>> JASON HIRSCHHORN: Welcome.
So just a few announcements before we start.
So sections, everyone should have sectioned already.
Sections normally run from Sunday to Tuesday.
So you guys should get your assignments.
And then I think next week you'll be with your actual TFs.
So if you have any questions, or trouble, or you forgot to section.
Some people do that.
Just let us know.
Shoot us an email at heads.
>> Then office hours.
We started office hours last week.
Office hours every week.
Monday is in Leverett 8:00 to 11:00.
Tuesdays in the quad, so Cabot 8:00 to 11:00.
Wednesday, Mather 8:30 to 11:30.
And Thursday Annenberg 8:00 to 11:00.
>> So Scratch, a lot of people don't need office hours
help for Scratch, which is completely fine.
If you do, that's absolutely fine as well.
But in the future, the problem sets, they get much harder.
So this is going to be your friend.
Definitely go to office hours.
Plan on going to office hours.
When you get to problem set four, five, six, seven,
you need to go to office hours.
That's where a lot the help and the work gets done.
>> I think last year, just for example, on Thursday night--
so I think the problem sets were due on Friday.
So Thursday night, we would have 200, 250 students at office hours.
So definitely make use of these.
These are your best friends.
Like this is where if you're stuck on a problem set,
this is where you'll probably get help.
So office hours.
OK so those are announcements.
Announcements are done, so let's start.
>> OK, the appliance.
Did everybody download the appliance yet?
No.
As in so so.
I kind of downloaded the appliance.
Right so the appliance is new this year, so we're probably
going to have some bugs in it.
So please download it as early as possible.
So problem set one is going to tell you exactly how to download it.
Getting started installing right here.
It's going to tell you how to download the appliance.
>> So definitely download the appliance sooner rather than later.
Instructions are in the P set spec.
So if you wait until Wednesday night, and then you have trouble,
and you send us an email at midnight on Wednesday night
that you can't download the appliance, that's
not a legit reason for an extension.
You need to do it now, and you will have trouble.
I tried to download it last night.
I ran into a little bit of trouble.
If you do run into trouble installing, definitely
hit retry, because I just hit retry a couple times, and eventually it worked,
which doesn't make any sense, but it does.
So definitely retry, but if you come up against a brick wall,
shoot us an email at heads, and we'll be more than happy to help you guys.
What's up?
>> STUDENT: Just a quick question.
If they check style, there's a part that says we didn't include this.
JASON HIRSCHHORN: Not yet.
STUDENT: It says execute it.
How do you execute that to make sure it's in your system?
>> JASON HIRSCHHORN: You're going to run a bash command.
I'll go over that later.
Cool.
So the appliance can be a little bit intimidating,
because you're used to kind of running in graphical user interface.
So Scratch was a graphical user interface, so what do I mean by that?
What I mean is that when you're programming,
you're basically using blocks of code.
You can see the code and stuff like that.
>> The appliance, you're going to be doing things in a command line environment.
And so for the rest of your programming lives,
you're going to be doing things in the command line environment.
So it's good to jump in early and get some experience.
So let's do it.
So that's Rob.
>> OK, so let's just jump into command line stuff.
So this is the appliance.
Can anybody tell me why we use an appliance?
What is the appliance?
Does anybody know?
What's up?
>> STUDENT: It's just a virtual machine that you run your machine so that you
can get over the OS disagreements between Mac and PC.
JASON HIRSCHHORN: Perfect.
STUDENT: It's a universal template.
JASON HIRSCHHORN: Yeah, it's like a universal template.
That's perfect.
So yeah, it's a virtual machine.
So it's basically an operating system.
This is an operating system.
This is equivalent to your Mac, or you PC, or your Linux.
This is the exact same.
And so you can do similar stuff.
If you wanted to, you could go online for example.
You can't really see it, but right here, Google Chrome.
You can go online if you want to.
This is an operating system.
And the reason we do that is because it's much easier when we're handing out
instructions and everything.
If it's just a uniform environment, it's much easier for us,
and it's much easier for you.
You're not going to run into it run into any idiosyncrasies
when you're programming.
We know exactly what you're going to run into.
So the appliance is here.
>> So when you're starting programming, you're
going to go down to the left hand corner.
There's a little box.
You're going to click it.
This is your terminal window.
So this is where you're going to be a lot of the semester.
So let me zoom in a little bit.
>> OK, so getting around a terminal window is a little bit different.
First, there's no icons.
I can't click anything.
There's nothing to click.
So you have to figure out a way to figure out where you are, what's here,
and how to move around.
>> So the two most useful commands probably are ls-- so what do you think ls does?
>> STUDENT: List.
JASON HIRSCHHORN: Lists, yeah.
It just lists algorithms within the directory.
And then cd.
So let's say I wanted to cd CS50, what did that do?
Yeah?
>> STUDENT: Changes your directory.
JASON HIRSCHHORN: Perfect, yeah.
It just changes the directory.
So let's go back.
So how do I go back?
So let's say I wanted to go back to the previous directory.
Yeah sure.
>> STUDENT: I think you write cd just without anything.
JASON HIRSCHHORN: You can do that.
So perfect.
So if I write cd, this is actually going to pop me
back up to the home directory.
So you see this tilde, that's going to pop me
way back up to where the tilde is.
So it's going to pop me up back to the home directory.
But let's say I did something like this.
Let's say cd CS50.
ls again.
I've got other stuff. cd super section.
cd file, so I'm going deep.
And then let's say I want to just pop back up to the top.
cd.
Let's say I don't want to do that.
Let's say I just want to pop back up to the directory that was right above me.
How do I do that?
So let's say cd CS50.
cd supersection.
So let's I'm here.
Let's say I just want to pop up one time.
How do I do that?
Yeah, what's up?
>> STUDENT: You just type cd supersection again.
>> JASON HIRSCHHORN: You could do that.
Well I'm in supersection.
>> STUDENT: Oh, the one before.
>> JASON HIRSCHHORN: Yeah, so you could do that.
You do cd ~CS50.
So if you look here, this is your path right here.
That's your path.
So you could seriously just go cd ~/CS50.
Boom, you're there.
What's an easier way to do it though?
That was perfectly correct, but what's a slightly easier way
to do it, because a lot of times you're going
to be jumping in directories, jumping out of directories.
So let's go back.
cd supersection.
So let's say I want to get back real quick.
You can do something like cd dot dot.
That's going to pop you back up real quick.
So cd supersection.
cd files.
Let's say I'm here.
By the way, clear is good, because my window was getting a little bit messy.
So ls, so I don't want to be here anymore. cd dot dot.
Take me one up.
Cd dot dot takes me one up.
Cd dot dot and back to my home.
>> OK, so cd, ls.
Probably the most important things, because you're
going to need to know where you are.
ls is going to tell you where you are, and cd
is how you're going to jump around.
>> You can also do some other cool stuff.
So for example ls.
ls is just going to show you the directories in your file.
It's also going to show you the files.
But it's just going to show you what's there.
>> If you wanted something a little bit more cool,
and you're going to actually do this in a couple weeks. ls dash l.
That prints out a little more information.
Can anybody guess on the left hand side the drwx, rwx and stuff like that.
Does anybody have an idea what that might mean?
>> STUDENT: Permissions.
>> JASON HIRSCHHORN: Yeah, perfect.
Permissions.
So you're going to have to deal with this
when you do web programming later on.
If you've ever gone online, and you've clicked on an image
or clicked on a field, and it said permission--
like permission not allowed, or like permission not granted or whatever,
that's because when they're programming, they
haven't set these permissions to let you do that.
So where that comes from.
>> OK, so that's cool.
So we jumped around.
We can look at stuff in our directory.
But how do we make stuff?
I've got directories here.
That's awesome.
How do I make a new directory?
Say I want to make a new directory for right now.
You can do something like this, mkdir.
So make directory.
So what do you want to call this directory?
STUDENT: Awesome.
JASON HIRSCHHORN: Awesome.
I love it.
Make directory awesome.
And there you go.
So we need a list.
You have a new directory.
And you can tell it's a directory in your appliance
because it's got this color, right?
So it's like a bluish purple or whatever.
So let's change.
Let's change directory.
Let's go into directory-- yeah, what's up?
STUDENT: Is there a list of all these [INAUDIBLE] online?
JASON HIRSCHHORN: Yeah, I'll put them up on-- we'll have slides.
It'll be a PDF.
We'll put them up for everybody.
Yeah, sorry it's a lot of commands.
if you want me to slow down, or go back, or anything like that,
definitely let me know.
It's a lot to absorb right away.
What's up?
STUDENT: Can I ask a potentially stupid question?
JASON HIRSCHHORN: There's no stupid questions.
STUDENT: There are.
A directory is for just looking at information where you store it.
>> JASON HIRSCHHORN: Yeah.
>> STUDENT: Is that essentially what you're doing?
So when we make something, hello world or whatever,
it's stored in the directory, yes?
>> JASON HIRSCHHORN: Yeah.
>> STUDENT: Is that true for everything.
Everything is stored in the directory.
>> JASON HIRSCHHORN: Yeah.
Whenever you make it, whatever directory you're in,
it's going to basically store it in that directory.
STUDENT: And when we're calling it, we're calling it from the directory.
>> JASON HIRSCHHORN: Yeah, so the directory is where it es.
So it's kind of like if you have a new folder on your desktop
and then you write a text file, put it in that folder,
that file is in that folder.
Does that make sense?
>> STUDENT: Yeah.
>> JASON HIRSCHHORN: Yeah, what's up?
>> STUDENT: Can I just ask you to make it a little bit bigger?
>> JASON HIRSCHHORN: Oh, yeah.
Sorry.
The projector is so bad.
We had so much trouble with this earlier.
Is that better?
Is that too big?
>> STUDENT: No, no it's not.
>> JASON HIRSCHHORN: No, it's not.
Is it too small?
What's up?
>> STUDENT: I'm sorry, what's the command for clearing?
>> JASON HIRSCHHORN: Oh, clear.
If you want to clear, just type clear. clear enter.
On the last appliance, what it basically does,
it doesn't clear your information, it just kind of shift everything
down one screen.
So if you typed clear-- for example let's say I'm scrolling up.
I can scroll up.
This is everything I did.
But if I type clear, and then I scroll up, here's my stuff.
So it's not erasing it, it's just basically giving you a new frame.
>> STUDENT: How do you [INAUDIBLE] a directory?
>> JASON HIRSCHHORN: We'll get there.
>> [LAUGHTER]
Slow your roll.
All right.
OK, so so we're in a directory right now.
So David, I think in lecture he did some cool stuff.
So let's write a program.
So you can do that a number of different ways.
You can use any kind of text editor.
You can use Nano, you can use Vim, Emacs.
Those are slightly more complicated.
Or you can use something in here that's called gedit.
Sorry.
gedit is not happy.
So gedit is basically just a text editor.
Just like I text edit word processor or something like that.
So it's got a nice interface just like this.
So you do something like that.
So actually let me give it name because I never did that.
>> So what do you want to name our first program?
It's just going to say hi.
>> STUDENT: [INAUDIBLE].
>> JASON HIRSCHHORN: You lack imagination.
OK, gedit.
Hi.
I'm going to do .txt.
OK, so here it is.
So let's write a program.
So I think you saw this in lecture.
You need a main function.
Boop.
So this is just a main function.
Bigger?
Yeah, sorry.
OK so you need a main function.
I think I'm jumping the gun a little bit,
but it's never bad to get used to this kind of stuff.
>> So a main function, every C program you're
going to write for the rest of this course will have a main function.
Does anybody know why?
What's up?
>> STUDENT: Start.
>> JASON HIRSCHHORN: Exactly, so it tells your program where to start.
So if you don't have a main function, and you compile this and stuff,
it's not going to know were to start.
Main is always where it starts.
So you have a main function, and then does anybody remember how to print?
>> STUDENT: printf.
JASON HIRSCHHORN: Yeah, printf.
printf hi.
OK, now I have set you guys up to fail, but will this compile.
>> STUDENT: No.
>> JASON HIRSCHHORN: Why?
There's multiple reasons why.
Sure.
>> STUDENT: It doesn't have the standard [INAUDIBLE].
>> JASON HIRSCHHORN: Perfect.
So what's this called up here?
>> STUDENT: [INAUDIBLE]
>> JASON HIRSCHHORN: It's a header file, right?
It's a header file.
And you're exactly right.
Both of you are right.
So to include that, I just do pound include standard input output .h.
Am I good to go now?
Would this compile?
>> STUDENT: No.
JASON HIRSCHHORN: Why not?
STUDENT: It's a [INAUDIBLE] text.
JASON HIRSCHHORN: Yeah, awesome.
So I just made it.
So I saved it, but gedit hi.txt.
So now if I do ls, here it is.
Right here.
Oh, it's not in awesome.
I must have gone out.
Oh, perfect.
How do we put this in awesome?
So I don't want it here in my home directory with everything else.
I want to put it in the folder.
How do I do that?
>> Not too hard.
Move.
So we've already learned ls, list.
cd to change directories.
cd dot dot to bump out of directories.
And now we're going to start moving around files.
So to move just move hi.txt, and I'm going to put it in awesome.
>> Just be careful.
Whenever you're writing in the command line, everything matters.
So uppercase matters.
If I had done lowercase a, it's not going to know what's going on.
So everything matters to be very specific.
>> So run that ls.
It's no longer here.
Let's change directory into awesome.
ls.
Boom, it's there.
And somebody mentioned why this would not compile.
Why is not going to compile again?
Yeah?
>> STUDENT: It's not in a .c.
>> JASON HIRSCHHORN: Perfect.
It's not a .c, s so how am I going to rename it to a .c?
What's an easy way to do it?
Let's use the command we just did.
So we're just going to move hi.txt to hi.c.
ls hi.c.
And then just for fun, let's just jump into this real quick.
hi.c.
You'll notice it's nice and colorful.
So these are just kind of helpful colors when you're programming.
So the int and the void, those are data types.
We'll go over those in a second.
Hi is a string.
But useful colors.
So if you ever accidentally choose the wrong extension,
you should have these colors.
What's up?
>> STUDENT: How can I copy into a file [INAUDIBLE] into another file?
JASON HIRSCHHORN: Oh, I'm going to show you that too.
So copy remove are on the dock.
Let's make this.
Make sure it works.
Make hi.
Works, run it.
So ./hi awesome.
That is your first program.
Let me make it bigger.
So a really good-- Yeah, what's up?
STUDENT: Why would it be that when I take gedit,
it doesn't give me another line for gedit hi.txt?
It just puts the cursor to a line.
STUDENT: Yeah.
The [INAUDIBLE].
>> JASON HIRSCHHORN: Wait, so say that again.
>> STUDENT: When I type gedit, the first time, the first line--
JASON HIRSCHHORN: Up there at the very top?
STUDENT: Has no J Harvard appliance, and the cursor
is just all the way to the left.
JASON HIRSCHHORN: All the way to the left?
STUDENT: Yeah.
Does anyone else have that?
>> JASON HIRSCHHORN: Oh, well J Harvard is just like the username.
So that's just the standard username.
>> STUDENT: It stops responding.
JASON HIRSCHHORN: Oh, it stops responding.
OK, so what you have to do is you have to close out of gedit.
So close out of gedit, and it should return.
So is gedit open on your appliance right now?
If you exit out of it, it should return and you should be good to go.
Did that work?
>> STUDENT: So did you use the gedit command to open--
JASON HIRSCHHORN: No, the first gedit command was a mistake.
The first gedit command, I to do gedit hi.txt.
So I give it a name, so gedit hi.txt.
hi.txt is the name of the file.
The first one was just a mistake, and then
I quickly closed out of the program.
So the question was if I do gedit, I'm not
going to be able to really do anything else in my command line
until I close that gedit, so I think that's what they ran into.
So close the gedit.
It'll return out of that program, and then you'll
be able to actually type in the command line again.
Is that good?
Dope.
So just make sure to close it if you run into trouble.
>> OK, so a really good question a second ago was copying.
So now I have hi.c.
But let's say I want to change it.
I want it to be slightly better.
I want to say hi class.
But I want to keep this template.
I don't have to rewrite the entire program again.
I want to keep the template.
How do I copy?
>> Easy.
So cp hi.c newhi.c.
Now I have two.
And then I can open up newhi.c, and instead of just saying hi,
I'll say hi class!
So I'll go back.
Exit out.
Go back to my command line.
ls make newhi.
Run it.
There.
It's better, much better.
So if you want to copy something, easy as that. cp is copy.
>> So we've gone over moving, making directories, copying.
Let's see what else should I go over?
Oh, yeah.
Let's go over this.
So this was awesome.
Let's say I wanted to-- oh wait.
I'm going to go back into awesome.
>> Let's say I don't want these files right here.
Let's say I want them in a new directory like in directory awesome.
I want to start organizing, so now I'm going
to have a new directory called hi.
It's got all the programs I write called hi.
So how do we do that?
>> STUDENT: Change directory.
>> JASON HIRSCHHORN: Yeah.
hi.
Boop.
mkdir hi_dir.
It complained because I already had a hi in here.
So now I have a hi directory.
So now I want to move everything, all the programs I wrote,
I want to move them into the hi directory.
Just clean stuff up.
How do I do that?
>> STUDENT: [INAUDIBLE].
>> JASON HIRSCHHORN: Moving it, right?
So let's move.
Yeah?
>> STUDENT: What's the difference between hi and hi.c that we already have.
JASON HIRSCHHORN: Oh, hi.c is the actual.
So that's the file that you wrote.
So if you were going to open it up in gedit or Nano,
that's what you're typing in.
When you compile it, then you get an executable file.
That's hi.
So if you opened up hi, that's going to be a bunch of gibberish to you.
It's basically going to be bunch of computer instructions.
What's up?
>> STUDENT: What do all the colors mean?
The blue is probably directory.
What's yellow?
>> JASON HIRSCHHORN: Those are the executable files.
So when I did make hi.c, it gave me-- so I can do it.
Jumping the gun.
So remove hi yes.
So then let's do ls.
I don't have hi anymore.
So when you make, that's when you take the program that you wrote
and you make a executable file.
So make hi.c-- well, make hi.
I think he went over this in lecture.
>> These are bunch of compiler instructions.
So instead of having to actually write clang with all these.
These are flags.
You just write make.
So this makes the executable files.
So when you do ls, now you have hi.
So this is what you're going to actually run.
And so you run that by doing ./hi, and it runs your program.
But if I try to do that with hi.c, no, it's not going to work.
Yeah, does that make sense?
>> STUDENT: Mm-hm.
>> JASON HIRSCHHORN: Cool.
Wait, I wanted to say something else.
Just real quick, in case anyone is interested,
when you jump out of a directory, it's the dot dot.
Does anybody have an idea of what maybe just the singular dot means?
STUDENT: [INAUDIBLE].
JASON HIRSCHHORN: It's actually the current directory.
So it's your current directory.
So I'm here.
I'm in awesome.
If I do cd dot, it doesn't do anything.
It takes me to my current directory.
So a little redundant, but you have to do that, so ./ your program name,
in order to run your program, you have to do that.
OK.
>> STUDENT: If you do cd dot dot dot, however many dots
will it just take you up that many directories?
>> JASON HIRSCHHORN: You mean like cd dot dot dot dot?
STUDENT: Yeah.
JASON HIRSCHHORN: No.
So actually Remember I did ls -l?
If I do something else, I can ls -a.
So what this says is -a shows you everything.
So it shows you things that are hidden and things that aren't hidden.
So if you notice that when I did just ls, I just saw that stuff,
the stuff I made.
If I do -a, it's going to show me everything.
And so current directory, directory above it.
Does that make sense?
Yeah?
>> STUDENT: But those aren't actually in the directory.
The current directory isn't in itself, so why show those with dots inside?
>> JASON HIRSCHHORN: You mean it's like it's not in the directory.
>> STUDENT: Right.
So we have a list of things that are in the directory.
>> JASON HIRSCHHORN: I think it is in the directory.
Like -a shows everything, even if it's hidden inside the directory.
So I think it's just a route to the current directory if that makes sense.
It's a little recursive.
It's like Inception.
What's up?
>> STUDENT: So you said -a shows [INAUDIBLE]?
JASON HIRSCHHORN: -a, yeah.
>> STUDENT: What makes it [INAUDIBLE]?
JASON HIRSCHHORN: Well, for example, let's go back up to the top.
It's usually stuff you shouldn't mess with and you don't need to worry about.
So this is kind of the home.
If I do -a, I get tons of stuff.
All that stuff.
And you don't need to mess with any of that stuff.
Definitely don't start going in here and just remove stuff.
>> [LAUGHTER]
>> Yeah, this was a little bit more comfy stuff, but it's good to know.
I think it's good to know.
But if you never want to type -a in the entirety of this course,
just forget it.
What's up?
>> STUDENT: What is the ls -l?
JASON HIRSCHHORN: That shows you permissions.
Basically you use that for permissions, so -l.
It shows you a little bit more information,
but again, this is a little bit more comfy stuff.
These are just permissions.
So the permissions of the folders.
So just for example, let's say I made a folder, so the d represents directory.
So those are directories.
And then read, write, execute.
So those are permissions for I think user, group, world.
Totally going too far.
You're going to go in this like weeks from now, but just so you know,
those are permissions.
What's up?
STUDENT: Can you clear rather than just skip to the next thing, can you--
JASON HIRSCHHORN: You mean actually delete?
I don't know.
I never do that.
Why would you want to do that?
Why would you do that?
I like it.
You live on the edge.
You're like I don't want any of this.
That's intense.
STUDENT: [INAUDIBLE].
JASON HIRSCHHORN: Yeah, we can do that.
Sorry, messing with the size is totally freaking me out right now.
Let's go up.
Yeah, so now I can't go up anymore.
So everything I taught you is gone forever.
Thank you.
We made our directories.
We made our files and stuff like that, so we still have all that stuff.
OK, I'll go fast.
>> So real quick.
So let's say I'm in awesome.
Let's say I wanted to move everything to the new directory
because I just want to organize it, right?
So how would I do that.
Move.
We're going to use move again.
So move hi.c.
And what you can do is you start listing stuff,
and the last thing you list is where it's going to move it.
So move hi.c, hi newhi newhi.c, hi_dir.
And so it moved everything into the last thing you mentioned.
So then change directory, hi_dir ls.
Everything is in there.
So it's nice and more organized.
>> OK, let's say I hate my original hi program.
I want to get rid of it.
How do I get rid of it?
What's up?
>> STUDENT: [INAUDIBLE].
JASON HIRSCHHORN: Let's say I just want to remove the program, so just hi.
No, you're right.
So it's rm, but you do rm hi.
It's going to give you a little warning.
It's going to say are you sure you want to delete this, and yes I do.
If you don't want to be prompted like that
because you don't have time for warnings, remove -f.
This is another flag.
It's like the -l.
It's like the -a.
These are just flags that you're giving to these commands.
So remove -f.
-f f means force.
So force it.
I do not want to be prompted.
So remove -f.
Let's do hi.c.
Just got rid of it.
It didn't tell me.
>> But let's say instead I'm done with this directory.
I want to get rid of directory.
So I do remove hi_dir.
No, that doesn't work.
No, so it won't let me remove a directory.
Does anybody have an idea?
>> STUDENT: Is it remove dir?
JASON HIRSCHHORN: So remove like this?
>> STUDENT: No.
Just the [INAUDIBLE].
JASON HIRSCHHORN: No, right now there's no hi.
There's just hi_dir.
So this is the directory.
So actually what you want to do is-- oh, do you have an idea?
I see like a half hand.
>> STUDENT: [INAUDIBLE].
JASON HIRSCHHORN: OK.
>> STUDENT: [INAUDIBLE].
>> JASON HIRSCHHORN: OK.
So if I want to remove a directory, remove -r.
That's recursive.
So remove that directory recursive.
So I want you to go into that directory, remove everything in the directory,
and then get rid of the directory.
And then I'm going to f as well, because I
don't want it to prompt me with warnings after it removes every single file.
So remove -rf hi_dir.
Boom, it's gone.
Just be careful where you use that.
I'm going to use it here too. -rf awesome.
It's gone.
Be careful where you use that.
If you use that here and type something like this, don't ever do that.
That will delete everything on your appliance without ever warning you.
And I've done that.
Grading problem sets at 4:00 AM, I was trying remove problem set directories,
and I did this.
And I was like oh my god.
Don't do-- I tried to cancel it.
You can cancel.
Command C just cancels everything, but I already deleted like 75%.
So I ruined it.
And people do that.
Everyone does this.
Don't do this though this semester.
It sucks.
I'm nervous having it here.
I'm going to delete it.
>> [LAUGHTER]
Can you imagine?
OK, questions.
Yes?
>> STUDENT: So if we do everything you're doing here, is this the same
as just going into actually file save as [INAUDIBLE].
JASON HIRSCHHORN: Oh, you mean through gedit.
STUDENT: Yeah.
JASON HIRSCHHORN: Yeah it is.
gedit is useful.
So gedit is very useful.
In this class, you can do a lot of things through gedit.
You can save.
You can rename.
You can move files and stuff like that, but just truthfully when you go to 50--
well you are in 50-- 51, if you go into 61, this is going to be your life.
This is the command line.
This is where you will program for the rest of your life,
so definitely use gedit if you want.
Definitely a good way to start.
For the first problem sets, definitely use, but every once in a while
try to get more used more comfortable with command line arguments.
But you can, definitely.
What's up?
>> STUDENT: So when we want to delete one of the files we've made,
there are two ways. [INAUDIBLE].
Is there a way when you're deleting a directory
to ask for prompt so you don't end up deleting it?
JASON HIRSCHHORN: rm -r.
So don't force it.
So rm -r.
It's going to go through recursively, delete everything, prompt you
after everything, which will drive you nuts.
Like I almost exclusively do rm -f and rm -rf, just constantly.
I'm like I don't have time.
I'm a busy man.
All right, what's up?
Yeah.
>> STUDENT: Is there an undo, or a trash bin, or anything like that?
JASON HIRSCHHORN: No.
[LAUGHTER]
I could go back and get my appliance that I just deleted, no.
No, no, no.
If you do rm, it should be gone.
What's up?
>> STUDENT: So you can [INAUDIBLE] together.
JASON HIRSCHHORN: Yeah, just like that.
So for example, you saw the rm -rf.
Let's say I want to do ls -la, it shows me
everything, even hidden things with their permissions.
>> STUDENT: So then r, the -r flag, if we don't have that,
we can't delete a directory, right?
>> JASON HIRSCHHORN: Right.
>> STUDENT: So that says that we want to do something with the directory?
Is that what that means?
>> JASON HIRSCHHORN: -r means recursively.
So a directory is going to have files in it, right?
So what you want to do is you want to go into that directory,
delete everything, all the files in that directory,
then pop out and delete the actual directory.
So the -r means recursive, do it all recursively if that makes sense.
Yeah.
>> STUDENT: So directories have [INAUDIBLE] and just delete it.
Or do you still--
>> JASON HIRSCHHORN: Let's try.
So mkdir empty.
Remove empty.
No.
>> STUDENT: No, so you still need the--
>> JASON HIRSCHHORN: Yeah, always.
So -r, -rf, empty.
Awesome.
Anymore questions?
I think that is your crash course on command line stuff.
Any questions, because we're going to jump over to data types, and loops,
and all that stuff?
What's up?
>> STUDENT: I don't have CS50 showing as one of my directories.
Is that normal?
>> JASON HIRSCHHORN: Oh, yeah.
That's mine.
Yeah, that's where my solutions to your stuff is.
>> [LAUGHTER]
>> So yeah, that's absolutely normal.
I think you'll probably just have these four.
I think that might be normal.
Yeah.
OK, any more questions?
I know that was super fast.
What's up?
>> STUDENT: I saw the blue green color.
What is that?
>> JASON HIRSCHHORN: That means directory.
STUDENT: And this blue?
>> JASON HIRSCHHORN: Oh, you mean this blue green?
>> STUDENT: No.
>> JASON HIRSCHHORN: This blue green?
>> STUDENT: No.
When you listed--
JASON HIRSCHHORN: Oh.
Like this?
>> STUDENT: Yes, the icons.
>> JASON HIRSCHHORN: Oh, this.
I don't know.
What does that mean?
So it seems to be pointing to a directory.
>> STUDENT: It almost looks like an alias.
>> JASON HIRSCHHORN: It might be.
You don't need to know that.
Anymore questions?
Awesome.
If you have any questions, I know that was a lot.
A lot of this information will be online.
I'll put it online for you guys.
Section B will be online as well.
Or if you have questions, just come up and ask me.
Here you go.
So now Sharon and Hannah will go over some
of the logic behind problem set one.
>> SHARON: Oh my god.
Troubles.
>> HANNAH: In the meantime, I'm Hanna.
This is Sharon.
We are both CS50 TFs, and we're going to give you
a little intro on a couple important things for problem set one.
Everything from data types to conditionals to for loops.
Loops in general.
>> SHARON: And in the end, we will take a look at P set one,
and what you have to know for it.
So let's start with data types.
You guys should be familiar with the ones that are highlighted in blue.
So we can start with ints.
ints are integers, so one, two, three, four. floats, floating points.
Like integers with decimals, so 5.2 or even 5.0.
Chars are characters like A, B, C. And a string, you guys all know.
Like CS50 or hello and world.
>> Bool is a Boolean, so we have true and false as our Boolean.
So if something computes to truth-- 5 equals 5, that computes to true,
and if we have 5 equals 4, that's false, so that computes to false.
And here we have the associated size of all of these data types.
And you will have to know this for your quizzes,
so this might be helpful to remember, but we'll post these.
Actually they're already posted.
So you should be familiar with the ones highlighted in blue.
>> JASON HIRSCHHORN: Real quick, just in case you
want to know the difference between a char and a string.
So when you're writing stuff, whenever you have something like "hi,"
that's going to be a string.
So if there are two quotes, it's a string.
But if I were to an "h" like that, that's a string.
But if I only have one quote, that's a char.
And so what's the difference you might ask.
Well, the difference is this is one byte of memory.
This is 4 bytes of memory.
>> SHARON: float?
Describe it again?
OK, it's kind of like a number that has a decimal point with it,
so 5.2 or even 5.0 is a float versus just 5 is an int.
STUDENT: [INAUDIBLE].
>> SHARON: Oh, it can have many decimal points, so 5.675 to 3.1415.
Yes?
>> STUDENT: Do we need to know all the sizes for this?
SHARON: Yes.
STUDENT: [INAUDIBLE].
>> SHARON: You don't worry about the white ones just yet.
>> HANNAH: The notes are posted at CS50.net/sections.
STUDENT: Just out of curiosity, is there a utility
to knowing the size of the files you're calling so that we should know it?
>> SHARON: So these are the data types, but yes.
So in this case, which one would take up more memory or more space?
>> JASON HIRSCHHORN: So for a concrete example,
in I think problem set four or five, we're
going to give you a bunch of data, and you basically
have to traverse that data.
So it's useful to know that if you have so many integers,
that's going to be what.
So let's say I gave you ten integers, what's the size of that?
STUDENT: 40.
JASON HIRSCHHORN: 40 bytes, right.
So you might need to know that OK, I need to jump 40 bytes.
So let's say instead it's 40 chars, then-- let's say I gave you 10 chars,
then you know, OK, I only need to jump 10 bytes.
So it's very useful to know the size of the actual data types,
because a lot times you'll be jumping around the data,
so you need to know how far to jump.
>> STUDENT: What's the difference between the quotes and one quote?
>> JASON HIRSCHHORN: OK, two quotes is a string.
One quote is a char.
>> STUDENT: Like operationally, what's the difference?
They both say hi.
JASON HIRSCHHORN: Oh, sorry.
This is just h.
Sorry, this is not clear.
>> STUDENT: So the top one--
JASON HIRSCHHORN: This is a string, yeah.
So it's two characters, right?
String, hi.
But let's say I just had a single character,
if I put two quotes around just h, that's a string.
This is a string h.
Does that make sense?
So this is four bytes.
But let's say I got rid of that an only put one.
One quote.
This is now a char, a char h.
So exact same thing, different data type.
This is only one byte, whereas the string was four bytes.
Yeah.
STUDENT: So if you were trying to store a word like David,
you would want to use a string because regardless of how long the string is,
it will only take four bytes unless it's a long long
or a double, in which case-- so if it extends beyond four characters, then
you want to make it into string.
Is that the general logic behind.
JASON HIRSCHHORN: If it extends beyond one character, it's a string.
>> STUDENT: So there's no [INAUDIBLE] to storing a four letter
word as four individual or three letter word as three individual characters
because that will only be three bytes, as opposed to the three part string.
JASON HIRSCHHORN: You could do that.
You could do that, but I don't think that's really worthwhile,
because you're only saving one byte.
You know what I'm saying?
In the big scheme of things, that won't really matter.
But for example, printf, if you're printing f and you
have three chars you can print that out-- let's say you C-A-T,
you can print out cat just by doing one char one char one char,
or can you print out string cat.
It's the exact same thing.
So you could do it like that, but--
>> STUDENT: It's not going to save you that much.
>> JASON HIRSCHHORN: Yeah.
The headache that it would induce is not worth it.
Yeah?
>> STUDENT: For example, you had the h with one
versus two, the single or double quotes.
Why would you want to save a single letter as a string.
What purpose does that ever serve?
JASON HIRSCHHORN: I don't know.
I mean, it really depends on the program.
For example, I think later on you'll get into command line stuff.
So for example, instead of me asking you for an integer-- sorry,
can I speak into you?
HANNAH: Yes.
>> JASON HIRSCHHORN: So for example, instead
of me asking for something with get end, which
I think you might have seen a lecture, there might be a command line.
You just type it in at the command line.
It would be like -/hello4 or something, or helloh h, whatever.
You do it at the command line instead of being asked for it,
that command line is always a string, whether it's a four or an h,
that is always a string.
So that's one example of when you might have a singular letter or a singular
number represented as a string.
SHARON: All right.
And then here are some basic operators.
Hopefully you're familiar with the first four just in general.
So there's adding, subtracting, multiplying, and dividing,
and make sure you use the right keystrokes.
>> And then there's also modulo, which some of you might not be very familiar with.
And what modulo does is that if we take this example 4 % 2,
it takes the remainder of what that does in division.
So 4 divided by 2 is 2, and there's no remainder.
4 divide by 3 is 1 and 1/3, and so remainder 1.
So it computes to one.
And then 4 % 5 is the fraction 4/5, and the remainder is 4.
Does that make sense?
OK, cool.
And we follow PEMDAS here too.
>> So some Boolean expressions.
So you guys have seen equals equals to compare, say, two numbers.
So if 5 equals equals 5, that computes to true.
Make sure you have only one equals sign.
For not equals, it's bang or exclamation.
>> HANNAH: Two equal signs when you do equality.
>> SHARON: Oh, yeah.
Make sure you don't have one equals sign.
And then for not equals, it's bang equals.
And then you can look through for less than, greater than.
And then we have logical and and logical or right here.
And what that does is, if you remember in Scratch
when you had that block that said and, and then you
could fit two different pieces there, that's what this and does.
And make sure you do two ampersands.
And this key you can find towards the right, right under your Delete button.
Question?
>> STUDENT: What was the [INAUDIBLE]?
>> SHARON: OK, so if you only do one, you're comparing bits.
So if you remember what bits are, they're 0's and 1's, and so
that's comparing something else.
So we're going to focus on this for now.
>> JASON HIRSCHHORN: So one equals is like assignment, right?
So like int x equals 4.
You're saying that variable x equals 4.
So 1 equals is assignment, but the ands and the pipes
or the bars, just like Sharon said, they're bitwise operators.
You don't need to worry about that.
>> STUDENT: When do you use logical and and logical or again?
I just forgot when you do that.
>> HANNAH: Sure, so if you ever want to check if two things are true,
for example, let's say I want to check that a number is between 5 and 15,
I would say make sure that the number is greater than 5.
So let's say if x is greater than 5 and less than 15.
So in order for that whole statement to evaluate to true,
you need both of the kind of sub statements to evaluate the true.
With or you only need one of the two or both.
STUDENT: Great, thank you.
>> SHARON: And also in Scratch, I'm sure in a lot of your projects
you had if touching the edge or if touching another sprite then bounce
or something.
HANNAH: All right, so why do we care about all of these Booleans?
We have these structures that you've seen in Scratch called conditionals.
And conditionals are anything of the form if some condition or some Boolean
is true, then do the code between these curly braces.
So you can see on the right here is the Scratch block.
You have this if then, and anything that goes into that little shape that
looks like this-- i don't know what you'd
call that-- that's going to be the Boolean or condition.
>> So again, a Boolean or condition is anything
that either evaluate to true or false.
And again, you can combine Booleans.
You can have, again, x is greater than 5 and x is less than 15.
Or you can just have one of those x is less than 5.
>> OK, so in C It's on the left.
It's just the keyword if.
In parentheses, the condition or the Boolean.
And then the code in between those two curly braces
will only execute, will only run if that condition or that Boolean is true.
Does that make sense?
Awesome.
OK.
And then as you may have seen in Scratch,
we can also add on an else, which is basically
anything between the curly braces under else
will only execute if the condition is false.
Make sense?
Any questions on these two?
Awesome.
>> Cool.
So here's a quick example.
Let's say we want to determine, based on the time,
whether we should say good morning or good evening.
I'll say if it's before 12:00 noon, we're going to say good morning,
otherwise we're going to say good evening.
And when I said that otherwise, that's equivalent in C to this else.
So we're going to check if the military time is less than 12:00,
say good morning.
Otherwise say good evening.
>> STUDENT: So we'd actually get that input for military time with the get string.
HANNAH: Right.
Did Somewhere else we'd actually have to provide that information.
Right here it has no value.
We never even declared it.
I assume that somewhere above this program I declared the time,
or asked the time, or-- anything else with this example?
Awesome.
>> OK, now we have in addition to that simple if else format,
we have two other different structures.
So first we have the switch statement, and here's
the general form of a switch statement.
You have the keyword switch the same way you have the keyword if.
And then an input, in this case n.
So this can only work with integers.
So that input n has to be an integer, OK?
And we're going to follow one of these cases
depending on what that value n is.
>> So in this case, you first compare is n equal to constant one,
if it is, do everything kind of indented over here.
I can point to it because I'm too short.
If it's not equal to constant one and is instead equal to constant two,
we're going to follow that second block, and we can do this for as many times
as we want, and then that default is if it didn't match any of the above cases,
execute that code.
Any questions here?
This one is a little more complicated.
Yeah?
>> STUDENT: Break signifying what?
>> HANNAH: Sure, so once we enter that block, if we find, let's say,
our n is in fact equal to constant one, we enter that block after the colon.
We do whatever is on that line of commenting,
and then we break, meaning we get out of this switch statement entirely.
STUDENT: OK.
>> HANNAH: Mm-hm?
>> STUDENT: Is the indentation necessary, or do you need [INAUDIBLE]?
HANNAH: Sure.
So, as always, indentation is not strictly necessary.
The computer doesn't care whether you indent.
We, as people who are reading your code, do care.
So it's a lot easier to look up at the screen
and see, oh, I know exactly what happens when n is equal to constant one.
If I had random indents, I wouldn't be able to tell so easily.
Yeah?
>> STUDENT: So if wanted a user to choose one of the options that
we have on the list, would we use, I guess, [INAUDIBLE],
but get an integer from them and put that into n?
HANNAH: Exactly right.
So let's say we wanted to check what integer.
We said oh, enter an integer between 0 and 5, let's say.
We could ask for that n, ask for that value,
and then have each of those cases.
And again, this is something we could very easily do with if statements,
right?
We could have if equal to case one, if equal to case two,
if equal to case three, so on and so forth.
This is a little bit faster and a little cleaner.
It's kind of just a nice structure now.
STUDENT: Faster as in writing?
Or does it run the computer a little faster?
>> HANNAH: It's runs a little faster.
Yeah?
>> STUDENT: So the double dash is just comment or--
>> HANNAH: Oh, I'm so sorry.
I didn't point that out.
OK, if you have that //, is a comment.
Just // is a comment.
So any time the computer sees that, it's going
to say OK, anything that follows this, I'm just going to ignore.
I'm going to pretend you never even wrote that.
>> STUDENT: So this [INAUDIBLE].
>> HANNAH: This does absolutely nothing.
But if I wrote something like on the first comment I wrote instead printf
say congratulations you entered constant one, that would be something.
Yeah?
>> STUDENT: So in real life, the case would just simply be a number
and then [INAUDIBLE] would be another integer.
>> HANNAH: Exactly.
>> STUDENT: And do you need the ellipses?
>> HANNAH: Sorry?
>> STUDENT: Do you need that?
>> HANNAH: Oh, no.
I'm sorry.
That was just to indicate that you can go on for as many cases as you want.
Let's do a concrete example that might make things a little more clear.
OK, so let's say I say, OK, give me an integer n
that represents a class number, specifically a computer science class
number.
So if you give me 50, I'm going to say great.
CS50 is an introduction to computer science, and then I'm going to break.
So that means I jump out of this whole switch statement,
so now I'm done running the code, OK?
>> If you gave me instead 51, I'd print the second statement.
And then if you gave me some number that wasn't 50 or 51,
I'm going to say sorry, I'm not familiar with that class.
Yeah?
>> STUDENT: You don't have break.
>> HANNAH: I'm sorry, I don't have break?
>> STUDENT: What if.
>> HANNAH: Oh, what if you don't have break?
Excellent question.
So what would happen is you would go in, and you would check am I equal to 50?
And let's say, yes, you were equal to 50, you'd print the statement.
And then you would continue executing, so you would say am I equal to 51?
And you would go on and go through every case like that?
Yes?
>> STUDENT: Is the default line analogous to saying else?
HANNAH: Exactly.
Very good.
It's like the kind of catch all.
>> STUDENT: So if you didn't have break and then one of the case statements
was true, and then it said increase n by 1,
then it would automatically make the next one.
It would check case 51, and then display that as well?
HANNAH: Yeah, I think that would work.
So you could kind of get messy, so a break is a good thing to have.
Yeah?
STUDENT: Without break would it do the default?
HANNAH: That is a good question.
JASON HIRSCHHORN: I think it just runs.
So if you don't have break-- so let's say I did 50 and it checked 50,
and it would print that out.
Print out CS50 as introduction to computer science.
You don't have break, it should go and keep going until it hits a break.
So if there's no break, it's going to keep going.
It's going to print everything else out.
>> HANNAH: So I guess that would be including the default.
>> JASON HIRSCHHORN: Yeah.
>> HANNAH: Good question.
Yeah?
STUDENT: Is case considered a function?
HANNAH: I'm sorry?
STUDENT: Is case considered a function?
HANNAH: Is case considered a function.
I would hesitate to say that.
>> JASON HIRSCHHORN: So normally when you see functions
they'll be in curly braces.
So for example, when you look at your code, for example, main.
It was main open paren, then void, close paren.
Functions-- they're basically always you'll see parentheses.
So case if case doesn't have any parentheses,
that's your clue that that's not a function.
But it's not.
Yeah?
>> STUDENT: So then does that make switch a function?
>> HANNAH: Does that makes switch a function.
>> STUDENT: In the sense that you put something in,
it gives you one of the cases.
JASON HIRSCHHORN: I wouldn't call it a function.
Not everything with parens is a function.
I mean that's a clue that it is a function.
I wouldn't say switch is a function because it's not really
returning anything.
You'll get into that more.
>> HANNAH: Yeah, just think of it as an if structure.
STUDENT: Can you nest this in an if structure?
HANNAH: Yeah, you nest pretty much anything.
>> STUDENT: Barry said the quotes are sort of like the same as else.
So can you replace the quotes with else?
>> HANNAH: Not in this particular structure because the switch
statement is expecting the word default.
The computer knows that default means something special, that
means the catch all.
Anything else on this.
We have one more to get through.
Yeah?
>> STUDENT: [INAUDIBLE].
HANNAH: Difference between using a semicolon and a colon.
So a semicolon is always to tell the computer I am done with this line.
You can go ahead and execute it.
This is a complete line.
A colon is going to, in this case, bring you into a particular block.
So semicolons are always use to end lines.
Colons are used for a variety of other cases.
>> STUDENT: Is this equivalent to just saying if n equals 50, then
blah blah blah.
HANNAH: Mm-hm.
STUDENT: It's just the same [INAUDIBLE].
HANNAH: Exactly.
And we said it runs a little bit faster.
If we did not have the break, it would be like if, if, if.
If we do have the break, it's like if else if else if else if.
Cool?
>> JASON HIRSCHHORN: So when coding, you can just use if else,
but I think last year on the quiz we had them write a switch statement,
so just in case.
HANNAH: But definitely nothing wrong.
We're not expecting you to worry about performance crazy things.
Just if else is totally fine.
These are just good things to be aware of.
>> And here's our last one that kind of falls under this conditional category.
We have something of the general form condition ?,
and then a little piece of code that happens if the condition is true.
And then colon, a little piece of code that
executes if the condition is false, semicolon.
We're done with that line.
So syntactically it's a little bit ugly.
We're going to go through an example I think that's most clear.
So we want to assign the string professor
to one of these two values, either David Malan or not David Malan, OK?
So you should be familiar with the string, professor =.
We're going to assign a string to the variable called professor.
>> Now we want to check a particular condition.
In this case, our condition is class_num = = 50.
And now might be a good time to point out
when we have string professor =, that's one equals sign.
That's assignment.
Whereas in class_num = = 50, that's two equal signs.
That's a quality check.
So we're going to say is the class number equal to 50?
If so, assign Professor David Malan.
If not, assign professor to not David Malan.
Any questions there?
Again, this is just something that's good to know.
You could do this with if else.
A good practice problem might be to, when
you go home, right the same exact conditional in an if else form,
because you can do that.
Any questions here?
All right, I think we're going to go on to loops.
Awesome.
>> SHARON: OK, let's talk about while loops.
So first on the left you see here-- OK that says while, while (condition).
And then you do this-- can we change this?
>> JASON HIRSCHHORN: It's cut off.
>> SHARON: OK, and then there are also curly braces.
So pretend that curly braces are there above
and below the do this again and again.
So if the condition in those parentheses evaluates to true,
then you should keep doing whatever is in the while loop.
>> So for example, we could do a practice problem.
I'm wearing two earrings right now, so let's say
if I'm wearing at least one hearing, clap your hands.
So if we're going to go through this, we're going--
>> [CLAPPING]
>> Right?
And I'm wearing one, and-- OK.
Yeah.
I'm not wearing earrings anymore, so no more.
OK, so then that's when you would stop.
And this you could say is equivalent to almost variables.
Let's say you have a variable number of earrings.
So while number of earrings is greater than or equal to 1, clap hands.
And then after clap hands, decrement earrings.
Do like earrings minus 1.
So decrement the number of earrings, and then you will go through the while loop
twice.
>> And if the condition is always true-- so if, let's say, 2 = = 2,
and 2 is always equal to 2, right?
Then you would always do something in there,
and that's equivalent to almost the forever loop that we had in Scratch.
Yes?
>> STUDENT: Are the two sides just different ways of writing a while loop?
>> SHARON: So we're focused on this one first.
And then so let's compare it with a do while loop.
So they're slightly different.
OK, so let's say the condition is still while I'm wearing at least one earring,
and I'm not wearing any earrings right now.
And let's say do and still clap while I'm wearing at least one earring.
What should happen?
>> [CLAPS]
>> Uh oh, guys.
OK, so you're supposed to clap once because basically you
go through the first part of the code.
You do it no matter what, and then you see the while condition,
and you go back into that loop if it's true.
Does that makes sense?
>> STUDENT: You always do it the first time.
SHARON: You always do it the first time regardless of whether
or not that condition is true or not.
JASON HIRSCHHORN: When do you think you could use this?
When does it make sense to use this?
HANNAH: Yeah?
STUDENT: [INAUDIBLE].
JASON HIRSCHHORN: Right.
HANNAH: Very good.
JASON HIRSCHHORN: So when you prompt a user,
you're going to always want to prompt a user one time.
You're always going to want to prompt the user one time.
So instead of putting in a while loop, you
put in do while loop, because you're always going to do it one time.
If they give you the correct answer, you're done.
If they don't, then you re-prompt them.
>> SHARON: All right, for loops.
So in Scratch, we had repeat blocks.
So we wanted to repeat something, let's say, seven times.
So we just said repeat seven, and say I'm here to help you Snow White!
In C, we have for loops if we want to go through something
a specific number of times.
Let's say if we initialize the variable dwarves
and make sure-- so the first block right there before the first semicolon,
we initialize our variable to, and we set it to 0.
And our variable there is an integer, int.
And the variable name is dwarves, and we set dwarves to 0.
>> And the second part between the two semicolons is our condition.
And so as long as dwarves is less than seven,
we'll keep going through this for loop.
And then the last part is what do we do at the end of this for loop?
dwarves ++, and that means we increment dwarves by one every time.
>> So what's going to happen here?
So first we're going to go through.
We have dwarves as 0, and then we're going
to print I'm here to help you Snow White!
And then dwarves is going to increase because we said dwarves ++.
Dwarves is going to be 1.
And then we compare-- dwarves is 1.
We compare it to is dwarves less than 7?
Yes.
I'm going to go through this again.
I'm here to help you Snow White!
And then dwarves becomes 2, and then we compare it.
Is 2 less that 7?
Yes.
We're going to keep going through.
And we're going to go through this seven times.
>> So in the end, we're going to have dwarves = 0 print out I'm
here to help you Snow White!
dwarves equals 1, 2, 3, 4, 5, and 6.
Remember we index at 0.
So we start with 0.
Yes?
>> STUDENT: So this is different than the do
because this doesn't print out initially at first.
So you could continue the same thing.
Could you do this also with the do loop?
Like the same process.
The condition is dwarves less than 7 or whatever.
Response is less than 7.
SHARON: So you could technically.
So if we go back to-- your talking about the while loop, right?
So the do while loop is slightly different
because we guarantee at least once that we'll go through it,
so that's the biggest difference.
But with the while loop we could say while dwarves is less than 7,
do this, and then increment dwarves by one.
And then we have to initialize dwarves before this whole statement
that it equals 0.
So yeah, we could do that with that.
Mm-hm?
>> STUDENT: Can you go back to [INAUDIBLE]?
SHARON: For loop?
STUDENT: Yeah, so with the dwarves ++, seems like that's what you do after
you've ran some other means.
SHARON: Correct.
STUDENT: Could you just not have that and put it--
SHARON: You could.
STUDENT: So is [INAUDIBLE] right after print out in the next line underneath.
SHARON: Yes, you could put it there.
But then you would just leave that empty.
>> JASON HIRSCHHORN: You still need the semi colon though.
>> SHARON: It looks a little awkward, but you could technically do that.
Technically.
Please don't.
Yes?
>> STUDENT: Are there any alternatives to ++?
Is there anything else [INAUDIBLE]?
>> SHARON: Technically it it's dwarves.
One equals sign we're going to set it to dwarves plus 1.
So technically that's what dwarves ++ means.
Does that make sense?
>> STUDENT: Yeah, but are there any alternatives?
Like if you ever--
>> SHARON: Yeah, you could do dwarves - -.
>> STUDENT: [INAUDIBLE].
>> SHARON: Yeah, you could do a host of things.
HANNAH: And you could increment by 2.
You could increment by 3.
Anything that's going to be changing, it will eventually make the condition.
>> STUDENT: So if you wanted to increment by 2, how would you write that?
>> HANNAH: You could either write dwarves-- you could write this whole thing out.
dwarves = dwarves + 2.
Or a slightly shorthand.
I'm just going to write + equals 2.
>> STUDENT: OK, and you'd write that right where dwarves ++ is there.
HANNAH: Exactly.
SHARON: Correct.
There was another question?
Yeah?
>> STUDENT: [INAUDIBLE] dwarves ++?
SHARON: So you don't--
JASON HIRSCHHORN: Yeah, don't need it there. [INAUDIBLE] very end.
So you do [INAUDIBLE] ones because you're
kind of splitting up the initialization, condition, and the change.
At the very end, you don't need it.
SHARON: Also notice you don't need the semicolon
after the whole entire for loop.
>> STUDENT: How would you start with an initial number of negative,
for example?
>> SHARON: You can initialize dwarves equal to negative 2.
>> STUDENT: Do you just do dash 2?
>> SHARON: Yes, so the negative sign, dash 2.
>> STUDENT: Is it [INAUDIBLE] to initialize as dwarves [INAUDIBLE]?
>> SHARON: Yes.
So if we had just initialized dwarves earlier, just do int dwarves semicolon,
and then there we can do dwarves set that equal to 0.
>> STUDENT: Could we do it earlier in the program saying int dwarves = 0,
and then just--
>> SHARON: Just not have-- so there would be a space again,
but you would still need a semicolon.
Yeah.
Mm-hm?
STUDENT: Does this code say what the value of dwarves is afterward.
If you ask it after all of this is done to print the value of dwarves.
SHARON: You could ask it to do that.
STUDENT: OK.
SHARON: But you would have to ask it to do that.
It doesn't do it for you.
I'm scared to put my earrings back in.
Do it at the end for the applause.
Just kidding.
>> JASON HIRSCHHORN: So that's the same.
So initialize at the top.
Condition, print, change.
So this is exactly the same as that if anybody can see that.
STUDENT: Why don't you have the semicolon on the first line?
SHARON: Where?
STUDENT: After the first line.
SHARON: After the 4 in the parentheses?
Because we want to go through into this loop.
And if you take a look at loops in general, they don't have semicolons.
It's not practical.
Are we good?
One more question, two more.
Yeah?
STUDENT: This might be a little bit advanced,
but do dwarves have a different [INAUDIBLE]
from a variable [INAUDIBLE].
SHARON: Very good.
STUDENT: [INAUDIBLE].
>> SHARON: Yes, it is.
What?
JASON HIRSCHHORN: Yeah, so it does.
So normally the scope of something is in the curly braces.
So scope is always going to be in the curly braces,
but that wouldn't make much sense for the for loop
because we initialized dwarves in that curly brace.
So normally the scope of dwarves, that variable wouldn't extend past that.
This is a special case though.
So you initialize it within those parentheses,
and then you have the scope later on.
So special case.
They should not have that, and that was-- That make sense?
>> STUDENT: No.
>> JASON HIRSCHHORN: I can go over it.
So what are you still confused about?
>> STUDENT: As in can you use dwarves-- you can obviously use dwarves [INAUDIBLE]
outside the formula too?
>> JASON HIRSCHHORN: No, no.
So the scope of it is only inside the curly braces for the for loop, yeah.
SHARON: But if you initialize dwarves outside of the for loop, then
you can use it elsewhere There's one more question?
No?
OK.
>> So we talked a little bit about nesting for loops or nesting conditionals,
if statements.
So here's an example of where we could nest a for loop.
Let's say we were printing this table of x's right here.
We might want to-- first if we just don't look at the code
and we just think about it, we want to go through every row
and print each column, right?
Does that make sense?
OK.
>> So here we're going through every row for basically each row,
and there are three rows.
And then within each row for each column, and there
are four columns, print an x.
so.
When row is 0 and column is 0, we print this x.
And then we keep going through the column loop.
Row is still 0, but column is one.
And then column is 2, and then column is 3.
And then we exit out of that loop because then column
is no longer less than 4.
And then we print a new line, and we go to the new line.
And then we go through the next row, and row gets incremented,
and we go through that again.
Does that make sense?
Yes?
>> STUDENT: So to nest [INAUDIBLE] is just putting it inside of the for loop?
>> SHARON: So nesting means that we have a for loop within a for loop,
like Inception.
>> STUDENT: You don't need a special rotation or anything?
You just stick it right inside of it?
SHARON: Correct.
Yes?
>> STUDENT: Might be [INAUDIBLE], but it looks like there's an extra space
between all the x's.
I don't know if that's-- If that were to actually do the program,
would it do something like that?
>> SHARON: So no.
That was pressing Enter.
That was bad on our part.
Apologize.
>> HANNAH: How would you change this program if you did one extra line?
>> STUDENT: [INAUDIBLE].
SHARON: Good job.
STUDENT: You can also print the same thing by switching around the column,
right?
>> SHARON: Hm?
>> STUDENT: So to print your little xxx, the first line can say print,
and instead of saying row, it says column.
>> SHARON: So you could change the variable names to do that.
Is that what you're saying?
>> STUDENT: No, I'm just saying that-- so you're
printing rows first and then printing columns.
Can you also print columns first and then rows to get the same diagram?
SHARON: You could, but then you would have
to be careful of where you put the new line.
And how would you jump back?
JASON HIRSCHHORN: If you just switched it
and the values were different, instead of having four-- what is it?
Three by four.
I You would have four rows and three columns.
Does that make sense?
>> STUDENT: Yeah.
>> JASON HIRSCHHORN: So yeah, you could do that.
Yeah, definitely.
SHARON: But that would be literally just switching the names of the variables.
Good?
OK.
All right, P set one.
OK, so the first part of it you will be doing this, Mario.
Just kidding.
It's more like this.
And so when we just looked at the nested for loops, making that table,
think about how you might be able to print out these hashtags in this way.
And then right here how would you print this right here, this open space.
>> STUDENT: [INAUDIBLE].
SHARON: Yeah, you just print a space.
OK, so just think about that.
HANNAH: And then the same part of the problem set
is a program called greedy.c, so that you're
going to want to think about conditionals
and making sure that you can make proper change.
And one little warning we have for you is be careful of floating point values.
If that means absolutely nothing to you, it'll be covered in lecture this week
and also in [INAUDIBLE] walk through, which
you guys will learn to love on the problem sets.
>> One thing that I really suggest, especially with Mario.c,
when you're doing the problem set, if you get stuck,
start by doing it on paper.
Write it out and actually sit there and pretend to be the computer,
and go through-- say I were the computer,
how would I follow this for loop through?
How would my variables in the for loop change?
So doing it on paper makes it 10 times easier
when you go to sit down at the computer.
So just my little plug.
>> SHARON: And also don't think that you have to code everything all at once.
Make sure you take an iterative process.
Do a little bit, print it out, see what happens.
Sometimes it's a little trial and error.
And come to office hours.
Super fun.
JASON HIRSCHHORN: So any questions?
All right guys, that was your first section.
HANNAH: Thanks for coming.
JASON HIRSCHHORN: Thanks for come.
SHARON: Thank you.
[APPLAUSE]