it's not always going to be perfect.

There's usually going to be something wrong with it.

And that is known as writing bugs.

And part of being a computer programmer is

learning how to debug, or remove the bugs, from programs.

And there are a lot of different techniques and strategies

that one might use to do this.

And what I want to do in this video is show you

three of the tools that are baked into CS50 IDE

that you can use to help with your debugging,

or help trying to figure out what's going on in your system.

And these three tools, which I'm going to just display the names of here,

are known as debug50, eprintf, and help50.

We're actually going to cover those in reverse order.

So we're going to start with help50.

So I have a couple of files pre-configured in my IDE.

I'm going to navigate into my help50 directory.

And just clear the terminal window here.

And what help50 basically does, it is designed

to be sort of a virtual teaching fellow of sorts.

It basically is trying to give you clues and hints about what might be going on

in your program or when you're trying to work with your program

and something goes wrong without just giving you the answer outright.

So it's trying to nudge you along in the right direction in the hopes

that it's an issue that, with a little bit of encountering and having

the computers sort of give you a little bit of back and forth,

it's an issue that you'll learn to be able to not recreate in the future.

So I have a couple of example programs here.

All of them are written in C. And let's just try and compile the first one.

So let's try and make example one.

Error.

So this is where help50 is going to come in handy.

Generally it's going to be used when you're working.

It's not so much a bug in your program that is

during runtime, a runtime error.

This is going to be a logical bug, or some kind

of bug that prevents your program from being compiled

is going to be the most common use case of help50.

So here I'm getting told I'm implicitly declaring library function

printf with blah, blah, blah.

There's a whole lot of text there.

Now, what help50 tries to do is it looks at that error message for you.

And if there are multiple ones it will look

at the first error message for you.

And it will try and translate that kind of arcane language

into something a little more, hopefully, intuitive to use.

And the way we use help50 is pretty straightforward.

We first type help50, and then we do exactly the command we tried to do.

So in this case it's help50, make example1.

Let's see if maybe help50 can give me a little more information about what

I might be doing wrong in my program.

So I run it again.

Now, I'm getting this sort of hint from the virtual TF,

which is written here in yellow.

"Did you forget to #include stdio.h in which printf

is declared atop your file?"

Well, let's take a look.

So in example 1.c, it looks like I did kind of forget to do that.

So if I just go ahead and add that, and I save,

and I try and make example one again maybe.

Hey, it compiled.

It worked just fine.

So I can now run the example1 program, hello world.

Now, of course, I have a logical bug here where I didn't put a backslash n.

So it kind of ran into a new line like that.

But that's OK.

That's something I can go ahead and fix pretty easily.

So let's try and compile example two.

And we'll go through this again where we won't look at the source code

beforehand.

We'll just try and compile it, thinking we're pretty close,

and see what happens.

So we'll try and make example2.

Oh.

Seven errors generated.

That's not good.

But, again, help50 can help us here.

And what it's going to do in this case is

it's going to look at the very first one and give us a hint for that one.

Now, oftentimes when you get a lot of errors from compiling

it's not necessarily that I've done seven things wrong.

In fact, if you look at the program you'll

see that it's not that many lines of code when we do.

But really it's just one error that sort of cascades and creates

another one because it doesn't understand what's happening before.

And if you can fix that one error at the beginning,

it might resolve one or more, or all, of the remaining errors.

So I'll clear my terminal window.

And we'll try this again.

But we'll preface this by saying help50 first.

So help50 make example2.

Let's see.

Use of undeclared identifier string.

Did you mean standard in?

Well, I'm assuming probably not.

Let's take a look at my code.

No.

I meant to use string there on line five.

So by undeclared identifier, Clang means you've used a name string

on line five which hasn't been defined.

Did you forget to #include cs50.h in which string is defined atop your file?

Well, here, again, I did.

I did forget to include cs50.h this time.

So it gave me another clue here.

But I had seven errors.

So hopefully this resolves most of them.

Lets clear this and see if it does.

We'll try and make example2 again.

And it did.

So even though it said I had seven errors, I really didn't.

I just had the one.

And because that one kind of threw everything else off,

it just kind of cascaded down and really messed us up.

All right.

Let's take a look at example three.

We'll try and make example three.

Seven errors again.

Ugh, man.

Let's see what I got here.

Looks like I have a for loop that's going from 1 to 10.

And it's supposed to print out a new integer on each line.

That looks OK to me, at least at first glance.

I don't really know what the issue could be here.

But help50 might be able to help us out again.

So we'll try one more time help50 make example3.

And, again, you'll see that help50 is commonly

going to be used most of the time-- although it's not all it can do.

And we'll take a look at a couple of examples of that in a second.

You're going to use help50 most of the time when there are compile time

errors that prevent your program from being compiled

and have to be resolved first.

So we'll try and help50 this one.

Looks like it says on line five it looks like you're

trying to declare a variable that's already been declared elsewhere.

I don't see that.

But then it tells me a little bit more.

If you meant to create a for loop be sure that each part is separated

with a semicolon rather than a comma.

So here I've got commas, and I'm supposed to have semicolons.

So it's, again, little things like this where it's little syntax things where

if you were able to get in touch with somebody

you could just kind of look at your code really quickly and be like,

oh, well, you really just need to put in a semicolon here instead of a comma.

That's the kind of thing where it stinks to get stuck in a jam where you're just

waiting for somebody to give you that sort of shining moment right

where suddenly the error is revealed.

You can use help50 to try and help you along and accelerate this

so that you don't get stuck waiting, and you can get a little resolution

for your errors a little more quickly.

Of course, we'll just confirm with trying to make it again.

And it compiled no problem.

I could run this program.

And it will print out the numbers 1 through 10, each on separate lines.

We can't see them all here because my terminal window is not big enough

to hold them, but they're all there.

All right.

I got one more example here.

And then I'll show you two examples of using help50 not in the make context

necessarily, or in the Clang context.

So let's try and make example4.

I have one error.

"Expression result unused, n times 5."

Not really sure what that means.

So let's just quickly pop open example4 here at the top.

Looks like what I want to do is have this print out 50.

So I set n as equal to 10.

I multiply n by 5 to get 50.

And I want to print out 50.

So I wonder what the issue could be.

So let's help50 make example4.

On line six of example 4.c you are performing an operation,

but not saving the result. Did you mean to print or store

the result in a variable?

So remember, when we are doing mathematical manipulations

to variables, we always have to assign the result to something.

And here what I probably meant to say is either n equals n times 5,

to change n from 10 into 50, or the more shorthand version,

n times equals 5, which would also work just fine.

So I can save this.

Hopefully this fixes it.

I'll try and recompile it.

And I'll try and run it to see if I get 50 printed out.

So that's a couple of uses of, in particular

this is help50 helping with Clang.

Although every time we were doing this we were typing make.

Remember that make is actually just a wrapper

for Clang, which is the name of the actual compiler

that we're using in CS50 IDE.

What happens though if I try and do this?

There's no file called example5.

So make example5.

Unlike the previous four, this is actually

going to be a help50 that helps with a message

from make as opposed to a message from Clang.

Now, obviously it's not there.

So you can probably figure out what it means.

But in case you didn't, I could preface it with help50.

"Do you actually have a file called example5.c in the current directory?"

Well, I don't.

So I'm trying to compile a program that doesn't exist.

So this is just help50, again, giving me a clue,

like, well, maybe you're making a little bit of a mistake here.

And it's true.

I am making a bit of a mistake.

One more example here.

Let's say that I want to change directories into another directory.

So I want to ls example6.

There is, again, no example6 here.

"ls cannot access example6, no such file or directory."

Again, help50 might be able to help with this.

So I'll preface it one more time.

Are you sure example6 exists?

Did you misspell example6?

It doesn't exist.

Maybe I had typed it wrong and I thought it was working.

So it's just a reminder.

Did you check that?

Oh, maybe I had misspelled it.

Maybe I meant to do something else, and then I can take a look.

Now, help50 is a tool that is still in development.

We actually have an open source GitHub repository.

So as you go through the class and you become more comfortable, particularly

as you start to learn Python, which is the tool that help50 is written in,

you can actually head to CS50's public GitHub repositories and take a look

and see what types of error messages that

are a little more arcane that we're trying to catch and have help50

explain in more human understandable terms

so that if you want to contribute to that you actually can.

And if you encounter an error that you're not

getting, that help50 doesn't understand, and you want to report that error