero--

397
00:17:34,640 --> 00:17:36,900
which, if we go back to
our diagram, is one--

398
00:17:36,900 --> 00:17:38,630
and filling in--

399
00:17:38,630 --> 00:17:39,700
oh, it says c.

400
00:17:39,700 --> 00:17:41,680
That should be %d.

401
00:17:41,680 --> 00:17:43,970
I will fix that in a bit when
I hand that out to you guys.

402
00:17:43,970 --> 00:17:48,670
But we're presenting an integer so it
should be %d right here, not %c.

403
00:17:48,670 --> 00:17:50,340
What is %c for?

404
00:17:50,340 --> 00:17:50,790
>> STUDENT: Character.

405
00:17:50,790 --> 00:17:52,680
>> JASON HIRSCHHORN: Character, exactly.

406
00:17:52,680 --> 00:17:53,690
What is the other one?

407
00:17:53,690 --> 00:17:56,260
If you don't want to do %d for an
integer, what else can you do?

408
00:17:56,260 --> 00:17:57,000
>> STUDENT: i.

409
00:17:57,000 --> 00:17:59,180
>> JASON HIRSCHHORN: %i, exactly.

410
00:17:59,180 --> 00:18:01,660
So any questions about iterating
through the array?

411
00:18:01,660 --> 00:18:05,500

412
00:18:05,500 --> 00:18:07,800
So what happens if we do this?

413
00:18:07,800 --> 00:18:10,290
Instead of doing i is less than
three, we do i is less

414
00:18:10,290 --> 00:18:11,390
than or equal to three.

415
00:18:11,390 --> 00:18:13,420
What happens when we run this code?

416
00:18:13,420 --> 00:18:17,510

417
00:18:17,510 --> 00:18:18,940
Yes, Annie?

418
00:18:18,940 --> 00:18:22,030
>> STUDENT: They're going to print out
weird stuff on the fourth line.

419
00:18:22,030 --> 00:18:23,850
>> JASON HIRSCHHORN: So you said, is it
going to print out weird stuff on the

420
00:18:23,850 --> 00:18:24,390
fourth line?

421
00:18:24,390 --> 00:18:25,750
Does anybody have an answer
to her question?

422
00:18:25,750 --> 00:18:28,940

423
00:18:28,940 --> 00:18:33,160
So the odds are that when
we get to the--

424
00:18:33,160 --> 00:18:36,700
so on the fourth round through our for
loop, just like you thought, Annie,

425
00:18:36,700 --> 00:18:39,430
it'll be scores bracket three, which
we've already figured out doesn't

426
00:18:39,430 --> 00:18:41,990
exist, and we're going to try
and access that, print it

427
00:18:41,990 --> 00:18:43,350
out, use it some way.

428
00:18:43,350 --> 00:18:46,060
So the computer will probably
get mad at us.

429
00:18:46,060 --> 00:18:46,760
You're exactly right.

430
00:18:46,760 --> 00:18:48,830
>> It might quit the program altogether.

431
00:18:48,830 --> 00:18:52,222
It might print out something
really weird.

432
00:18:52,222 --> 00:18:53,420
You can't say for sure.

433
00:18:53,420 --> 00:18:56,210
Odds are it'll probably get mad at
you and just quit right away.

434
00:18:56,210 --> 00:19:00,100
>> But this is a common mistake you'll have
where you're just off by one at

435
00:19:00,100 --> 00:19:05,110
the end, and this is a bug that will
plague you throughout your coding.

436
00:19:05,110 --> 00:19:09,350

437
00:19:09,350 --> 00:19:14,150
One way I like to do is just on a
whiteboard, go through each iteration

438
00:19:14,150 --> 00:19:18,190
of the loop on hand or on paper to see
what's happening, and then you

439
00:19:18,190 --> 00:19:20,440
generally realize, oh wait, I shouldn't
have gone to less than or

440
00:19:20,440 --> 00:19:20,870
equal to three.

441
00:19:20,870 --> 00:19:24,730
I should have stopped at two, which is
what happens if your condition is less

442
00:19:24,730 --> 00:19:26,206
than three.

443
00:19:26,206 --> 00:19:29,120
Any questions so far?

444
00:19:29,120 --> 00:19:31,010
Sweet.

445
00:19:31,010 --> 00:19:33,360
>> So you all have your computers open.

446
00:19:33,360 --> 00:19:36,760
We're going to give you two to three
minutes to write a program that

447
00:19:36,760 --> 00:19:40,460
creates an array with the integers one
through five and then prints out each

448
00:19:40,460 --> 00:19:41,720
integer on a new line.

449
00:19:41,720 --> 00:19:44,330

450
00:19:44,330 --> 00:19:46,000
And if you have any questions,
raise your hand.

451
00:19:46,000 --> 00:19:48,140
I'm happy to come by.

452
00:19:48,140 --> 00:19:50,630
Like I said, let's take three
minutes to write this.

453
00:19:50,630 --> 00:20:41,300

454
00:20:41,300 --> 00:20:45,110
Is everyone OK if I go to a
new window and close this?

455
00:20:45,110 --> 00:22:13,420

456
00:22:13,420 --> 00:22:15,520
>> I'm sorry if you haven't had a chance
to finish, but we're going to do it

457
00:22:15,520 --> 00:22:17,310
together right now.

458
00:22:17,310 --> 00:22:23,640
So up on the screen, I have #include
cs50.h, #include stdio.h.

459
00:22:23,640 --> 00:22:26,090
Can somebody tell me what
those two lines do?

460
00:22:26,090 --> 00:22:32,250

461
00:22:32,250 --> 00:22:33,155
Michael?

462
00:22:33,155 --> 00:22:40,030
>> STUDENT: They include the libraries
that CS50 made and that Steve has

463
00:22:40,030 --> 00:22:44,210
saved for you to use to
make your life easier.

464
00:22:44,210 --> 00:22:46,820
>> JASON HIRSCHHORN: So you said they
include the libraries, one that CS50

465
00:22:46,820 --> 00:22:51,230
made, cs50.h, and one that is
just standard, comes with

466
00:22:51,230 --> 00:22:53,510
the C package, stdio.h.

467
00:22:53,510 --> 00:22:54,890
What do you mean include
the libraries, though?

468
00:22:54,890 --> 00:22:55,770
Can you be a bit more specific?

469
00:22:55,770 --> 00:23:01,180
>> STUDENT: It makes it so that you can
use certain code words to have a

470
00:23:01,180 --> 00:23:05,710
function happen without having to go
and make the function c yourself.

471
00:23:05,710 --> 00:23:08,730
>> JASON HIRSCHHORN: Can you
give me an example?

472
00:23:08,730 --> 00:23:11,160
>> STUDENT: Like printf
is one you can use.

473
00:23:11,160 --> 00:23:11,860
>> JASON HIRSCHHORN: Printf.

474
00:23:11,860 --> 00:23:14,680
So you said printf is one of those
functions where rather than having to

475
00:23:14,680 --> 00:23:19,840
write printf on your own, printf is
included actually in stdio.h, so you

476
00:23:19,840 --> 00:23:22,010
can just use it and assume
that it will work.

477
00:23:22,010 --> 00:23:28,270
In stdio.h, if you opened up that file,
would it include how printf

478
00:23:28,270 --> 00:23:31,620
works, all of the code for printf?

479
00:23:31,620 --> 00:23:33,105
Why are you shaking your head?

480
00:23:33,105 --> 00:23:35,710
>> STUDENT: Because the header file just
includes the function declaration.

481
00:23:35,710 --> 00:23:36,400
>> JASON HIRSCHHORN: Exactly right.

482
00:23:36,400 --> 00:23:39,680
The header file just includes
the function declaration.

483
00:23:39,680 --> 00:23:42,950
So basically, when you compile this,
hit Make Count, which we'll do in a

484
00:23:42,950 --> 00:23:48,070
second, the compiler knows that if
you use printf, printf exists.

485
00:23:48,070 --> 00:23:49,370
It's going to be defined somewhere.

486
00:23:49,370 --> 00:23:51,200
It's not a problem for you using it.

487
00:23:51,200 --> 00:23:55,490
And then later on, it'll include the
ones and zeros that actually contain

488
00:23:55,490 --> 00:23:58,360
printf's implementation and
merge them with your code.

489
00:23:58,360 --> 00:24:01,935
How do you make sure it includes the
ones and zeros for, say, get string?

490
00:24:01,935 --> 00:24:05,670

491
00:24:05,670 --> 00:24:07,210
>> STUDENT: Get string is in cs50.h.

492
00:24:07,210 --> 00:24:07,830
>> JASON HIRSCHHORN: Right.

493
00:24:07,830 --> 00:24:11,110
So that says get string exists,
the function declaration.

494
00:24:11,110 --> 00:24:15,410
But then the ones and zeros, the binary
file that actually includes the

495
00:24:15,410 --> 00:24:17,370
implementation of get string?

496
00:24:17,370 --> 00:24:19,310
How do you make sure that
goes with your program?

497
00:24:19,310 --> 00:24:23,220
>> STUDENT: Pass the linker in
the command line argument.

498
00:24:23,220 --> 00:24:25,430
>> JASON HIRSCHHORN: Pass the linker
in the command line argument.

499
00:24:25,430 --> 00:24:26,700
How do you do that?

500
00:24:26,700 --> 00:24:29,730
>> STUDENT: It's the -l line in the clang

501
00:24:29,730 --> 00:24:30,480
>> JASON HIRSCHHORN: Exactly right.

502
00:24:30,480 --> 00:24:32,900
You say -l cs50, right?

503
00:24:32,900 --> 00:24:34,940
That includes the ones and zeros.

504
00:24:34,940 --> 00:24:36,460
>> STUDENT: But Make does
that for you, right?

505
00:24:36,460 --> 00:24:37,010
>> JASON HIRSCHHORN: Exactly.

506
00:24:37,010 --> 00:24:40,390
Make does that for you, but eventually,
you won't have Make, or

507
00:24:40,390 --> 00:24:43,220
you'll have to make a Make
on your own, so that's a

508
00:24:43,220 --> 00:24:43,940
good thing to remember.

509
00:24:43,940 --> 00:24:47,350
If you were going to use one of these
other libraries, not only do you have

510
00:24:47,350 --> 00:24:49,490
to write #include and
then the library.

511
00:24:49,490 --> 00:24:51,420
You also need to link it in.

512
00:24:51,420 --> 00:24:54,480
And those two things are different.
-l cs50 is different

513
00:24:54,480 --> 00:24:59,260
than #include cs50.h.

514
00:24:59,260 --> 00:25:02,870
>> Then we have int main void curly braces,
and now we're going to code

515
00:25:02,870 --> 00:25:03,590
inside main.

516
00:25:03,590 --> 00:25:07,090
The first line I have here is a comment
that says, create an array

517
00:25:07,090 --> 00:25:08,820
with ints one through five.

518
00:25:08,820 --> 00:25:12,600
What is a good word to describe what I
have right now on the screen, just

519
00:25:12,600 --> 00:25:14,170
comments inside main?

520
00:25:14,170 --> 00:25:15,722
What do we call that?

521
00:25:15,722 --> 00:25:16,820
>> STUDENT: Pseudocode.

522
00:25:16,820 --> 00:25:17,840
>> JASON HIRSCHHORN: Pseudocode.

523
00:25:17,840 --> 00:25:18,420
Thank you, Jeffrey.

524
00:25:18,420 --> 00:25:19,540
That is exactly right.

525
00:25:19,540 --> 00:25:20,005
Pseudocode.

526
00:25:20,005 --> 00:25:22,854
Have you guys seen pseudocode before?

527
00:25:22,854 --> 00:25:28,340
Raise your hand if you've used
pseudocode before problem set one.

528
00:25:28,340 --> 00:25:29,400
About half of you.

529
00:25:29,400 --> 00:25:32,700
>> I would highly recommend using
pseudocode to break down problems into

530
00:25:32,700 --> 00:25:35,210
smaller, more easy to bite off pieces.

531
00:25:35,210 --> 00:25:38,630
Also, if you do the pseudocode, you
have your comments already largely

532
00:25:38,630 --> 00:25:40,760
built in, so you don't have to
go back and try to figure out

533
00:25:40,760 --> 00:25:42,450
what you were doing.

534
00:25:42,450 --> 00:25:44,730
So can somebody give me this first line
to create an array with ints one

535
00:25:44,730 --> 00:25:45,980
through five?

536
00:25:45,980 --> 00:25:48,700

537
00:25:48,700 --> 00:25:49,130
You?

538
00:25:49,130 --> 00:25:53,350
>> STUDENT: Int and then you can call
whatever number, and then square

539
00:25:53,350 --> 00:25:54,830
bracket five.

540
00:25:54,830 --> 00:25:57,820
>> JASON HIRSCHHORN: So int numbers,
square bracket five.

541
00:25:57,820 --> 00:25:59,160
>> STUDENT: Equals, and
then curly braces.

542
00:25:59,160 --> 00:26:00,830
>> JASON HIRSCHHORN: Equals,
curly braces.

543
00:26:00,830 --> 00:26:01,830
>> STUDENT: One, two, three, four.

544
00:26:01,830 --> 00:26:05,566
>> JASON HIRSCHHORN: One, two,
three, four, five.

545
00:26:05,566 --> 00:26:07,390
>> STUDENT: And then curly braces
and then semicolon.

546
00:26:07,390 --> 00:26:09,240
>> JASON HIRSCHHORN: And curly
braces and semicolon.

547
00:26:09,240 --> 00:26:10,330
Excellent.

548
00:26:10,330 --> 00:26:12,210
Print out each int on a new line.

549
00:26:12,210 --> 00:26:17,140
Can somebody else give
me that code, please?

550
00:26:17,140 --> 00:26:17,530
[? Matu? ?]

551
00:26:17,530 --> 00:26:18,560
Go ahead.

552
00:26:18,560 --> 00:26:20,440
>> STUDENT: For (

553
00:26:20,440 --> 00:26:21,750
>> JASON HIRSCHHORN: For (

554
00:26:21,750 --> 00:26:23,740
>> STUDENT: Int i=0

555
00:26:23,740 --> 00:26:26,070
>> JASON HIRSCHHORN: Int i=0

556
00:26:26,070 --> 00:26:28,190
>> STUDENT: Semicolon i less than five.

557
00:26:28,190 --> 00:26:31,680
>> JASON HIRSCHHORN: Semicolon,
i less than 5.

558
00:26:31,680 --> 00:26:33,600
>> STUDENT: Semicolon, i++.

559
00:26:33,600 --> 00:26:35,140
>> JASON HIRSCHHORN: Semicolon, i++.

560
00:26:35,140 --> 00:26:37,720
>> STUDENT: Closed parentheses,
then open brace.

561
00:26:37,720 --> 00:26:39,410
>> JASON HIRSCHHORN: Closed parentheses
and two braces.

562
00:26:39,410 --> 00:26:40,510
Inside the braces?

563
00:26:40,510 --> 00:26:47,640
>> STUDENT: And then printf "%d/n"

564
00:26:47,640 --> 00:26:48,695
>> JASON HIRSCHHORN: Sorry, hold on.

565
00:26:48,695 --> 00:26:52,330
Printf "%d/n, closed quote.

566
00:26:52,330 --> 00:26:55,980
>> STUDENT: And then closed quote,
comma, count, bracket i.

567
00:26:55,980 --> 00:26:58,350
>> JASON HIRSCHHORN: Comma,
count, bracket i.

568
00:26:58,350 --> 00:26:59,970
>> STUDENT: Sorry, numbers bracket i.

569
00:26:59,970 --> 00:27:01,416
>> JASON HIRSCHHORN: Numbers, bracket i.

570
00:27:01,416 --> 00:27:03,200
>> STUDENT: Sorry.

571
00:27:03,200 --> 00:27:04,750
And then closed parentheses,
semicolon.

572
00:27:04,750 --> 00:27:07,156
>> JASON HIRSCHHORN: Closed
parentheses, semicolon.

573
00:27:07,156 --> 00:27:08,680
I'm going to save that.

574
00:27:08,680 --> 00:27:12,670
We're going to make count.

575
00:27:12,670 --> 00:27:14,742
And how do I run count?

576
00:27:14,742 --> 00:27:16,470
>> STUDENT: Backslash.

577
00:27:16,470 --> 00:27:17,180
>> STUDENT: Dot forward slash--

578
00:27:17,180 --> 00:27:18,050
>> JASON HIRSCHHORN: ./count, exactly.

579
00:27:18,050 --> 00:27:19,380
Thank you. ./count.

580
00:27:19,380 --> 00:27:21,060
One, two, three, four,
five on new lines.

581
00:27:21,060 --> 00:27:22,110
Looks like it worked.

582
00:27:22,110 --> 00:27:23,200
Yes, Noah?

583
00:27:23,200 --> 00:27:26,660
>> STUDENT: You could also do less than or
equal to four, like i is less than

584
00:27:26,660 --> 00:27:27,630
or equal to four?

585
00:27:27,630 --> 00:27:31,190
Is there any advantage to doing less
than five, or just a different way of

586
00:27:31,190 --> 00:27:31,610
writing it?

587
00:27:31,610 --> 00:27:35,280
>> JASON HIRSCHHORN: So the question is you
could have done i is less than or

588
00:27:35,280 --> 00:27:37,450
equal to four instead of
i is less than five.

589
00:27:37,450 --> 00:27:38,935
Would that have worked the same?

590
00:27:38,935 --> 00:27:39,760
>> STUDENT: It should.

591
00:27:39,760 --> 00:27:39,860
>> JASON HIRSCHHORN: Yes.

592
00:27:39,860 --> 00:27:41,660
It totally would have work the same.

593
00:27:41,660 --> 00:27:44,780
Stylistically, it's generally less than,
and you'll see in a second when

594
00:27:44,780 --> 00:27:48,160
we do strings, you can use something
called strlen, which you've seen

595
00:27:48,160 --> 00:27:50,380
before, to get the length
of the string.

596
00:27:50,380 --> 00:27:54,090
And you can't do less than
or equal strlen.

597
00:27:54,090 --> 00:27:56,500
You could do less than or
equal strlen minus one.

598
00:27:56,500 --> 00:27:59,260
So it's just nicer to
do less than strlen.

599
00:27:59,260 --> 00:28:00,240
Yes?

600
00:28:00,240 --> 00:28:03,270
>> STUDENT: And in the actual quizzes, do
we have to provide comments on our

601
00:28:03,270 --> 00:28:04,620
code as well?

602
00:28:04,620 --> 00:28:06,130
>> JASON HIRSCHHORN: The question
was in quizzes, do you

603
00:28:06,130 --> 00:28:07,760
have to provide comments?

604
00:28:07,760 --> 00:28:12,050
So we'll talk more about quizzes when
they come up, but generally, a good

605
00:28:12,050 --> 00:28:14,530
rule of thumb is whenever you're
writing code, provide comments.

606
00:28:14,530 --> 00:28:18,130
If you get the code wrong but the
comment right, you might get some

607
00:28:18,130 --> 00:28:22,280
credit both from me if I was grading
you on a problem set or on a quiz.

608
00:28:22,280 --> 00:28:24,660
So comments are always
good to provide.

609
00:28:24,660 --> 00:28:25,040
Sweet.

610
00:28:25,040 --> 00:28:25,460
Count.

611
00:28:25,460 --> 00:28:26,720
Excellent job.

612
00:28:26,720 --> 00:28:28,570
Let's move on.

613
00:28:28,570 --> 00:28:29,920
>> Strings.

614
00:28:29,920 --> 00:28:30,630
Strings.

615
00:28:30,630 --> 00:28:31,730
We've seen this before in lecture.

616
00:28:31,730 --> 00:28:33,390
This blew my mind when I
first found this out.

617
00:28:33,390 --> 00:28:37,240
It probably didn't blow anybody's
mind here, but that's OK.

618
00:28:37,240 --> 00:28:41,260
>> Strings are arrays of characters, and
they end with, as you said earlier,

619
00:28:41,260 --> 00:28:45,710
the null terminator, which
is this /0 character.

620
00:28:45,710 --> 00:28:47,740
If we're going to draw a string on the
board, we'll draw it over here.

621
00:28:47,740 --> 00:28:53,080

622
00:28:53,080 --> 00:28:56,420
So how long is this string?

623
00:28:56,420 --> 00:28:56,680
Sorry.

624
00:28:56,680 --> 00:28:57,660
How long is this string?

625
00:28:57,660 --> 00:28:57,910
>> STUDENT: [INAUDIBLE].

626
00:28:57,910 --> 00:28:59,010
>> JASON HIRSCHHORN: Four?

627
00:28:59,010 --> 00:28:59,880
Why is it four?

628
00:28:59,880 --> 00:29:02,240
>> STUDENT: Because the fifth spot
is for the null character.

629
00:29:02,240 --> 00:29:04,070
>> JASON HIRSCHHORN: So the fifth spot
is for the null character.

630
00:29:04,070 --> 00:29:07,130
So if I were to run strlen on
this, it would give me four.

631
00:29:07,130 --> 00:29:07,850
Correct.

632
00:29:07,850 --> 00:29:11,640
And the array, though, if I were to
initialize this array, how many spaces

633
00:29:11,640 --> 00:29:13,444
would I have to ask for?

634
00:29:13,444 --> 00:29:13,821
>> STUDENT: Five.

635
00:29:13,821 --> 00:29:14,200
>> JASON HIRSCHHORN: Five.

636
00:29:14,200 --> 00:29:17,650
I would have to ask for five, but I can
only put four characters in here

637
00:29:17,650 --> 00:29:19,750
because I need to put that null
terminator on the end.

638
00:29:19,750 --> 00:29:25,260
So we're going to write in "ohai" and
put the null terminator in at the end.

639
00:29:25,260 --> 00:29:31,820

640
00:29:31,820 --> 00:29:34,940
We'll say this is string
s equals this.

641
00:29:34,940 --> 00:29:36,610
What is s bracket zero?

642
00:29:36,610 --> 00:29:40,210

643
00:29:40,210 --> 00:29:41,550
s bracket zero, Annie?

644
00:29:41,550 --> 00:29:41,950
>> STUDENT: O.

645
00:29:41,950 --> 00:29:45,400
>> JASON HIRSCHHORN: O, exactly.
s bracket one?

646
00:29:45,400 --> 00:29:48,200
s bracket five?

647
00:29:48,200 --> 00:29:52,670

648
00:29:52,670 --> 00:29:53,955
s bracket five?

649
00:29:53,955 --> 00:29:54,740
>> STUDENT: Doesn't exist.

650
00:29:54,740 --> 00:29:55,040
>> JASON HIRSCHHORN: Doesn't exist.

651
00:29:55,040 --> 00:29:57,050
Exactly right.

652
00:29:57,050 --> 00:29:57,940
Cool.

653
00:29:57,940 --> 00:30:03,120
Any questions about understanding
strings as arrays of characters?

654
00:30:03,120 --> 00:30:03,870
OK, then.

655
00:30:03,870 --> 00:30:05,440
One more program for you guys.

656
00:30:05,440 --> 00:30:09,190
Write a program that asks the user
for a string then prints out each

657
00:30:09,190 --> 00:30:11,880
character on a new line.

658
00:30:11,880 --> 00:30:14,160
So let's take three minutes to write
this program, and then we'll code it

659
00:30:14,160 --> 00:30:15,410
as a group.

660
00:30:15,410 --> 00:31:04,730

661
00:31:04,730 --> 00:31:04,830
>> Sorry.

662
00:31:04,830 --> 00:31:06,470
Did you need to see this?

663
00:31:06,470 --> 00:31:07,410
>> STUDENT: No.

664
00:31:07,410 --> 00:31:08,660
>> JASON HIRSCHHORN: OK.

665
00:31:08,660 --> 00:31:36,390

666
00:31:36,390 --> 00:31:40,605
Let's take one more minute to code this,
or at least get started on it.

667
00:31:40,605 --> 00:32:13,900

668
00:32:13,900 --> 00:32:17,020
>> So you can keep working, but I'm going
to start going through it.

669
00:32:17,020 --> 00:32:22,560
So like before, I have on the screen
#include cs50.h, #include stdio.h.

670
00:32:22,560 --> 00:32:24,020
int main void.

671
00:32:24,020 --> 00:32:28,880
Inside main's curly braces, I have
two comments, pseudocode.

672
00:32:28,880 --> 00:32:30,980
One is ask for a string from the user.

673
00:32:30,980 --> 00:32:33,900
The second is print out each
character on a new line.

674
00:32:33,900 --> 00:32:39,780
So Anna, can you give me ask
for a string from the user?

675
00:32:39,780 --> 00:32:41,970
>> STUDENT: Printf.

676
00:32:41,970 --> 00:32:42,720
>> JASON HIRSCHHORN: Printf?

677
00:32:42,720 --> 00:32:45,050
>> STUDENT: Yes.

678
00:32:45,050 --> 00:32:46,165
(

679
00:32:46,165 --> 00:32:47,610
>> JASON HIRSCHHORN: (

680
00:32:47,610 --> 00:32:49,300
>> STUDENT: "give me a string.

681
00:32:49,300 --> 00:32:52,380
>> JASON HIRSCHHORN: "give
me a str-- oops--

682
00:32:52,380 --> 00:32:53,710
string.

683
00:32:53,710 --> 00:32:59,980
>> STUDENT: /n") semicolon.

684
00:32:59,980 --> 00:33:02,300
>> JASON HIRSCHHORN: End the string,
closed parentheses, semicolon.

685
00:33:02,300 --> 00:33:03,550
>> STUDENT: Yes.

686
00:33:03,550 --> 00:33:09,080
And then next line, string
s = get string.

687
00:33:09,080 --> 00:33:11,240
>> JASON HIRSCHHORN: String
s = get string.

688
00:33:11,240 --> 00:33:11,665
>> STUDENT: Yes.

689
00:33:11,665 --> 00:33:13,080
And then ( )

690
00:33:13,080 --> 00:33:14,365
>> JASON HIRSCHHORN: ( )

691
00:33:14,365 --> 00:33:14,720
>> STUDENT: Semicolon.

692
00:33:14,720 --> 00:33:15,540
>> JASON HIRSCHHORN: Semicolon.

693
00:33:15,540 --> 00:33:16,890
Fantastic.

694
00:33:16,890 --> 00:33:19,000
So I'm going to get a string and
I'm going to save it in what?

695
00:33:19,000 --> 00:33:21,830
What's the variable called
that you created?

696
00:33:21,830 --> 00:33:23,290
>> STUDENT: String s.

697
00:33:23,290 --> 00:33:24,580
>> JASON HIRSCHHORN: The variable
is called s.

698
00:33:24,580 --> 00:33:25,700
It's type string.

699
00:33:25,700 --> 00:33:27,200
Exactly.

700
00:33:27,200 --> 00:33:30,080
Next thing we want to is print out
each character on a new line.

701
00:33:30,080 --> 00:33:31,710
Who can give me that code?

702
00:33:31,710 --> 00:33:32,110
[INAUDIBLE]?

703
00:33:32,110 --> 00:33:33,000
Can you give it to me?

704
00:33:33,000 --> 00:33:33,580
>> STUDENT: Sure.

705
00:33:33,580 --> 00:33:35,420
>> JASON HIRSCHHORN: Let's do it.

706
00:33:35,420 --> 00:33:36,080
And sorry.

707
00:33:36,080 --> 00:33:38,896
I don't like to repeat what you say
because you're getting it right, but I

708
00:33:38,896 --> 00:33:42,100
have to repeat it so that when they
transcribe this, they can get all of

709
00:33:42,100 --> 00:33:44,870
the text out on the transcriptions.

710
00:33:44,870 --> 00:33:47,180
So you can just keep talking.

711
00:33:47,180 --> 00:33:48,590
Unfortunately, I have
to say it over you.

712
00:33:48,590 --> 00:33:49,510
I'm sorry, but--

713
00:33:49,510 --> 00:33:50,610
>> STUDENT: That's fine.

714
00:33:50,610 --> 00:33:53,010
For int i=0

715
00:33:53,010 --> 00:33:54,170
>> JASON HIRSCHHORN: For int i=0

716
00:33:54,170 --> 00:33:56,740
>> STUDENT: I less than--

717
00:33:56,740 --> 00:33:58,206
>> JASON HIRSCHHORN: Semicolon.

718
00:33:58,206 --> 00:34:00,650
>> STUDENT: Before this, can we put a
variable so you get int name length?

719
00:34:00,650 --> 00:34:01,870
>> JASON HIRSCHHORN: Oh, you don't
want to put the semicolon?

720
00:34:01,870 --> 00:34:02,310
OK.

721
00:34:02,310 --> 00:34:05,120
So what do you want to do after zero?

722
00:34:05,120 --> 00:34:09,340
>> STUDENT: I want to get the
string length of s and

723
00:34:09,340 --> 00:34:10,410
set that to a variable?

724
00:34:10,410 --> 00:34:11,760
>> JASON HIRSCHHORN: OK,
so how do I do that?

725
00:34:11,760 --> 00:34:17,636
>> STUDENT: So you do int length =

726
00:34:17,636 --> 00:34:20,010
>> JASON HIRSCHHORN: Int length =

727
00:34:20,010 --> 00:34:22,230
>> STUDENT: S-T-R-L-E-N.

728
00:34:22,230 --> 00:34:24,260
>> JASON HIRSCHHORN: S-T-R-L-E-N--

729
00:34:24,260 --> 00:34:25,380
>> STUDENT: Of s.

730
00:34:25,380 --> 00:34:25,790
Brackets.

731
00:34:25,790 --> 00:34:27,920
>> JASON HIRSCHHORN: (s).

732
00:34:27,920 --> 00:34:33,040
>> STUDENT: And then I think you have to
put a semicolon between the two ints.

733
00:34:33,040 --> 00:34:34,590
>> JASON HIRSCHHORN: A semicolon
after zero?

734
00:34:34,590 --> 00:34:35,260
>> STUDENT: Yeah.

735
00:34:35,260 --> 00:34:40,670
Then put a semicolon after that, and
you say, i less than length.

736
00:34:40,670 --> 00:34:43,250
i less than length, which
is the variable.

737
00:34:43,250 --> 00:34:44,590
>> STUDENT: And then semicolon.

738
00:34:44,590 --> 00:34:45,790
>> JASON HIRSCHHORN: Semicolon.

739
00:34:45,790 --> 00:34:46,290
>> STUDENT: i++.

740
00:34:46,290 --> 00:34:46,909
>> JASON HIRSCHHORN: i++.

741
00:34:46,909 --> 00:34:47,530
There's our for loop.

742
00:34:47,530 --> 00:34:48,860
OK, inside the for loop.

743
00:34:48,860 --> 00:34:51,670
>> STUDENT: And then do printf.

744
00:34:51,670 --> 00:34:53,340
>> JASON HIRSCHHORN: Printf.

745
00:34:53,340 --> 00:34:53,889
>> STUDENT: Brackets.

746
00:34:53,889 --> 00:34:55,130
>> JASON HIRSCHHORN: Parentheses, quotes.

747
00:34:55,130 --> 00:34:59,930
>> STUDENT: Parentheses, quotes, %c/n.

748
00:34:59,930 --> 00:35:00,760
>> JASON HIRSCHHORN: %c, OK.

749
00:35:00,760 --> 00:35:01,670
>> STUDENT: /n.

750
00:35:01,670 --> 00:35:03,470
>> JASON HIRSCHHORN: /n.

751
00:35:03,470 --> 00:35:04,410
>> STUDENT: Closed quotes.

752
00:35:04,410 --> 00:35:05,270
>> JASON HIRSCHHORN: Closed quotes.

753
00:35:05,270 --> 00:35:05,810
>> STUDENT: Comma.

754
00:35:05,810 --> 00:35:06,860
>> JASON HIRSCHHORN: Comma.

755
00:35:06,860 --> 00:35:07,800
>> STUDENT: S.

756
00:35:07,800 --> 00:35:08,070
>> JASON HIRSCHHORN: S.

757
00:35:08,070 --> 00:35:08,690
>> STUDENT: Bracket i.

758
00:35:08,690 --> 00:35:10,230
>> JASON HIRSCHHORN: Bracket i.

759
00:35:10,230 --> 00:35:11,520
>> STUDENT: And then closed parentheses.

760
00:35:11,520 --> 00:35:12,540
>> JASON HIRSCHHORN: Closed parentheses.

761
00:35:12,540 --> 00:35:13,660
>> STUDENT: And then semicolon.

762
00:35:13,660 --> 00:35:13,770
>> JASON HIRSCHHORN: Semicolon.

763
00:35:13,770 --> 00:35:14,380
Fantastic.

764
00:35:14,380 --> 00:35:16,890
So let's save this.

765
00:35:16,890 --> 00:35:19,770
Make spell.

766
00:35:19,770 --> 00:35:20,040
Oh boy.

767
00:35:20,040 --> 00:35:22,275
I've got a couple of errors.

768
00:35:22,275 --> 00:35:33,120
First error on line 11, it says,
expected expression, and it's giving

769
00:35:33,120 --> 00:35:36,910
me an error that the second
int, there's the error.

770
00:35:36,910 --> 00:35:38,430
There some error around there.

771
00:35:38,430 --> 00:35:41,060
Does anybody know what error that is?

772
00:35:41,060 --> 00:35:41,640
Kurt?

773
00:35:41,640 --> 00:35:44,108
>> STUDENT: I think it's supposed to
be a comma and not a semicolon.

774
00:35:44,108 --> 00:35:46,010
>> JASON HIRSCHHORN: OK, a comma
instead of a semicolon.

775
00:35:46,010 --> 00:35:47,340
So is that all we need to fix?

776
00:35:47,340 --> 00:35:47,655
>> STUDENT: No.

777
00:35:47,655 --> 00:35:48,930
>> JASON HIRSCHHORN: What else
do we need to fix?

778
00:35:48,930 --> 00:35:50,920
>> STUDENT: We have to include string.h.

779
00:35:50,920 --> 00:35:51,680
>> JASON HIRSCHHORN: Include string.h.

780
00:35:51,680 --> 00:35:53,730
Where do I need to include that?

781
00:35:53,730 --> 00:35:57,085
>> STUDENT: Same place you're including
the other stuff, right

782
00:35:57,085 --> 00:35:59,560
after include stdio.h.

783
00:35:59,560 --> 00:36:01,610
>> JASON HIRSCHHORN: Let's see if this runs
first because generally, I like

784
00:36:01,610 --> 00:36:06,360
to fix one thing first, and then run
it, and then see the errors we get.

785
00:36:06,360 --> 00:36:11,620
So we're going to clear the terminal
with control n, let's do make spell.

786
00:36:11,620 --> 00:36:15,190

787
00:36:15,190 --> 00:36:18,770
So another error on line 11.

788
00:36:18,770 --> 00:36:20,540
Expected semicolon.

789
00:36:20,540 --> 00:36:22,640
And I'm still getting
errors on this line.

790
00:36:22,640 --> 00:36:25,270
So it hasn't given me an error saying it
doesn't understand what strlen is,

791
00:36:25,270 --> 00:36:28,980
but it is giving me errors on this
line, so what else is wrong?

792
00:36:28,980 --> 00:36:29,690
Mark?

793
00:36:29,690 --> 00:36:30,940
Marcus.

794
00:36:30,940 --> 00:36:32,570
>> STUDENT: Do you have to remove
the second int declaration?

795
00:36:32,570 --> 00:36:34,990
>> JASON HIRSCHHORN: Remove
the second int.

796
00:36:34,990 --> 00:36:35,710
Let's try this.

797
00:36:35,710 --> 00:36:37,600
Now clear the terminal.

798
00:36:37,600 --> 00:36:40,130
Make spell.

799
00:36:40,130 --> 00:36:40,490
Error.

800
00:36:40,490 --> 00:36:42,890
Implicitly declaring library
function strlen.

801
00:36:42,890 --> 00:36:44,270
So now we do see that error.

802
00:36:44,270 --> 00:36:45,760
What do we need to include?

803
00:36:45,760 --> 00:36:46,660
>> STUDENT: String.h.

804
00:36:46,660 --> 00:36:51,880
>> JASON HIRSCHHORN: Include string.h,
we'll do #include string.h at the top

805
00:36:51,880 --> 00:36:52,920
of the screen.

806
00:36:52,920 --> 00:36:55,450
Let's save, make spell.

807
00:36:55,450 --> 00:36:56,970
Now compiled.

808
00:36:56,970 --> 00:37:01,970
So again, the things we needed to
remember line 12 in our for loop when

809
00:37:01,970 --> 00:37:08,010
we're setting it up, int i=0,
length = strlen (s).

810
00:37:08,010 --> 00:37:09,350
>> This is a fantastic strategy.

811
00:37:09,350 --> 00:37:12,590
The style of this is great, the
design of this is great.

812
00:37:12,590 --> 00:37:15,620
We can go into those reasons later,
but this way you only need to call

813
00:37:15,620 --> 00:37:19,040
strlen once instead of every single
time you run through the loop.

814
00:37:19,040 --> 00:37:22,255
However, you need to use a comma there
and you don't need int again.

815
00:37:22,255 --> 00:37:24,180
If you're declaring both those
variables together, you

816
00:37:24,180 --> 00:37:26,850
don't need int again.

817
00:37:26,850 --> 00:37:29,820
And then if you're going to use strlen,
which is another library

818
00:37:29,820 --> 00:37:33,560
function not included in
stdio.h or cs50.h, make

819
00:37:33,560 --> 00:37:35,310
sure you include string.h.

820
00:37:35,310 --> 00:37:35,900
Kurt?

821
00:37:35,900 --> 00:37:39,085
>> STUDENT: So then if you don't need to
say what type length is, does that

822
00:37:39,085 --> 00:37:40,290
mean it has to be an int?

823
00:37:40,290 --> 00:37:41,930
You couldn't have two different
types there?

824
00:37:41,930 --> 00:37:43,290
>> JASON HIRSCHHORN: So you could
have two different types.

825
00:37:43,290 --> 00:37:45,730
Length is an int here, though.

826
00:37:45,730 --> 00:37:48,590
Length is an int because it's comma
and then the same thing.

827
00:37:48,590 --> 00:37:50,460
>> STUDENT: So it assumes that it's going
to be the same as the first one?

828
00:37:50,460 --> 00:37:52,423
>> JASON HIRSCHHORN: Exactly.

829
00:37:52,423 --> 00:37:54,970
We're not going to do this now, but you
can include two variables on the

830
00:37:54,970 --> 00:37:57,080
same line using one data type.

831
00:37:57,080 --> 00:38:01,415
So int x, y would create
two variables.

832
00:38:01,415 --> 00:38:05,830

833
00:38:05,830 --> 00:38:10,770
So int x, y, if we make that, it gives
me an unused variable x and y because

834
00:38:10,770 --> 00:38:12,020
we don't use them.

835
00:38:12,020 --> 00:38:17,950

836
00:38:17,950 --> 00:38:21,650
So putting void before is just a
fancy trick if you don't use

837
00:38:21,650 --> 00:38:22,810
something, to use it.

838
00:38:22,810 --> 00:38:26,300
But as you see, I create x and y
on the same line with a comma.

839
00:38:26,300 --> 00:38:28,450
It's creating two integers
right there.

840
00:38:28,450 --> 00:38:31,560
So yes, totally possible to create
two things on the same line

841
00:38:31,560 --> 00:38:33,720
separated by a comma.

842
00:38:33,720 --> 00:38:35,470
Let's get rid of those, though.

843
00:38:35,470 --> 00:38:36,370
>> So there's spell.

844
00:38:36,370 --> 00:38:39,200
Thank you guys for coding it for us.

845
00:38:39,200 --> 00:38:43,040
Let's make it one more time, run it.

846
00:38:43,040 --> 00:38:44,580
Give me a string.

847
00:38:44,580 --> 00:38:45,580
[INAUDIBLE], you did most of it.

848
00:38:45,580 --> 00:38:47,910
What's the string?

849
00:38:47,910 --> 00:38:48,250
>> STUDENT: Hello.

850
00:38:48,250 --> 00:38:50,110
>> JASON HIRSCHHORN: Hello.

851
00:38:50,110 --> 00:38:52,810
H-E-L-L-O, each character on one line.

852
00:38:52,810 --> 00:38:53,700
Looks great.

853
00:38:53,700 --> 00:38:54,750
Let's run it again.

854
00:38:54,750 --> 00:38:56,050
Always going to check
for corner cases.

855
00:38:56,050 --> 00:38:57,750
What's a good corner case to check?

856
00:38:57,750 --> 00:39:00,250

857
00:39:00,250 --> 00:39:00,890
>> STUDENT: A number?

858
00:39:00,890 --> 00:39:02,240
>> JASON HIRSCHHORN: A number.

859
00:39:02,240 --> 00:39:06,580
So I'll put in 1, and it prints out 1.

860
00:39:06,580 --> 00:39:08,970
It's not asking for letters, so
it seems like numbers work.

861
00:39:08,970 --> 00:39:09,890
Good corner case to check.

862
00:39:09,890 --> 00:39:11,740
What's another good corner
case to check?

863
00:39:11,740 --> 00:39:12,420
>> STUDENT: A blank string.

864
00:39:12,420 --> 00:39:12,910
>> JASON HIRSCHHORN: Hm?

865
00:39:12,910 --> 00:39:13,880
>> STUDENT: Like an empty string.

866
00:39:13,880 --> 00:39:14,400
>> JASON HIRSCHHORN: Empty string.

867
00:39:14,400 --> 00:39:16,730
So let's not put in anything.

868
00:39:16,730 --> 00:39:18,660
And what gets printed out?

869
00:39:18,660 --> 00:39:19,090
>> STUDENT: Nothing.

870
00:39:19,090 --> 00:39:19,870
>> JASON HIRSCHHORN: Nothing.

871
00:39:19,870 --> 00:39:20,555
Seems like it works.

872
00:39:20,555 --> 00:39:23,790
You could try more corner cases, but
generally numbers, a different data is

873
00:39:23,790 --> 00:39:26,160
great to try, as well is nothing.

874
00:39:26,160 --> 00:39:29,180
Nothing, zero, negatives are always
great things to try and things you

875
00:39:29,180 --> 00:39:33,140
should test, things certainly we'll test
for and Check 50 will test for.

876
00:39:33,140 --> 00:39:34,720
Any questions about spell.c?

877
00:39:34,720 --> 00:39:37,752

878
00:39:37,752 --> 00:39:39,310
>> OK, let's head back.

879
00:39:39,310 --> 00:39:42,420
I have one more program
for you guys to write.

880
00:39:42,420 --> 00:39:44,700
It's called students.c.

881
00:39:44,700 --> 00:39:49,020
This is a program that asks the user for
five names, then randomly chooses

882
00:39:49,020 --> 00:39:51,600
and prints out one of the names.

883
00:39:51,600 --> 00:39:54,840
This program is a little bit more
involved than the last two we wrote,

884
00:39:54,840 --> 00:39:58,080
so why don't we take five minutes to
code it individually, and then we'll

885
00:39:58,080 --> 00:40:00,130
code it as a group.

886
00:40:00,130 --> 00:40:03,795
>> This program also involve some things
you may not have seen before, so if

887
00:40:03,795 --> 00:40:06,390
you have a question about, how do
I do that, raise your hand.

888
00:40:06,390 --> 00:40:08,530
I'll come over and point you
in the right direction.

889
00:40:08,530 --> 00:40:10,820
But we're also doing it because this
will be a good way to introduce some

890
00:40:10,820 --> 00:40:12,070
topics that are tricky.

891
00:40:12,070 --> 00:44:14,600

892
00:44:14,600 --> 00:44:15,560
>> So it works, right?

893
00:44:15,560 --> 00:44:17,580
>> STUDENT: Yeah.

894
00:44:17,580 --> 00:44:19,180
>> JASON HIRSCHHORN: Whenever you turn in a
problem set, there's many ways to do

895
00:44:19,180 --> 00:44:20,790
it, but some are more
elegant than others.

896
00:44:20,790 --> 00:44:29,000

897
00:44:29,000 --> 00:44:31,110
Let us dive in.

898
00:44:31,110 --> 00:44:32,880
#include cs50.h.

899
00:44:32,880 --> 00:44:34,250
#include stdio.h.

900
00:44:34,250 --> 00:44:37,240
int main void, curly braces.

901
00:44:37,240 --> 00:44:39,430
Inside the curly braces,
I have two comments.

902
00:44:39,430 --> 00:44:41,500
The first one is, ask the
user for five names.

903
00:44:41,500 --> 00:44:46,580
The second one is, randomly choose
and print out one of the names.

904
00:44:46,580 --> 00:44:50,250
So let's do the first one, ask
the user for five names.

905
00:44:50,250 --> 00:44:53,730
Who has asked the user for five names
and can tell me how to do that?

906
00:44:53,730 --> 00:44:56,760

907
00:44:56,760 --> 00:44:58,570
Only two people did the first part.

908
00:44:58,570 --> 00:45:00,150
Three people did the first part.

909
00:45:00,150 --> 00:45:00,670
Four, five.

910
00:45:00,670 --> 00:45:03,140
Tom, how do you do it?

911
00:45:03,140 --> 00:45:09,940
>> STUDENT: You would declare a string
and then name, bracket five.

912
00:45:09,940 --> 00:45:12,070
>> JASON HIRSCHHORN: String
name bracket five.

913
00:45:12,070 --> 00:45:15,350
So what is this line doing,
string name bracket five?

914
00:45:15,350 --> 00:45:17,800
>> STUDENT: You would declare
an array of strings.

915
00:45:17,800 --> 00:45:19,000
>> JASON HIRSCHHORN: This is
an array of strings.

916
00:45:19,000 --> 00:45:21,590
How many strings are going
to fit in here?

917
00:45:21,590 --> 00:45:22,010
>> STUDENT: Five.

918
00:45:22,010 --> 00:45:22,320
>> JASON HIRSCHHORN: Five.

919
00:45:22,320 --> 00:45:22,660
Excellent.

920
00:45:22,660 --> 00:45:26,360
We're going to call it Names because
we're going to get multiple ones.

921
00:45:26,360 --> 00:45:27,790
OK, next?

922
00:45:27,790 --> 00:45:29,270
>> STUDENT: And then you
have a for loop--

923
00:45:29,270 --> 00:45:30,060
>> JASON HIRSCHHORN: For loop.

924
00:45:30,060 --> 00:45:33,035
>> STUDENT: --for i=0.

925
00:45:33,035 --> 00:45:37,040
>> JASON HIRSCHHORN: For (int i=0.

926
00:45:37,040 --> 00:45:38,270
>> STUDENT: Semicolon.

927
00:45:38,270 --> 00:45:38,880
>> JASON HIRSCHHORN: Semicolon.

928
00:45:38,880 --> 00:45:39,790
>> STUDENT: I less than five.

929
00:45:39,790 --> 00:45:41,310
>> JASON HIRSCHHORN: I less than five.

930
00:45:41,310 --> 00:45:42,230
>> STUDENT: Semicolon.

931
00:45:42,230 --> 00:45:42,560
>> JASON HIRSCHHORN: Semicolon.

932
00:45:42,560 --> 00:45:43,380
>> STUDENT: i++.

933
00:45:43,380 --> 00:45:46,050
>> JASON HIRSCHHORN: i++.

934
00:45:46,050 --> 00:45:47,100
>> STUDENT: And then closed parentheses.

935
00:45:47,100 --> 00:45:48,600
>> JASON HIRSCHHORN: Closed parentheses,
and then some curly braces.

936
00:45:48,600 --> 00:45:50,050
Inside the curly braces?

937
00:45:50,050 --> 00:45:51,423
>> STUDENT: And then printf.

938
00:45:51,423 --> 00:45:52,535
>> JASON HIRSCHHORN: Printf.

939
00:45:52,535 --> 00:45:56,200
>> STUDENT: Open parentheses, name %d.

940
00:45:56,200 --> 00:45:57,090
>> JASON HIRSCHHORN: Sorry?

941
00:45:57,090 --> 00:45:57,660
>> STUDENT: Name.

942
00:45:57,660 --> 00:45:58,200
>> JASON HIRSCHHORN: Name.

943
00:45:58,200 --> 00:46:01,700
>> STUDENT: And then space, and then %d.

944
00:46:01,700 --> 00:46:02,950
>> JASON HIRSCHHORN: Name, space, %d.

945
00:46:02,950 --> 00:46:05,630

946
00:46:05,630 --> 00:46:10,820
>> STUDENT: And then /n, and then closed.

947
00:46:10,820 --> 00:46:15,070
>> JASON HIRSCHHORN: So name, space,
%d/n, closed parentheses.

948
00:46:15,070 --> 00:46:17,730
>> STUDENT: Comma, i + 1.

949
00:46:17,730 --> 00:46:19,390
I'm sorry.

950
00:46:19,390 --> 00:46:21,040
Closed quote, and then comma, i +1.

951
00:46:21,040 --> 00:46:24,400
>> JASON HIRSCHHORN: Closed
quote, comma, i+1.

952
00:46:24,400 --> 00:46:25,850
>> STUDENT: Yes.

953
00:46:25,850 --> 00:46:27,650
And then the next line--

954
00:46:27,650 --> 00:46:29,400
>> JASON HIRSCHHORN: i + 1.

955
00:46:29,400 --> 00:46:31,600
>> STUDENT: Yes.

956
00:46:31,600 --> 00:46:36,440
And then the next slide
is names bracket i.

957
00:46:36,440 --> 00:46:39,000
>> JASON HIRSCHHORN: Next line
is names bracket i.

958
00:46:39,000 --> 00:46:41,360
>> STUDENT: = get string.

959
00:46:41,360 --> 00:46:43,460
>> JASON HIRSCHHORN: = Get--

960
00:46:43,460 --> 00:46:44,510
capital G-E-T--

961
00:46:44,510 --> 00:46:45,530
string.

962
00:46:45,530 --> 00:46:47,800
Oops, sorry.

963
00:46:47,800 --> 00:46:49,050
>> STUDENT: Open, closed parentheses
and then--

964
00:46:49,050 --> 00:46:50,940
>> JASON HIRSCHHORN: Open, closed paren.

965
00:46:50,940 --> 00:46:51,030
>> STUDENT: Semicolon.

966
00:46:51,030 --> 00:46:52,500
>> JASON HIRSCHHORN: Semicolon.

967
00:46:52,500 --> 00:46:57,545
OK, so you initialize the array of
strings, names, and then go through a

968
00:46:57,545 --> 00:47:02,720
for loop, and five times print out some
prompt to the user and then store

969
00:47:02,720 --> 00:47:03,360
five strings.

970
00:47:03,360 --> 00:47:08,090
Does anybody have any
adjustments to this?

971
00:47:08,090 --> 00:47:10,030
Tom, I think everybody
agrees with you.

972
00:47:10,030 --> 00:47:10,880
Looks pretty good.

973
00:47:10,880 --> 00:47:11,660
OK.

974
00:47:11,660 --> 00:47:14,830
Why did you do i+1, though?

975
00:47:14,830 --> 00:47:18,000
>> STUDENT: I mean, doesn't have
to be, but i+1 so that it's

976
00:47:18,000 --> 00:47:19,700
name one to name five.

977
00:47:19,700 --> 00:47:21,930
Otherwise, it would be name
zero to name four.

978
00:47:21,930 --> 00:47:24,400
>> JASON HIRSCHHORN: So you did i+1
because otherwise it would have

979
00:47:24,400 --> 00:47:27,060
printed out zero through four because
that's how computers think, but humans

980
00:47:27,060 --> 00:47:28,530
don't really read like that.

981
00:47:28,530 --> 00:47:30,190
That is another thing
you'll encounter.

982
00:47:30,190 --> 00:47:33,210
Sometimes when you print out something
to the user, you'll want to make it a

983
00:47:33,210 --> 00:47:35,733
little bit prettier, and doing
i+1 in this case is a

984
00:47:35,733 --> 00:47:37,430
way to make it prettier.

985
00:47:37,430 --> 00:47:41,380
>> I also see five twice, string name
is five, i is less than five.

986
00:47:41,380 --> 00:47:42,430
I know where you got the five.

987
00:47:42,430 --> 00:47:44,890
You got it from me telling
you five names.

988
00:47:44,890 --> 00:47:49,000
But if we wanted to make the code a
little bit more robust, one thing we

989
00:47:49,000 --> 00:47:53,110
could do is out here, outside of int,
we could do something like this,

990
00:47:53,110 --> 00:47:57,070
#define, and we could do NAMES 5.

991
00:47:57,070 --> 00:48:01,580

992
00:48:01,580 --> 00:48:05,390
>> So basically, wherever I write capital
N, capital A, capital M, capital E,

993
00:48:05,390 --> 00:48:08,890
capital S, that string, it will replace
that with 5 in the code.

994
00:48:08,890 --> 00:48:11,530

995
00:48:11,530 --> 00:48:13,445
Or let's just call it-- we'll
just call it NUMBER.

996
00:48:13,445 --> 00:48:17,190
So I'm defining all caps
NUMBER to be 5.

997
00:48:17,190 --> 00:48:22,830
>> So now wherever I have the number 5, I
can replace it with all caps NUMBER.

998
00:48:22,830 --> 00:48:26,880
And now, if I want to change the number
of names, all I need to do is

999
00:48:26,880 --> 00:48:31,350
this one line, line four, where I have
#define number 5, change that to 10,

1000
00:48:31,350 --> 00:48:34,340
or change that to 4.

1001
00:48:34,340 --> 00:48:37,400
When your code gets longer, you're not
going to remember all the places you

1002
00:48:37,400 --> 00:48:41,630
put how many names you wanted, and in
order for you not to miss one, this is

1003
00:48:41,630 --> 00:48:45,490
a good way to do it, to find something
up top and then just use that phrase

1004
00:48:45,490 --> 00:48:46,400
everywhere throughout your code.

1005
00:48:46,400 --> 00:48:46,780
Marcus?

1006
00:48:46,780 --> 00:48:49,780
>> STUDENT: Is that stylistically preferred
than globally defining a

1007
00:48:49,780 --> 00:48:51,270
constant n?

1008
00:48:51,270 --> 00:48:53,760
>> JASON HIRSCHHORN: So is that
stylistically preferred than globally

1009
00:48:53,760 --> 00:48:54,980
defining a constant n?

1010
00:48:54,980 --> 00:48:59,036
So you're saying you could
do, int number = 5?

1011
00:48:59,036 --> 00:49:01,300
>> STUDENT: Do they have consonants
in C or no?

1012
00:49:01,300 --> 00:49:02,720
>> JASON HIRSCHHORN: Or const-- in equal.

1013
00:49:02,720 --> 00:49:04,275
So you're saying just
a global variable?

1014
00:49:04,275 --> 00:49:04,990
>> STUDENT: Yeah.

1015
00:49:04,990 --> 00:49:08,670
>> JASON HIRSCHHORN: So stylistically,
there's no necessarily one right

1016
00:49:08,670 --> 00:49:12,050
answer, but both of those are better
than just throwing these random

1017
00:49:12,050 --> 00:49:12,960
numbers throughout your code.

1018
00:49:12,960 --> 00:49:14,650
We call them magic numbers.

1019
00:49:14,650 --> 00:49:16,450
When I look at your code or when
anybody's reading through it, they

1020
00:49:16,450 --> 00:49:18,790
don't want just these random numbers
that don't make sense.

1021
00:49:18,790 --> 00:49:22,420
Even the number 26, which generally is
the number of letters in the alphabet,

1022
00:49:22,420 --> 00:49:28,870
it's better to define ALPHABET up top
and save the number 26 in that string,

1023
00:49:28,870 --> 00:49:29,550
if that makes sense.

1024
00:49:29,550 --> 00:49:33,210
>> Or on problem set one, rather than--
again, we didn't talk about this at

1025
00:49:33,210 --> 00:49:37,020
the time, but instead of just throwing
in a 25 everywhere or a 10, you can

1026
00:49:37,020 --> 00:49:41,860
save the number 25 as QUARTER or 10 as
DIME, and that makes it easier for

1027
00:49:41,860 --> 00:49:44,140
whoever's reading through your code to
understand why you chose that number.

1028
00:49:44,140 --> 00:49:46,540
And also for you, if you ever want to
update your code, it makes it a bit

1029
00:49:46,540 --> 00:49:47,290
more robust.

1030
00:49:47,290 --> 00:49:48,070
Yeah?

1031
00:49:48,070 --> 00:49:50,830
>> STUDENT: Is this code also going
to be available for us after?

1032
00:49:50,830 --> 00:49:52,453
>> JASON HIRSCHHORN: All of these codes
that we have written together are

1033
00:49:52,453 --> 00:49:54,620
going to be emailed to you
guys and available.

1034
00:49:54,620 --> 00:49:55,740
Yes.

1035
00:49:55,740 --> 00:49:58,040
So ask the user for five names.

1036
00:49:58,040 --> 00:50:00,280
Randomly choose and print
one of the names.

1037
00:50:00,280 --> 00:50:00,940
Who did this line?

1038
00:50:00,940 --> 00:50:02,230
This is the trickiest line here.

1039
00:50:02,230 --> 00:50:02,450
Sorry, Tom.

1040
00:50:02,450 --> 00:50:05,510
You did a really good job, but
this is a bit trickier.

1041
00:50:05,510 --> 00:50:06,600
Marcus, go ahead.

1042
00:50:06,600 --> 00:50:10,180
>> STUDENT: Well first, you're going to
need to include stdlib.h and time.h.

1043
00:50:10,180 --> 00:50:13,200
>> JASON HIRSCHHORN: So you're saying at
the top, include two more libraries.

1044
00:50:13,200 --> 00:50:18,830
You want me to include S-T-D-L-I-B
dot H, Standard Library, and what

1045
00:50:18,830 --> 00:50:19,760
was the other one?

1046
00:50:19,760 --> 00:50:21,010
>> STUDENT: Time.h.

1047
00:50:21,010 --> 00:50:25,130

1048
00:50:25,130 --> 00:50:27,840
>> JASON HIRSCHHORN: #include time.h.

1049
00:50:27,840 --> 00:50:29,470
So these are two more libraries.

1050
00:50:29,470 --> 00:50:31,790
I assume now we're going to be using
functions that are contained in these

1051
00:50:31,790 --> 00:50:34,240
two, which is why you needed
to include them?

1052
00:50:34,240 --> 00:50:36,980
>> STUDENT: So then, first you're
going to want to write srand.

1053
00:50:36,980 --> 00:50:38,770
>> JASON HIRSCHHORN: Srand.

1054
00:50:38,770 --> 00:50:42,070
>> STUDENT: (time(0)

1055
00:50:42,070 --> 00:50:42,700
>> JASON HIRSCHHORN: Sorry.

1056
00:50:42,700 --> 00:50:45,580
Srand (time(

1057
00:50:45,580 --> 00:50:46,890
>> STUDENT: 0.

1058
00:50:46,890 --> 00:50:47,230
>> JASON HIRSCHHORN: 0.

1059
00:50:47,230 --> 00:50:49,990
>> STUDENT: )), semicolon.

1060
00:50:49,990 --> 00:50:51,320
>> JASON HIRSCHHORN: Another closed paren?

1061
00:50:51,320 --> 00:50:51,650
Semicolon.

1062
00:50:51,650 --> 00:50:53,610
I'm going to just replace
your zero with NULL.

1063
00:50:53,610 --> 00:50:55,980
It'll do about the same thing, but
generally, you'll see NULL, which

1064
00:50:55,980 --> 00:50:57,720
we'll talk about this
line in a second.

1065
00:50:57,720 --> 00:50:58,580
>> STUDENT: Stylistically better?

1066
00:50:58,580 --> 00:51:02,240
>> JASON HIRSCHHORN: Generally, people
right NULL, but same thing.

1067
00:51:02,240 --> 00:51:05,010
>> STUDENT: And then int name.

1068
00:51:05,010 --> 00:51:06,490
>> JASON HIRSCHHORN: Int name.

1069
00:51:06,490 --> 00:51:07,280
>> STUDENT: =

1070
00:51:07,280 --> 00:51:07,780
>> JASON HIRSCHHORN: =

1071
00:51:07,780 --> 00:51:08,580
>> STUDENT: Rand.

1072
00:51:08,580 --> 00:51:09,340
>> JASON HIRSCHHORN: Rand.

1073
00:51:09,340 --> 00:51:10,420
>> STUDENT: ()

1074
00:51:10,420 --> 00:51:11,500
>> JASON HIRSCHHORN: ()

1075
00:51:11,500 --> 00:51:12,850
>> STUDENT: Modulo number.

1076
00:51:12,850 --> 00:51:14,310
>> JASON HIRSCHHORN: What's modulo?

1077
00:51:14,310 --> 00:51:14,970
The character?

1078
00:51:14,970 --> 00:51:16,000
The percent character?

1079
00:51:16,000 --> 00:51:17,790
>> STUDENT: Percent character.

1080
00:51:17,790 --> 00:51:19,521
Modulo NUMBER that you defined up top.

1081
00:51:19,521 --> 00:51:22,030
>> JASON HIRSCHHORN: Modulo NUMBER.

1082
00:51:22,030 --> 00:51:24,052
>> STUDENT: And then semicolon.

1083
00:51:24,052 --> 00:51:24,660
>> JASON HIRSCHHORN: Semicolon.

1084
00:51:24,660 --> 00:51:27,820
>> STUDENT: And then new line, printf.

1085
00:51:27,820 --> 00:51:28,460
>> JASON HIRSCHHORN: Printf.

1086
00:51:28,460 --> 00:51:29,730
>> STUDENT: (

1087
00:51:29,730 --> 00:51:30,620
>> JASON HIRSCHHORN: (

1088
00:51:30,620 --> 00:51:31,650
>> STUDENT: "

1089
00:51:31,650 --> 00:51:32,630
>> JASON HIRSCHHORN: "

1090
00:51:32,630 --> 00:51:35,320
>> STUDENT: %s/n

1091
00:51:35,320 --> 00:51:37,220
>> JASON HIRSCHHORN: %s/n

1092
00:51:37,220 --> 00:51:38,420
>> STUDENT: "

1093
00:51:38,420 --> 00:51:40,640
>> JASON HIRSCHHORN: %s/n"

1094
00:51:40,640 --> 00:51:43,540
>> STUDENT: Comma, space names.

1095
00:51:43,540 --> 00:51:44,350
>> JASON HIRSCHHORN: Space--

1096
00:51:44,350 --> 00:51:46,300
>> STUDENT: Oh, I guess, what
did we call it here?

1097
00:51:46,300 --> 00:51:48,640
>> JASON HIRSCHHORN: We called it names.

1098
00:51:48,640 --> 00:51:52,360
>> STUDENT: Open bracket, name.

1099
00:51:52,360 --> 00:51:55,090
>> JASON HIRSCHHORN: Open bracket, name.

1100
00:51:55,090 --> 00:51:55,850
>> STUDENT: Closed bracket.

1101
00:51:55,850 --> 00:51:56,490
>> JASON HIRSCHHORN: Closed bracket.

1102
00:51:56,490 --> 00:51:57,460
>> STUDENT: )

1103
00:51:57,460 --> 00:51:58,800
>> JASON HIRSCHHORN: )

1104
00:51:58,800 --> 00:51:59,440
>> STUDENT: Semicolon.

1105
00:51:59,440 --> 00:52:00,410
>> JASON HIRSCHHORN: Semicolon.

1106
00:52:00,410 --> 00:52:01,500
OK, let's save.

1107
00:52:01,500 --> 00:52:04,410
Let's make this code.

1108
00:52:04,410 --> 00:52:06,365
Students.

1109
00:52:06,365 --> 00:52:07,100
Let's run this code.

1110
00:52:07,100 --> 00:52:08,480
Students.

1111
00:52:08,480 --> 00:52:11,630
Not ./student. ./students.

1112
00:52:11,630 --> 00:52:12,870
Name one.

1113
00:52:12,870 --> 00:52:15,490
We'll say David.

1114
00:52:15,490 --> 00:52:18,070
Name two, RJ.

1115
00:52:18,070 --> 00:52:20,561
Name three, Rob.

1116
00:52:20,561 --> 00:52:22,490
Name four, Lucas.

1117
00:52:22,490 --> 00:52:26,060
Name five, Joseph.

1118
00:52:26,060 --> 00:52:28,850
And then it prints out Joseph again.

1119
00:52:28,850 --> 00:52:31,730
>> So let me run this again.

1120
00:52:31,730 --> 00:52:32,460
Let me do the same names.

1121
00:52:32,460 --> 00:52:39,760
David, RJ, Rob, Lucas, Joseph.

1122
00:52:39,760 --> 00:52:41,070
And it gave me David this time.

1123
00:52:41,070 --> 00:52:43,030
So I did the same thing, but it gave
me a different thing, and that's

1124
00:52:43,030 --> 00:52:45,410
because, of course, we're asking
for a random name.

1125
00:52:45,410 --> 00:52:50,950
So I think, based off of me wandering
around the room, that most people

1126
00:52:50,950 --> 00:52:55,960
probably got the first part right,
setting up that array and then getting

1127
00:52:55,960 --> 00:52:56,430
all the names.

1128
00:52:56,430 --> 00:52:59,320
I'm talking about lines 10 through 16.

1129
00:52:59,320 --> 00:53:02,150
But lines 18 through 21 are probably
some things you haven't seen before,

1130
00:53:02,150 --> 00:53:03,750
so let's go through those.

1131
00:53:03,750 --> 00:53:10,390
>> This line, srand time NULL
is essentially--

1132
00:53:10,390 --> 00:53:14,300
sorry, let's skip down
a line, rand ().

1133
00:53:14,300 --> 00:53:17,320
That gives you a random number,
a random integer.

1134
00:53:17,320 --> 00:53:19,630
And if you hit man rand, which
a lot of you did--

1135
00:53:19,630 --> 00:53:21,530
M-A-N space R-A-N-D--

1136
00:53:21,530 --> 00:53:25,910
in the terminal like this, M-A-N
space R-A-N-D, hit Enter.

1137
00:53:25,910 --> 00:53:30,490
You would come up to the manual and
would be able to read that the rand

1138
00:53:30,490 --> 00:53:34,260
command outputs num pseudo random bytes
after seeding the random number

1139
00:53:34,260 --> 00:53:35,510
generator once.

1140
00:53:35,510 --> 00:53:38,450

1141
00:53:38,450 --> 00:53:39,700
>> So that's a description.

1142
00:53:39,700 --> 00:53:43,860

1143
00:53:43,860 --> 00:53:46,800
That is what rand does, gives
you a random number when

1144
00:53:46,800 --> 00:53:48,610
you call that function.

1145
00:53:48,610 --> 00:53:53,140
Unfortunately, it's not really random
because if you just call random

1146
00:53:53,140 --> 00:53:55,970
without getting it some seed, some
starting point, it will always choose

1147
00:53:55,970 --> 00:53:59,100
the same one and it uses a formula
to determine the next one.

1148
00:53:59,100 --> 00:54:01,730
So if you call rand, it'll just
give you the same thing.

1149
00:54:01,730 --> 00:54:02,610
It will be very predictable.

1150
00:54:02,610 --> 00:54:03,790
It actually won't be random.

1151
00:54:03,790 --> 00:54:09,610
>> So what we need to do, per Marcus's
instructions to me, were seed rand

1152
00:54:09,610 --> 00:54:13,810
with something else, which is what
the srand function does.

1153
00:54:13,810 --> 00:54:18,055
It seeds rand with something a bit more
random than the same thing, and

1154
00:54:18,055 --> 00:54:23,360
he's seeding it with time NULL, which
is, just by convention, the current

1155
00:54:23,360 --> 00:54:24,650
time in seconds.

1156
00:54:24,650 --> 00:54:29,790
>> So you're assuming that everybody's not
going to run the code at the exact

1157
00:54:29,790 --> 00:54:33,620
same second, so it's a safe bet that
time NULL is going to be different

1158
00:54:33,620 --> 00:54:35,250
every time this code is run.

1159
00:54:35,250 --> 00:54:39,020
Safe bet that thus, srand will seed rand
with something different every

1160
00:54:39,020 --> 00:54:39,990
single time.

1161
00:54:39,990 --> 00:54:44,110
So by convention, when we do want a
random number, we write this line,

1162
00:54:44,110 --> 00:54:50,540
srand time NULL, to give us a much
more random seed, and then we run

1163
00:54:50,540 --> 00:54:52,080
rand, which gives us a random number.

1164
00:54:52,080 --> 00:54:52,840
Yeah?

1165
00:54:52,840 --> 00:54:54,140
>> STUDENT: Can you say what
you mean by seed?

1166
00:54:54,140 --> 00:54:55,630
>> JASON HIRSCHHORN: Seed as in--

1167
00:54:55,630 --> 00:54:58,730
so rand is going to pick some number
and it's going to use some

1168
00:54:58,730 --> 00:55:00,060
formula to get there.

1169
00:55:00,060 --> 00:55:03,250
So it's going to start with something
and then turn it into something based

1170
00:55:03,250 --> 00:55:03,890
off of that formula.

1171
00:55:03,890 --> 00:55:06,090
If you always give it the same thing,
it's always going to turn it into the

1172
00:55:06,090 --> 00:55:06,790
same thing.

1173
00:55:06,790 --> 00:55:08,450
So that first thing you
give it is the seed.

1174
00:55:08,450 --> 00:55:10,880
So you want to give it something
different to start out with, and so it

1175
00:55:10,880 --> 00:55:12,610
will turn it into something different.

1176
00:55:12,610 --> 00:55:15,840
>> And then finally, line 20,
this % is called the

1177
00:55:15,840 --> 00:55:17,470
modulo, and that is confusing.

1178
00:55:17,470 --> 00:55:19,850
That's something that is a little bit
different here that you don't really

1179
00:55:19,850 --> 00:55:22,240
have in real life math.

1180
00:55:22,240 --> 00:55:27,430
But essentially, when you take a giant
number and you divide in C, what

1181
00:55:27,430 --> 00:55:29,482
happens to the remainder?

1182
00:55:29,482 --> 00:55:30,510
>> STUDENT: It gets truncated.

1183
00:55:30,510 --> 00:55:31,220
>> JASON HIRSCHHORN: It gets truncated.

1184
00:55:31,220 --> 00:55:35,200
So 20 divided by 3 is what?

1185
00:55:35,200 --> 00:55:37,950
What does that return to
you, 20 divided by 3.

1186
00:55:37,950 --> 00:55:39,260
Sorry, 10 divided by 3.

1187
00:55:39,260 --> 00:55:41,062
What does that return to you?

1188
00:55:41,062 --> 00:55:41,540
>> STUDENT: [INAUDIBLE].

1189
00:55:41,540 --> 00:55:45,170
>> JASON HIRSCHHORN: 10 divided by 3 just
returns 3, not 3 remainder 1.

1190
00:55:45,170 --> 00:55:51,490
So what 10 modulo 3 returns
is just the remainder, 1.

1191
00:55:51,490 --> 00:55:54,020
So if you wanted to know what 10 divided
by 3 actually was, you would

1192
00:55:54,020 --> 00:55:58,950
do 10 divided by 3, you'd get your 3,
and then you'd do 10 modulo 3, and

1193
00:55:58,950 --> 00:55:59,860
you'd get a 1.

1194
00:55:59,860 --> 00:56:04,630
So you'd realize that 10 divided
by 3 is 3, remainder 1.

1195
00:56:04,630 --> 00:56:07,990
>> And in this line, rand could be
any integer, essentially.

1196
00:56:07,990 --> 00:56:12,660
So modulo is saying, divide it by
number, which is in this case 5.

1197
00:56:12,660 --> 00:56:16,420
And if you ever divide by a number, the
integer always has to be less than

1198
00:56:16,420 --> 00:56:17,590
that number.

1199
00:56:17,590 --> 00:56:22,200
So if you're dividing by a number 5,
the remainder is always going to be

1200
00:56:22,200 --> 00:56:23,396
zero through four.

1201
00:56:23,396 --> 00:56:25,520
That's just how the math works out.

1202
00:56:25,520 --> 00:56:29,700
>> So a large number moduloed by a smaller
number will always return

1203
00:56:29,700 --> 00:56:33,530
remainder and will always return some
number that's less than the number you

1204
00:56:33,530 --> 00:56:34,693
divided by.

1205
00:56:34,693 --> 00:56:37,920
Does anybody have any questions about
modulo, because it will come in very,

1206
00:56:37,920 --> 00:56:41,450
very handy, so I want to make sure
everybody understands it?

1207
00:56:41,450 --> 00:56:43,710
Cool.

1208
00:56:43,710 --> 00:56:46,000
And we saw that this program worked.

1209
00:56:46,000 --> 00:56:50,280
OK, moving on.

1210
00:56:50,280 --> 00:56:52,970
>> So next, we're going to go into
functions, and we're going to go

1211
00:56:52,970 --> 00:56:55,580
through the rest of this stuff pretty
quickly because I want to get to

1212
00:56:55,580 --> 00:56:57,150
command line arguments.

1213
00:56:57,150 --> 00:57:00,330
So functions are essentially,
as you've seen, black boxes.

1214
00:57:00,330 --> 00:57:04,290
They take things in called parameters,
they do something to them, either

1215
00:57:04,290 --> 00:57:06,990
change them or create some side effects,
and finally, they spit

1216
00:57:06,990 --> 00:57:09,530
something out, which is
the return value.

1217
00:57:09,530 --> 00:57:12,495
Can anybody give me one reason
why we would use functions?

1218
00:57:12,495 --> 00:57:15,870

1219
00:57:15,870 --> 00:57:16,360
Anybody?

1220
00:57:16,360 --> 00:57:19,360
One reason why we'd use functions?

1221
00:57:19,360 --> 00:57:21,280
Anyone?

1222
00:57:21,280 --> 00:57:22,700
I saw two hands.

1223
00:57:22,700 --> 00:57:23,590
What's a good reason?

1224
00:57:23,590 --> 00:57:24,930
Marcus?

1225
00:57:24,930 --> 00:57:27,730
>> STUDENT: If you have to call the same
chunk of code multiple times.

1226
00:57:27,730 --> 00:57:29,150
>> JASON HIRSCHHORN: OK, if you need
to call the same chunk of

1227
00:57:29,150 --> 00:57:30,040
code multiple times.

1228
00:57:30,040 --> 00:57:32,470
Jeff, did you have a different reason?

1229
00:57:32,470 --> 00:57:34,230
>> STUDENT: Just so that
you can code less.

1230
00:57:34,230 --> 00:57:35,070
>> JASON HIRSCHHORN: You can code less.

1231
00:57:35,070 --> 00:57:35,320
Right.

1232
00:57:35,320 --> 00:57:38,720
If you want to do something a lot--

1233
00:57:38,720 --> 00:57:41,720
in the previous code, I could have
written get string five times, but a

1234
00:57:41,720 --> 00:57:44,620
for loop makes it look nicer and
you have to write less code.

1235
00:57:44,620 --> 00:57:45,760
Functions are a similar idea.

1236
00:57:45,760 --> 00:57:48,720
If I want to do the same thing a lot,
move it to a function and call that

1237
00:57:48,720 --> 00:57:49,610
function each time.

1238
00:57:49,610 --> 00:57:51,190
That will do that thing for you.

1239
00:57:51,190 --> 00:57:53,740
That's a great reason
to use functions.

1240
00:57:53,740 --> 00:57:54,870
>> So anatomy of a function.

1241
00:57:54,870 --> 00:57:56,670
A function will generally
look like this.

1242
00:57:56,670 --> 00:57:59,280
It'll start with the return type,
then you'll get the name.

1243
00:57:59,280 --> 00:58:02,935
It'll take the arguments, otherwise
known as the parameters, and inside

1244
00:58:02,935 --> 00:58:05,290
those curly braces, you'll
write your code.

1245
00:58:05,290 --> 00:58:07,150
>> We've seen one function a lot.

1246
00:58:07,150 --> 00:58:09,160
That's main.

1247
00:58:09,160 --> 00:58:13,820
Here is main superimposed onto
that anatomy I had earlier.

1248
00:58:13,820 --> 00:58:15,525
What does int right here signify?

1249
00:58:15,525 --> 00:58:18,534

1250
00:58:18,534 --> 00:58:19,450
What does int mean?

1251
00:58:19,450 --> 00:58:21,026
When I write int main (void).

1252
00:58:21,026 --> 00:58:21,740
>> STUDENT: Return type?

1253
00:58:21,740 --> 00:58:22,350
>> JASON HIRSCHHORN: Sorry?

1254
00:58:22,350 --> 00:58:23,170
>> STUDENT: The return type.

1255
00:58:23,170 --> 00:58:24,820
>> JASON HIRSCHHORN: Int is the
return type, exactly right.

1256
00:58:24,820 --> 00:58:25,870
Main returns an int.

1257
00:58:25,870 --> 00:58:28,678
What is main?

1258
00:58:28,678 --> 00:58:29,510
>> STUDENT: Function name.

1259
00:58:29,510 --> 00:58:30,800
>> JASON HIRSCHHORN: Main is the
name of the function.

1260
00:58:30,800 --> 00:58:31,510
Exactly right.

1261
00:58:31,510 --> 00:58:31,840
And void?

1262
00:58:31,840 --> 00:58:33,190
What is void inside the parentheses?

1263
00:58:33,190 --> 00:58:36,650

1264
00:58:36,650 --> 00:58:42,030
>> STUDENT: It doesn't take
any external value.

1265
00:58:42,030 --> 00:58:42,930
>> JASON HIRSCHHORN: So void--

1266
00:58:42,930 --> 00:58:43,240
right.

1267
00:58:43,240 --> 00:58:45,940
Inside the parentheses, we put the
parameters or the arguments the

1268
00:58:45,940 --> 00:58:48,480
function takes, what goes
inside the black box.

1269
00:58:48,480 --> 00:58:50,760
When you write void, you're saying
this takes nothing in.

1270
00:58:50,760 --> 00:58:53,802
So int main void means main's
not taking anything in.

1271
00:58:53,802 --> 00:58:56,950
>> Inside the curly braces, we have some
code, and then we see at the end

1272
00:58:56,950 --> 00:58:57,640
return zero.

1273
00:58:57,640 --> 00:59:01,220
What does return zero mean?

1274
00:59:01,220 --> 00:59:02,470
Well, we talked about return type.

1275
00:59:02,470 --> 00:59:03,780
Main needs to return an int.

1276
00:59:03,780 --> 00:59:04,960
That's what this line does.

1277
00:59:04,960 --> 00:59:07,330
It returns an int, zero.

1278
00:59:07,330 --> 00:59:13,600
>> Of course, we haven't seen that a lot
because in the programming we do with

1279
00:59:13,600 --> 00:59:16,070
the machines we do and the compilers
we use nowadays, you

1280
00:59:16,070 --> 00:59:16,990
don't need that line.

1281
00:59:16,990 --> 00:59:20,070
Main is special enough that it knows
when you reach the end of main, just

1282
00:59:20,070 --> 00:59:22,060
return zero if everything went well.

1283
00:59:22,060 --> 00:59:25,560
In fact, if something doesn't go well,
main will often return a different

1284
00:59:25,560 --> 00:59:28,500
number signifying that something
went wrong with your program.

1285
00:59:28,500 --> 00:59:32,470
>> So generally when you see code, you
don't write return zero, but it's good

1286
00:59:32,470 --> 00:59:34,770
to know that it's happening behind the
scenes because whenever you write

1287
00:59:34,770 --> 00:59:36,370
another type of function,
you are going to need to

1288
00:59:36,370 --> 00:59:37,400
include a return line.

1289
00:59:37,400 --> 00:59:39,710
If you're saying I'm going to
return an int, you have to

1290
00:59:39,710 --> 00:59:41,250
write return something.

1291
00:59:41,250 --> 00:59:45,160
If you say void up here at the
beginning, void main void, for

1292
00:59:45,160 --> 00:59:48,670
example, then it's not returning
anything-- void means nothing--

1293
00:59:48,670 --> 00:59:50,320
so you don't need that return line.

1294
00:59:50,320 --> 00:59:53,610
But you ever write something that's not
void for the return type, you need

1295
00:59:53,610 --> 00:59:55,950
a line that says return and you
need to put something there

1296
00:59:55,950 --> 00:59:57,650
that matches the type.

1297
00:59:57,650 --> 00:59:58,900
Any questions about that?

1298
00:59:58,900 --> 01:00:03,110

1299
01:00:03,110 --> 01:00:04,680
Sweet.

1300
01:00:04,680 --> 01:00:05,930
>> Scope.

1301
01:00:05,930 --> 01:00:07,360
This is something we've
touched on as well.

1302
01:00:07,360 --> 01:00:10,720
Every variable, as we know, has a
certain scope, and that's basically

1303
01:00:10,720 --> 01:00:14,160
talking about where the variable
may or may not be referenced.

1304
01:00:14,160 --> 01:00:16,310
And I like to say what happens
in curly braces stays

1305
01:00:16,310 --> 01:00:18,000
inside the curly braces.

1306
01:00:18,000 --> 01:00:21,490
So for example, if we look at this code,
we're defining a global variable

1307
01:00:21,490 --> 01:00:24,510
outside of main, int a,
up here at the top.

1308
01:00:24,510 --> 01:00:27,380
We're calling another
int a inside main.

1309
01:00:27,380 --> 01:00:29,760
We have some more curly braces, and
we're calling another int a.

1310
01:00:29,760 --> 01:00:31,690
>> We have three a's in code.

1311
01:00:31,690 --> 01:00:32,550
These aren't the same thing.

1312
01:00:32,550 --> 01:00:34,880
I've created three distinct variables.

1313
01:00:34,880 --> 01:00:39,370
Then inside these inner curly
braces, I say a = 4.

1314
01:00:39,370 --> 01:00:44,130
Which a is that talking about, the
first one, the second one,

1315
01:00:44,130 --> 01:00:47,045
or the third one?

1316
01:00:47,045 --> 01:00:47,920
>> STUDENT: The third one.

1317
01:00:47,920 --> 01:00:49,320
>> JASON HIRSCHHORN: It's talking
about the third one.

1318
01:00:49,320 --> 01:00:52,850
All of those a's actually can be
talked about in this particular

1319
01:00:52,850 --> 01:00:58,660
instance because their scope all goes
into these innermost curly braces.

1320
01:00:58,660 --> 01:01:02,360
However, when you call things the same
name, there's something called hiding.

1321
01:01:02,360 --> 01:01:09,670
So you only deal with or fix or use the
most recent variable, or the one

1322
01:01:09,670 --> 01:01:11,090
closest to you.

1323
01:01:11,090 --> 01:01:14,760
So inside here, this third a is closest
to that line, a = 4, so it

1324
01:01:14,760 --> 01:01:16,550
affects the third one.

1325
01:01:16,550 --> 01:01:19,890
And the last line, a = 2, which a is
that talking about, the first, second,

1326
01:01:19,890 --> 01:01:20,805
or third a?

1327
01:01:20,805 --> 01:01:22,530
>> STUDENT: Second.

1328
01:01:22,530 --> 01:01:24,430
>> JASON HIRSCHHORN: It's talking
about the second a.

1329
01:01:24,430 --> 01:01:27,970
Of course, the third a is closest to
it in the code, but the third a

1330
01:01:27,970 --> 01:01:30,200
doesn't exist outside
those curly braces.

1331
01:01:30,200 --> 01:01:32,650
Once you get to that closed curly
brace, it's like it's gone,

1332
01:01:32,650 --> 01:01:33,570
essentially.

1333
01:01:33,570 --> 01:01:36,150
So the only two you have left are those
first two, and of course, now

1334
01:01:36,150 --> 01:01:37,960
it's closest to the second one.

1335
01:01:37,960 --> 01:01:39,410
>> So that's a good thing to
remember about scope.

1336
01:01:39,410 --> 01:01:42,900
For example, I'm sure you've encountered
this in a do while loop.

1337
01:01:42,900 --> 01:01:46,680
If you initialize the variable inside
the do while loop and then you try to

1338
01:01:46,680 --> 01:01:50,360
access it outside, you probably
got some warning like, this

1339
01:01:50,360 --> 01:01:51,680
variable does not exist.

1340
01:01:51,680 --> 01:01:54,370
That's because of the scope idea,
that things only exist

1341
01:01:54,370 --> 01:01:55,320
inside those curly braces.

1342
01:01:55,320 --> 01:01:59,320
Curly braces are like a giant hug that
you're giving to your code, is another

1343
01:01:59,320 --> 01:02:01,380
way to think about it.

1344
01:02:01,380 --> 01:02:05,280
>> So this is another function, something
for you guys to code, hellofunction.c.

1345
01:02:05,280 --> 01:02:08,240
We're going to skip that for now, but
this is something I encourage you to

1346
01:02:08,240 --> 01:02:09,740
practice on your own.

1347
01:02:09,740 --> 01:02:12,510
Write a program in which main calls
another function that prints out a

1348
01:02:12,510 --> 01:02:13,750
greeting to the user.

1349
01:02:13,750 --> 01:02:17,750
It's essentially taking hello world
and abstracting that printf into

1350
01:02:17,750 --> 01:02:18,680
another function.

1351
01:02:18,680 --> 01:02:22,360
But we're not going to do that right now
because we don't have as much time

1352
01:02:22,360 --> 01:02:23,350
left as I would have liked.

1353
01:02:23,350 --> 01:02:26,250
>> So function declaration.

1354
01:02:26,250 --> 01:02:28,080
We've seen this before as well.

1355
01:02:28,080 --> 01:02:31,180
If you want to write another function,
you need to let the computer know and

1356
01:02:31,180 --> 01:02:32,600
main know that it exists.

1357
01:02:32,600 --> 01:02:37,140
So what we do is above main, we write
the return type, the name, and the

1358
01:02:37,140 --> 01:02:42,970
parameters, and then down below main,
down here, we actually copy that line

1359
01:02:42,970 --> 01:02:45,080
and then write the actual code.

1360
01:02:45,080 --> 01:02:50,990
So this way, if you call the function
in main at the top, main knows that

1361
01:02:50,990 --> 01:02:54,650
hello exists, for example, if I were
to call hello, and then it looks to

1362
01:02:54,650 --> 01:02:56,710
the implementation down below.

1363
01:02:56,710 --> 01:02:59,680
What happens if I don't include this
line up top, void hello (void)?

1364
01:02:59,680 --> 01:03:03,820

1365
01:03:03,820 --> 01:03:06,560
>> If I don't include this line
and main calls hello, I'm

1366
01:03:06,560 --> 01:03:07,640
going to get an error.

1367
01:03:07,640 --> 01:03:10,090
I'm going to get an error because
the compiler doesn't know

1368
01:03:10,090 --> 01:03:13,070
that this thing exists.

1369
01:03:13,070 --> 01:03:16,680
This is similar to why we
do #include cs50.h.

1370
01:03:16,680 --> 01:03:21,390
What essentially cs50.h is doing
is putting get string up there.

1371
01:03:21,390 --> 01:03:23,120
It's putting get int up there.

1372
01:03:23,120 --> 01:03:26,570
It's putting all of those function
prototypes up before main so that when

1373
01:03:26,570 --> 01:03:29,900
you call them in main, main knows
that these things exist.

1374
01:03:29,900 --> 01:03:31,630
That's what those .h files do.

1375
01:03:31,630 --> 01:03:33,850
And so if you're going to make your own
function, you've got to make sure

1376
01:03:33,850 --> 01:03:36,690
you put that prototype up there.

1377
01:03:36,690 --> 01:03:38,380
Any questions about this so far?

1378
01:03:38,380 --> 01:03:41,630

1379
01:03:41,630 --> 01:03:42,490
>> Command line arguments.

1380
01:03:42,490 --> 01:03:44,780
This is what we're going to finish
on, and then hopefully we'll

1381
01:03:44,780 --> 01:03:46,180
squeeze in some pset.

1382
01:03:46,180 --> 01:03:48,700
I don't know if we'll be able to squeeze
that in because of time, but

1383
01:03:48,700 --> 01:03:51,560
with you guys who are here, and if you
guys want to say after for a bit, we

1384
01:03:51,560 --> 01:03:54,290
can go over it outside.

1385
01:03:54,290 --> 01:03:56,020
But we're going to finish with
command line arguments

1386
01:03:56,020 --> 01:03:58,510
for the last 15 minutes.

1387
01:03:58,510 --> 01:03:59,930
We awake, Michael?

1388
01:03:59,930 --> 01:04:01,180
You're awake?

1389
01:04:01,180 --> 01:04:01,660
Good.

1390
01:04:01,660 --> 01:04:03,760
I don't know if you sleep
with your eyes closed.

1391
01:04:03,760 --> 01:04:05,800
It would be tricky.

1392
01:04:05,800 --> 01:04:07,180
>> Command line arguments.

1393
01:04:07,180 --> 01:04:10,430
Those are one way to pass information
into a program.

1394
01:04:10,430 --> 01:04:12,780
Eventually, we'll encounter others.

1395
01:04:12,780 --> 01:04:15,600
But essentially, we're going to stop
writing int main (void) if we want to

1396
01:04:15,600 --> 01:04:16,720
take some command line arguments.

1397
01:04:16,720 --> 01:04:19,850
We're going to write this thing
called int main (int argc,

1398
01:04:19,850 --> 01:04:22,430
string argv, brackets.

1399
01:04:22,430 --> 01:04:23,710
That is confusing.

1400
01:04:23,710 --> 01:04:26,070
That's a lot of letters that aren't in
human English, so we're going to

1401
01:04:26,070 --> 01:04:27,680
unpack them right now.

1402
01:04:27,680 --> 01:04:30,450
>> Argc stands for argument count--

1403
01:04:30,450 --> 01:04:32,600
and again, all of these will
be available online--

1404
01:04:32,600 --> 01:04:34,280
and that's the number of arguments.

1405
01:04:34,280 --> 01:04:37,940
Argv stands for argument vector, and
those are the arguments themselves.

1406
01:04:37,940 --> 01:04:38,830
It's an array of strings.

1407
01:04:38,830 --> 01:04:40,880
We've actually seen an array
of strings before today.

1408
01:04:40,880 --> 01:04:44,390
We created one in the last
program we coded.

1409
01:04:44,390 --> 01:04:45,720
>> So here's an example.

1410
01:04:45,720 --> 01:04:50,230
I write ./ohai cs50 section
on my command line.

1411
01:04:50,230 --> 01:04:54,090
Argc in this case equals 3.

1412
01:04:54,090 --> 01:05:01,170
Argv bracket 0 is ohai, argv bracket 1
is cs50, argv bracket 2 is section.

1413
01:05:01,170 --> 01:05:05,160
Again, argc is the number of arguments,
and to trip you up a little

1414
01:05:05,160 --> 01:05:08,710
bit, we always count the name of the
function as the first argument.

1415
01:05:08,710 --> 01:05:12,350
So in this case, there are three
arguments, the name of the function

1416
01:05:12,350 --> 01:05:14,590
and then those two that I typed in.

1417
01:05:14,590 --> 01:05:18,900
>> Next, we're going to save those
arguments themselves in argv.

1418
01:05:18,900 --> 01:05:22,690
And again, just as we saw before, we did
this in the last program we coded.

1419
01:05:22,690 --> 01:05:27,110
You can move through the array of
argv and get those strings.

1420
01:05:27,110 --> 01:05:28,830
That's essentially what we did at
the end of our last program.

1421
01:05:28,830 --> 01:05:33,190
We picked a random name and then went to
names bracket whatever and printed

1422
01:05:33,190 --> 01:05:34,580
out the string located there.

1423
01:05:34,580 --> 01:05:39,870

1424
01:05:39,870 --> 01:05:43,670
>> On top of just the one array, because
strings themselves are arrays, as

1425
01:05:43,670 --> 01:05:46,320
talked about, you can have arrays
of arrays, which is

1426
01:05:46,320 --> 01:05:48,530
essentially what argv is.

1427
01:05:48,530 --> 01:05:51,370
One way you can think about
this is rows and columns.

1428
01:05:51,370 --> 01:05:54,750
I can draw that right now.

1429
01:05:54,750 --> 01:05:57,370
So argv--

1430
01:05:57,370 --> 01:05:58,630
that's not erasing.

1431
01:05:58,630 --> 01:05:59,880
Let's draw under ohai.

1432
01:05:59,880 --> 01:06:09,320

1433
01:06:09,320 --> 01:06:12,490
>> So if I wrote in, ohai cs50 section,
I'm going to truncate

1434
01:06:12,490 --> 01:06:14,520
section for this example.

1435
01:06:14,520 --> 01:06:29,026
But essentially, my multi-dimensional
array, ohai cs50,

1436
01:06:29,026 --> 01:06:32,200
we're going to do sect.

1437
01:06:32,200 --> 01:06:34,610
So here, I can access--

1438
01:06:34,610 --> 01:06:40,930

1439
01:06:40,930 --> 01:06:45,710
if I just give argv one number--

1440
01:06:45,710 --> 01:06:46,390
this is wrong.

1441
01:06:46,390 --> 01:06:49,020
What's wrong in this diagram?

1442
01:06:49,020 --> 01:06:49,690
>> STUDENT: Start at zero.

1443
01:06:49,690 --> 01:06:50,350
>> JASON HIRSCHHORN: It should
start with zero.

1444
01:06:50,350 --> 01:06:52,940
I should probably also rotate it, but
since I've already drawn it on, I'm

1445
01:06:52,940 --> 01:06:54,120
going to leave it on there for now.

1446
01:06:54,120 --> 01:06:55,160
But it starts at zero.

1447
01:06:55,160 --> 01:06:56,410
That's right.

1448
01:06:56,410 --> 01:06:58,980

1449
01:06:58,980 --> 01:07:00,120
I'm sorry if this is small.

1450
01:07:00,120 --> 01:07:01,370
Can you guys kind of see?

1451
01:07:01,370 --> 01:07:03,620

1452
01:07:03,620 --> 01:07:06,670
So if I do argv 0, I'll
get all of ohai.

1453
01:07:06,670 --> 01:07:08,555
If I do argv 1, I'll get all of cs50.

1454
01:07:08,555 --> 01:07:10,720
If I do argv 2, I'll get all of sect.

1455
01:07:10,720 --> 01:07:13,370
>> But now, as we said, strings
are arrays themselves.

1456
01:07:13,370 --> 01:07:19,740
So what I can do is I can treat this
as a multi-dimensional array, and I

1457
01:07:19,740 --> 01:07:24,190
can do something along the lines of
argv 0, 0, which gives me just the

1458
01:07:24,190 --> 01:07:25,400
character 0.

1459
01:07:25,400 --> 01:07:28,760
Argv 0, 1, which gives me just h.

1460
01:07:28,760 --> 01:07:33,960
Argv 1, 3, which gives me just
this 0 down here in cs50.

1461
01:07:33,960 --> 01:07:37,690
>> So because strings themselves
are arrays, we're putting

1462
01:07:37,690 --> 01:07:38,540
that in another array.

1463
01:07:38,540 --> 01:07:42,850
We now have array of arrays, and we
can index into our first array and

1464
01:07:42,850 --> 01:07:44,840
then index into our second array.

1465
01:07:44,840 --> 01:07:48,650
And as you would expect, we do that
by just putting those two--

1466
01:07:48,650 --> 01:07:51,760
we do the square brackets just
right next to one another.

1467
01:07:51,760 --> 01:07:58,930
So in this example, argv 1 is cs50 and
argv 1, 2, like we said, is just 5.

1468
01:07:58,930 --> 01:08:02,120
Does anybody have any questions about
multi-dimensional arrays?

1469
01:08:02,120 --> 01:08:05,090

1470
01:08:05,090 --> 01:08:06,380
You guys are doing awesome.

1471
01:08:06,380 --> 01:08:07,750
Sweet.

1472
01:08:07,750 --> 01:08:11,980
>> So we have another program right here,
and this is the final thing we're

1473
01:08:11,980 --> 01:08:13,690
going to do right now.

1474
01:08:13,690 --> 01:08:18,160
So I want you to write a program that
takes a user's full name, two and only

1475
01:08:18,160 --> 01:08:21,649
two command line arguments, and finally,
print out a greeting to the

1476
01:08:21,649 --> 01:08:24,180
user that includes their first name.

1477
01:08:24,180 --> 01:08:27,800
So why don't you take three or four
minutes to do this, and then we'll go

1478
01:08:27,800 --> 01:08:29,870
over it as a group, and
then we'll wrap up?

1479
01:08:29,870 --> 01:09:44,189

1480
01:09:44,189 --> 01:09:48,084
>> And again, here, when we say take a
user's full name per the description

1481
01:09:48,084 --> 01:09:54,473
of this problem, it should be command
line arguments, not using get string.

1482
01:09:54,473 --> 01:09:55,450
Yeah, Marcus?

1483
01:09:55,450 --> 01:09:58,260
>> STUDENT: So when you say two command
line arguments, is that in addition to

1484
01:09:58,260 --> 01:10:00,000
the ./run, or--

1485
01:10:00,000 --> 01:10:02,980
>> JASON HIRSCHHORN: Good question.

1486
01:10:02,980 --> 01:10:05,995
Do, including the ./run, three
command line arguments.

1487
01:10:05,995 --> 01:10:08,550

1488
01:10:08,550 --> 01:10:11,320
Or including the ./personalized, in
this case, because that's the

1489
01:10:11,320 --> 01:10:12,360
name of this file.

1490
01:10:12,360 --> 01:10:13,300
So argc would be three.

1491
01:10:13,300 --> 01:10:19,480
But I want only two arguments
given to the program.

1492
01:10:19,480 --> 01:10:20,400
I should specify that.

1493
01:10:20,400 --> 01:10:23,380
I'll make that change before I post
these and send them to you guys.

1494
01:10:23,380 --> 01:10:32,747

1495
01:10:32,747 --> 01:10:34,740
>> STUDENT: What if you
have a middle name?

1496
01:10:34,740 --> 01:10:36,800
>> JASON HIRSCHHORN: If you have a middle
name, you're out of luck.

1497
01:10:36,800 --> 01:10:38,050
I'm sorry.

1498
01:10:38,050 --> 01:10:40,320

1499
01:10:40,320 --> 01:10:43,744
Or if you only go by Shakira,
you're also out of luck.

1500
01:10:43,744 --> 01:10:55,672

1501
01:10:55,672 --> 01:10:56,425
Yeah?

1502
01:10:56,425 --> 01:11:00,760
>> STUDENT: So when people don't have
cs50.h, do they define argv

1503
01:11:00,760 --> 01:11:03,450
as char star argv?

1504
01:11:03,450 --> 01:11:05,160
Or is it still called string argv?

1505
01:11:05,160 --> 01:11:07,430
>> JASON HIRSCHHORN: Yes.

1506
01:11:07,430 --> 01:11:10,480
So string, again, we mask over what
exactly is happening there.

1507
01:11:10,480 --> 01:11:14,040
That's going to be unpacked maybe
on Wednesday, but soon.

1508
01:11:14,040 --> 01:11:16,380
It's like training wheels.

1509
01:11:16,380 --> 01:11:18,370
Because essentially, it's
the same thing.

1510
01:11:18,370 --> 01:11:37,040

1511
01:11:37,040 --> 01:11:42,560
>> So let's take another minute to program
this, and then we will go over

1512
01:11:42,560 --> 01:11:45,130
it as a group, and then
like I said, wrap up.

1513
01:11:45,130 --> 01:11:47,770
And I think they're still extra candy,
so you guys should hopefully take

1514
01:11:47,770 --> 01:11:49,790
that, or please take that.

1515
01:11:49,790 --> 01:12:20,380

1516
01:12:20,380 --> 01:12:22,560
>> Sorry if you're still writing,
but I want to dive in and we

1517
01:12:22,560 --> 01:12:24,010
can finish it together.

1518
01:12:24,010 --> 01:12:30,470
So I have written in, #include
cs50.h, #include stdio.h.

1519
01:12:30,470 --> 01:12:33,970
int main (void), curly brace, curly
brace, inside three lines of

1520
01:12:33,970 --> 01:12:38,760
pseudocode, take a user's full name,
two and only two command line

1521
01:12:38,760 --> 01:12:42,670
arguments, print out a greeting to the
user that includes their first name.

1522
01:12:42,670 --> 01:12:45,810
>> So the first one, take
a user's full name.

1523
01:12:45,810 --> 01:12:47,290
Who can help me take
a user's full name?

1524
01:12:47,290 --> 01:12:50,882

1525
01:12:50,882 --> 01:12:52,780
How do I do that part?

1526
01:12:52,780 --> 01:12:57,685

1527
01:12:57,685 --> 01:12:59,500
Did you guys do that first part?

1528
01:12:59,500 --> 01:13:02,587

1529
01:13:02,587 --> 01:13:03,894
Take a user's full name.

1530
01:13:03,894 --> 01:13:05,860
Well, for that one command line
argument, what should I write?

1531
01:13:05,860 --> 01:13:09,624
What should I change about
my function declaration?

1532
01:13:09,624 --> 01:13:11,140
>> STUDENT: You don't want
it to be void anymore.

1533
01:13:11,140 --> 01:13:11,410
>> JASON HIRSCHHORN: Exactly.

1534
01:13:11,410 --> 01:13:12,385
I don't want it to be void.

1535
01:13:12,385 --> 01:13:13,630
What do I want it to be?

1536
01:13:13,630 --> 01:13:14,650
>> STUDENT: Int argc.

1537
01:13:14,650 --> 01:13:15,720
>> JASON HIRSCHHORN: Int argc.

1538
01:13:15,720 --> 01:13:17,430
>> STUDENT: String argv.

1539
01:13:17,430 --> 01:13:19,055
>> JASON HIRSCHHORN: String argv.

1540
01:13:19,055 --> 01:13:19,705
>> STUDENT: Bracket, bracket.

1541
01:13:19,705 --> 01:13:20,070
>> JASON HIRSCHHORN: Bracket, bracket.

1542
01:13:20,070 --> 01:13:21,780
That's exactly right, and that's
actually how I'm going to get

1543
01:13:21,780 --> 01:13:22,820
a user's full name.

1544
01:13:22,820 --> 01:13:24,070
They're going to put it in there.

1545
01:13:24,070 --> 01:13:27,720

1546
01:13:27,720 --> 01:13:28,780
So that one's done.

1547
01:13:28,780 --> 01:13:30,780
So now I want two and only two
command line arguments.

1548
01:13:30,780 --> 01:13:35,065
So Noah, what is one way I can
check the number of command

1549
01:13:35,065 --> 01:13:36,315
line arguments I have?

1550
01:13:36,315 --> 01:13:39,676

1551
01:13:39,676 --> 01:13:40,725
>> STUDENT: You do an if statement.

1552
01:13:40,725 --> 01:13:42,270
>> JASON HIRSCHHORN: If
statement, exactly.

1553
01:13:42,270 --> 01:13:44,830
What do I want as the condition?

1554
01:13:44,830 --> 01:13:48,370
>> STUDENT: If argc greater than 3.

1555
01:13:48,370 --> 01:13:52,485
>> JASON HIRSCHHORN: If argc
is greater than 3.

1556
01:13:52,485 --> 01:13:58,430
If argc greater than 3, what do I do?

1557
01:13:58,430 --> 01:13:59,400
>> STUDENT: Return 1.

1558
01:13:59,400 --> 01:14:01,856
>> JASON HIRSCHHORN: Return 1.

1559
01:14:01,856 --> 01:14:04,050
Will that ensure I have
two and only two?

1560
01:14:04,050 --> 01:14:07,280

1561
01:14:07,280 --> 01:14:07,820
>> STUDENT: No.

1562
01:14:07,820 --> 01:14:13,110
If argc is greater than
3 and less than 2.

1563
01:14:13,110 --> 01:14:17,770
>> JASON HIRSCHHORN: If argc is greater
than 3 and argc is less than 2.

1564
01:14:17,770 --> 01:14:21,540
What's one way I can write this in just
one line, or without using that

1565
01:14:21,540 --> 01:14:23,890
and Boolean operator?

1566
01:14:23,890 --> 01:14:25,942
>> STUDENT: It's not equal to 2.

1567
01:14:25,942 --> 01:14:28,790
>> JASON HIRSCHHORN: If argc
is not equal to 2.

1568
01:14:28,790 --> 01:14:29,260
Exactly.

1569
01:14:29,260 --> 01:14:34,890
So if argc is not equal to 2, that
will give me this program,

1570
01:14:34,890 --> 01:14:37,250
personalized, when I run it, and
that will give me how many

1571
01:14:37,250 --> 01:14:38,740
command line arguments?

1572
01:14:38,740 --> 01:14:42,110
How many will I check for?

1573
01:14:42,110 --> 01:14:42,370
>> Sorry.

1574
01:14:42,370 --> 01:14:43,400
Let me reword that.

1575
01:14:43,400 --> 01:14:46,620
If argc = 2, how many things did
the user write into the line?

1576
01:14:46,620 --> 01:14:47,500
>> STUDENT: One.

1577
01:14:47,500 --> 01:14:49,120
>> JASON HIRSCHHORN: Just one.

1578
01:14:49,120 --> 01:14:50,430
So before, you were right, Noah.

1579
01:14:50,430 --> 01:14:52,192
What do I actually need to put there?

1580
01:14:52,192 --> 01:14:53,640
>> STUDENT: That equals 3.

1581
01:14:53,640 --> 01:14:55,970
>> JASON HIRSCHHORN: Argc
does not equal 3.

1582
01:14:55,970 --> 01:15:00,250
I want to equal 3 because I want
personalized plus the other two.

1583
01:15:00,250 --> 01:15:02,560
So if argc does not equal 3, return 1.

1584
01:15:02,560 --> 01:15:04,475
Why did you say return 1?

1585
01:15:04,475 --> 01:15:06,180
>> STUDENT: Because that
means it's wrong.

1586
01:15:06,180 --> 01:15:08,420
>> JASON HIRSCHHORN: That means it's wrong,
and what's that going to do?

1587
01:15:08,420 --> 01:15:10,360
>> STUDENT: Re-prompt the user.

1588
01:15:10,360 --> 01:15:11,940
>> JASON HIRSCHHORN: It's going to quit
the program, so it will have to run

1589
01:15:11,940 --> 01:15:14,520
again, but we don't have anything
in here to ask for them again.

1590
01:15:14,520 --> 01:15:14,840
I didn't ask for that.

1591
01:15:14,840 --> 01:15:15,990
So that's exactly right.

1592
01:15:15,990 --> 01:15:17,680
Return 1 just quits the program.

1593
01:15:17,680 --> 01:15:18,520
Great.

1594
01:15:18,520 --> 01:15:21,600
>> And print out a greeting to the user
that includes their first name.

1595
01:15:21,600 --> 01:15:24,010
How can I do that?

1596
01:15:24,010 --> 01:15:25,990
Anna?

1597
01:15:25,990 --> 01:15:26,580
>> STUDENT: Printf.

1598
01:15:26,580 --> 01:15:27,785
>> JASON HIRSCHHORN: Printf.

1599
01:15:27,785 --> 01:15:28,640
>> STUDENT: ("

1600
01:15:28,640 --> 01:15:30,460
>> JASON HIRSCHHORN: (

1601
01:15:30,460 --> 01:15:31,600
>> STUDENT: "hello

1602
01:15:31,600 --> 01:15:33,210
>> JASON HIRSCHHORN: "hello

1603
01:15:33,210 --> 01:15:34,800
>> STUDENT: %s

1604
01:15:34,800 --> 01:15:37,230
>> JASON HIRSCHHORN: %s

1605
01:15:37,230 --> 01:15:38,840
>> STUDENT: /n

1606
01:15:38,840 --> 01:15:41,030
>> JASON HIRSCHHORN: /n

1607
01:15:41,030 --> 01:15:43,120
>> STUDENT: ",

1608
01:15:43,120 --> 01:15:45,130
>> JASON HIRSCHHORN: ",

1609
01:15:45,130 --> 01:15:46,620
>> STUDENT: Argv

1610
01:15:46,620 --> 01:15:47,080
>> JASON HIRSCHHORN: Sorry?

1611
01:15:47,080 --> 01:15:47,750
>> STUDENT: Argv

1612
01:15:47,750 --> 01:15:48,840
>> JASON HIRSCHHORN: Argv

1613
01:15:48,840 --> 01:15:50,190
>> STUDENT: Square brackets 1.

1614
01:15:50,190 --> 01:15:51,230
>> JASON HIRSCHHORN: Square brackets?

1615
01:15:51,230 --> 01:15:51,870
>> STUDENT: 1.

1616
01:15:51,870 --> 01:15:54,050
>> JASON HIRSCHHORN: 1.

1617
01:15:54,050 --> 01:15:55,850
>> STUDENT: And ) semicolon.

1618
01:15:55,850 --> 01:15:58,290
>> JASON HIRSCHHORN: ) semicolon.

1619
01:15:58,290 --> 01:16:00,250
Let us run this.

1620
01:16:00,250 --> 01:16:03,730
Make personalized.

1621
01:16:03,730 --> 01:16:06,392
./personalized.

1622
01:16:06,392 --> 01:16:07,420
It just quit.

1623
01:16:07,420 --> 01:16:10,390
Why did it quit?

1624
01:16:10,390 --> 01:16:11,310
>> STUDENT: You didn't put any arguments.

1625
01:16:11,310 --> 01:16:12,560
>> JASON HIRSCHHORN: I didn't
put any arguments in.

1626
01:16:12,560 --> 01:16:16,360

1627
01:16:16,360 --> 01:16:16,920
Looks great.

1628
01:16:16,920 --> 01:16:18,130
You guys nailed it.

1629
01:16:18,130 --> 01:16:22,420
So again, you went into argv and you
checked at the beginning argc.

1630
01:16:22,420 --> 01:16:24,390
Those are two important
things to remember.

1631
01:16:24,390 --> 01:16:29,250
So in our last two minutes before we
leave, we won't get to pset two, but

1632
01:16:29,250 --> 01:16:33,585
like I said, if you want to stay after
for 10 minutes, I would he happy to go

1633
01:16:33,585 --> 01:16:35,710
over with you guys the logic
for these problem sets.

1634
01:16:35,710 --> 01:16:37,930
We had a lot to cover today
so I didn't get to them.

1635
01:16:37,930 --> 01:16:39,820
Generally, I like to include
that at the end.

1636
01:16:39,820 --> 01:16:42,150
So if you want to stick around,
stick around for 10 minutes.

1637
01:16:42,150 --> 01:16:42,990
We'll do that.

1638
01:16:42,990 --> 01:16:46,250
>> Then, as I mentioned at the beginning,
pset zero and one, we'll get that

1639
01:16:46,250 --> 01:16:48,040
feedback back to you as
soon as possible.

1640
01:16:48,040 --> 01:16:51,260
The goal is to get pset one feedback,
the comments part,

1641
01:16:51,260 --> 01:16:52,850
before pset two is due.

1642
01:16:52,850 --> 01:16:56,730
So if you made some mistakes or
stylistically, things could be better,

1643
01:16:56,730 --> 01:17:01,330
you'll have that information so you
won't do the same things in pset two.

1644
01:17:01,330 --> 01:17:06,140
And then in the future, you'll get all
of your pset feedback and comments

1645
01:17:06,140 --> 01:17:08,190
back before the next one is due.

1646
01:17:08,190 --> 01:17:12,695
>> Finally, if I could put in another
plug for feedback.

1647
01:17:12,695 --> 01:17:13,935
This is your first section.

1648
01:17:13,935 --> 01:17:15,920
It was my first section
teaching this year.

1649
01:17:15,920 --> 01:17:16,930
Go there.

1650
01:17:16,930 --> 01:17:19,800
Write stuff, good and bad,
and I will use it to make

1651
01:17:19,800 --> 01:17:21,320
section better next time.

1652
01:17:21,320 --> 01:17:24,870
And last, again, if you need
anything, let me know.

1653
01:17:24,870 --> 01:17:26,410
I'm more than happy to help you guys.

1654
01:17:26,410 --> 01:17:28,420
If you just want candy, I
will have extra candy.

1655
01:17:28,420 --> 01:17:29,800
I'm happy to bring it to you.

1656
01:17:29,800 --> 01:17:31,190
>> This is going to be an amazing course.

1657
01:17:31,190 --> 01:17:34,900
This is one of, if not my favorite
course, I've taken in my entire life,

1658
01:17:34,900 --> 01:17:37,220
and I hope it will be that
wonderful for you.

1659
01:17:37,220 --> 01:17:40,090
So I want to do everything in my power
to make it that awesome for you.

1660
01:17:40,090 --> 01:17:46,590
I'm going to send out an email if you're
in my section or Noah, and

1661
01:17:46,590 --> 01:17:50,310
going to try and schedule with each of
you guys a time to check in over the

1662
01:17:50,310 --> 01:17:53,820
next couple of weeks so I can get a
sense of what your CS background is,

1663
01:17:53,820 --> 01:17:58,170
and it will also help me tailor these
sections to better suit your needs.

1664
01:17:58,170 --> 01:17:59,740
So that's what I've got lined up.

1665
01:17:59,740 --> 01:18:02,260
Again, I'll see you outside
if you want to be there.

1666
01:18:02,260 --> 01:18:03,700
This was week two section.

1667
01:18:03,700 --> 01:18:04,950
Thank you guys so much.

1668
01:18:04,950 --> 01:18:09,067