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[MUSIC PLAYING]

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DAVID MALAN: All right.

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This is CS50's Introduction
to Programming with Python.

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My name is David Malan.

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And this is a look at debugging.

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Odds are, by now, you've
written quite a bit of code.

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And within that code is at
least one, maybe even more,

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bugs, that is mistakes that you
accidentally introduce to your code.

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Now, maybe your code
doesn't even run at all.

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And you have some form of
syntax error, or maybe you've

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gotten past that hurdle.

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And you just have one or more
logical bugs, so to speak,

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whereby when you run your code,
it doesn't produce the output

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that you actually intended.

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So it's not quite correct.

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There are mistakes therein.

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And how do you go about,
then, solving those problems?

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Well, if you can't quite
wrap your mind around what's

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going wrong by just reading
with your own human eyes

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your own code from top to bottom,
left to right, and you don't have,

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for instance, a teacher, a
friend, a colleague, a rubber duck

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to help you reason through what might be
wrong in your code, what kind of tools

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do we actually have for
chasing down bugs in code

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and ultimately fixing them?

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Well, already, we actually have
some tools at our disposal.

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And for instance, let me propose that
we build something like this in code.

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So pictured here is Mario from Super
Mario Brothers, the original Nintendo

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game, jumping over a pyramid of sorts.

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And that period is implemented
with these bricks of sorts.

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Let's go ahead and create
a textual version thereof,

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whereby we use maybe hash
symbols, simple hashes,

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to represent each of these bricks.

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And therefore, the goal in printing
out a pyramid textually that

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resembles that of Mario here would be
to print out one hash, and then two

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hashes, and then three hashes,
and perhaps more if the pyramid

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itself is higher.

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Well, let me propose that if I open
up VS Code here-- and, in advance,

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I've created a file called mario.py.

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Let me propose that this is my
intended solution to this problem.

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Let's go ahead and walk
through what's here.

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I first, at the top of my
file, have a main function

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defined, inside of which is a second
line of code that calls input,

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asking the user for the
height of this pyramid.

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Immediately, the return value of
input is passed to the input of int

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so as to convert that
textual input, presumably,

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to the corresponding integer.

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And then I'm storing on the left-hand
side of that equal sign the value

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that the user typed in, be
it 1, 2, 3, or anything else.

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And then I'm using that
variable as an input

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to a function that's apparently called
pyramid so that I can ultimately

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print out a pyramid of that height.

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All right.

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Well, what is this pyramid function?

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Well, it looks like, on this line
six, that the pyramid function expects

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one argument called n,
n presumably for number.

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And how am I using that number?

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Well, inside of this
function, I have a for loop

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that's going to iterate,
it would seem, n times.

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And this is indeed our common
paradigm, doing for i in the range of n

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allows us to iterate from zero on up.

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And then that innermost
line of code is just

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going to go ahead and print
out some number of hashes.

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And recall this trick, whereby we're
using the multiplication operator

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but really just to automatically
concatenate one or more of these hashes

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together.

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So hopefully, the net
effect of this program,

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once we actually run it per these
final two lines, and main is invoked,

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and then pyramid is
invoked, is that we'll get

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a pyramid of the corresponding height.

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But suppose now that I
very enthusiastically

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went about trying to run this program.

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And so I went to my terminal
window and ran python of mario.py.

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And then I went ahead
and proposed a height

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of three, the goal being to have one
hash, two hashes, three hashes so

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that the total pyramid
is of height three.

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I'm a little disappointed already
to see that it's not quite correct,

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this output.

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I seem to have a pyramid
really of height two

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because there's a blank line, and then
a single hash, and then two hashes.

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But, again, what I wanted was one hash,
then two hashes, then three hashes,

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forming a pyramid that's
in total of height three.

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Now, you might have an immediate
instinct as to how to fix this problem.

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And indeed, relatively speaking, this
isn't the most complicated program,

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at least if you're a few weeks into
learning and writing code in Python.

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But suppose that it's
just not obvious to me,

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or this is representative of an even
more complicated problem down the road.

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And so I'd really like to start
figuring out, systematically,

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how to solve this particular bug.

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Well, what's one tool in my toolkit?

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Well, let me propose
first that I do this.

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Let me clear my terminal window to
get rid of the output from before.

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And let me propose that it might be
helpful for me to maybe figure out

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if maybe i is wrong, right? i
is my variable in this for loop

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that's responsible for
printing out, presumably,

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one hash, then two hashes, then
three hashes, and so forth.

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So maybe just to check
my understanding here

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I should-- maybe why don't I
temporarily just print out i in addition

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to these hashes just so that I can
make sure that i is what I expect next

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to the appropriate number of hashes?

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Which is to say, Print
itself, the function

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we've been using now
for quite some time,

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is a very reasonable, effective tool
for debugging, at least in some cases.

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It's a nice way to quickly
and dirtily, kind of,

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see what's going on inside of your code.

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Well, let me do this.

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In addition to printing
out that number of hashes,

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let me also print out the value of i.

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But rather than print out
a new line, which I think

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is going to completely
mess up the pyramid,

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let me just end the
line with a single space

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just to kind of push the
cursor over to the right

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so that my pyramid is printed to
the right of all of these integers.

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All right.

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Let me go down to my terminal window
again and run python of mario.py Enter.

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I'm going to, again,
type in 3 for the height.

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And now, I see essentially
the same output.

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But I've prepended, so to speak,
to each line the value of i.

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And now, maybe it makes a
little more sense to me.

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Now, maybe the proverbial light
bulb just went off over my head

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because, oh, the reason I'm
seeing a blank line and then only

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one and only two hashes on the
screen instead of one, two, and three

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respectively is because i,
of course-- now I remember.

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--starts from zero, certainly
when doing a for loop like the one

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I have here on line seven.

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So how do I go about fixing this?

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Well, I think, instinctively,
I could just do this.

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Let me go ahead and
remove the previous print.

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Indeed, when using print to debug
your code, it's usually temporary.

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So I should now undo
that change because I

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don't want those numbers to appear
ultimately in my program's output.

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Let me go ahead now and print out--

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OK.

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I don't want to print out zero
hashes, and then one, and then two.

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I want to print out one more
than the current value of i.

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And I bet there's a bunch
of ways I can solve this.

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But I think the easiest
way might be this.

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Let me go ahead and in parentheses do
i + 1 so that what I'm really doing

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is multiplying this
single hash by i + 1.

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So instead of being zero, one,
and two hashes respectively,

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now it's one, two, and three.

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And I've parenthesized it
just to make clear to myself

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that indeed the math is
going to happen first,

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just like in math class,
inside of the parentheses.

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And then I'm going to use that total
value for the concatenation of all

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of those hashes.

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All right.

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Let me go ahead now back
to my terminal window,

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clearing it, running python
of mario.py once more.

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Let's again type in a height of 3.

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And now I have the intended pyramid.

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So a minor bug, if you will--

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Certainly, if you're several
weeks into learning python,

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already that kind of bug might have
jumped out at you pretty quickly.

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But that's not always
going to be the case.

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And so Print is the
first of your friends

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when it comes to debugging your code.

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Using Print, you can
temporarily, but pretty easily,

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just display the values of variables
or other things in your program

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just so you can help see what the
computer sees underneath the hood,

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so to speak.

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But it turns out there's
more powerful tools.

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And Print can start to get a little
tedious, especially if you're

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adding a print up here, and up
here, and up here, and up here,

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and all over the place in
more complicated programs

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because then you have to clean
up a whole mess that you've made.

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And you might have put the
Print in not the right location.

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And you have to remember what
output is coming from one Print.

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So eventually, Print is
not so much your friend.

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And we need better tools than that.

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So let me go ahead and clear
my terminal window again.

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Let me undo these
changes so that I again

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have the same bug, where I'm now
printing out i number of hashes, so

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zero, one, and two again.

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And let me propose that
we use another tool that

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comes with a lot of programming
environments nowadays,

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VS Code included.

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Indeed, typically, I've hidden
the so-called activity bar

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to the left-hand side here of VS Code.

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But I've revealed it
today so that we can

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see some of the
functionality that actually

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comes with VS Code, and other
text editors, and other IDEs.

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Integrated Development Environments
often come with built-in debuggers.

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And indeed, a debugger is a special
type of program whose purpose in life

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is to do just that, to help
you debug your program.

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It doesn't necessarily
solve the problems for you.

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So it's perhaps a bit of a misnomer.

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But it helps you.

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It empowers you to solve
bugs yourself and eliminate

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them more methodically than
maybe Print alone would allow.

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So I'm going to do this first.

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I'm going to introduce this notion of
what's called, generally, a breakpoint.

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A breakpoint is simply a mechanism
when using a text editor or an IDE that

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allows you to specify on what
line or what lines of code

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do you want to pause or break
execution of the program

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just so you can start poking
around at that line of code.

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In other words, if I keep running
mario.py in my terminal window,

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it pretty much runs from top to
bottom, left to right, again and again,

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pausing of course for
my own human input.

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But it's so darn fast,
Python, that I can't really

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see what's happening on each
individual line of code.

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So let me go back to VS Code
and this time set a breakpoint.

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Wouldn't it be nice if
I could pump the brakes,

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so to speak, and slow down
the execution of my program

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so that I can tell the computer
when I, the human, am ready to step,

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step, step through each
of my lines of code?

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So it turns out that to the left of the
line numbers here in VS Code-- and this

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is a popular approach in
other programs as well.

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Notice if you hover over to
the left of a line number,

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it becomes a grayed out form of red.

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And if you click on it
once, it becomes really red.

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And what that indicates
is that I've set one

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of these things called a breakpoint.

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I've indicated to VS code, in this
case, that I want it to break,

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that is pause execution of my
code just before main is called.

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And I can actually set it on
almost any of these lines of code.

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But I'm starting at the very
bottom one because, of course,

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that's the line of code that
we know is going to kick off

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the whole process of running my code.

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But it's going to allow
me, therefore, to step

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through any and all of the
specific lines therein.

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So let's go ahead and do this.

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But now, I'm not going to go ahead
and just run python of mario.py.

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I need to actually run the debugger.

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And the debugger in VS
Code is pretty smart

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to figure out exactly
what I want to run.

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And you can even configure it
to be even more customized.

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I'm going to go ahead and, perhaps
aptly, click on this icon here, a.k.a.

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Run and Debug.

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And it's the little Play icon with
what appears to be an actual bug on it.

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And when I click that, this
additional panel opens up in VS Code.

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And its a big button there.

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It's called Run and Debug.

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00:11:13,210 --> 00:11:16,180
Now, I could follow more of the
directions and customize it further.

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00:11:16,180 --> 00:11:18,730
But I'm just going to go
ahead and click Run and Debug.

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00:11:18,730 --> 00:11:23,020
And using some default settings that
I might have specified earlier too,

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it's going to run my program
but pause it on this line here.

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Notice that in yellow on line 12,
that's where I set the breakpoint.

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00:11:31,960 --> 00:11:35,350
And the yellow highlight of
line 12 means that line of code

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has not yet executed.

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00:11:36,700 --> 00:11:39,880
But it's about to be executed
if and when I am ready.

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There's a bit of noise, or messy output,
in my terminal window potentially.

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00:11:43,480 --> 00:11:46,510
But that's just part of the debugging
process in my terminal window.

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Eventually will I see that
same prompt for height,

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00:11:48,970 --> 00:11:52,600
but I don't yet because I'm
not at that line of code.

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00:11:52,600 --> 00:11:55,900
Now let me draw your attention to all
of these icons at the top of the screen.

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The first of these, which looks
like a Play button with a line

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to the left of it, is
this Continue button.

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00:12:01,570 --> 00:12:05,470
If I click that button, the
breakpoint is going to be left behind.

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And my code is just going to continue
on to the end of the program.

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00:12:08,647 --> 00:12:10,480
That's probably not
what I want in this case

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00:12:10,480 --> 00:12:13,600
because I actually do want to
step through it line by line.

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00:12:13,600 --> 00:12:17,380
There's this next icon here, an
arrow going over a single dot.

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00:12:17,380 --> 00:12:19,390
That's the Step Over command.

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00:12:19,390 --> 00:12:23,140
This would allow me to
step over this line 12.

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00:12:23,140 --> 00:12:25,240
It will execute it, but that's it.

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00:12:25,240 --> 00:12:27,280
It's not going to step
into that function

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00:12:27,280 --> 00:12:31,000
so that I can go line by line through
main or, in turn, pyramid either.

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00:12:31,000 --> 00:12:35,290
The one I really want to use now is
this one in the middle, Step Into.

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00:12:35,290 --> 00:12:37,690
And this is the line pointing
down at the dot which

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00:12:37,690 --> 00:12:41,230
is a symbol that indicates that if
I click this button in a moment,

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00:12:41,230 --> 00:12:44,170
I'm going to step into line 12.

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00:12:44,170 --> 00:12:47,230
That is, I'm going to step
inside of the main function.

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00:12:47,230 --> 00:12:51,790
And from there, I can continue to
step-by-step walk through my code.

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00:12:51,790 --> 00:12:54,130
So watch what happens on
the yellow on the screen

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00:12:54,130 --> 00:12:57,130
here when I click Step Into like this.

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00:12:57,130 --> 00:13:01,120
You'll see that now line
12 has begun executing.

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00:13:01,120 --> 00:13:04,450
But when we step into
that function, notice now

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00:13:04,450 --> 00:13:07,960
that the debugger has broken,
or paused, on line two.

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00:13:07,960 --> 00:13:10,150
Line two has not yet executed.

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00:13:10,150 --> 00:13:13,842
But as soon as I step
over this one, I think

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00:13:13,842 --> 00:13:15,550
we're going to start
to see some results.

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00:13:15,550 --> 00:13:19,720
And I was careful there to say
step over instead of step into.

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00:13:19,720 --> 00:13:20,290
Why?

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00:13:20,290 --> 00:13:24,490
Well, notice on this line of code two,
there's two functions, input and int.

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00:13:24,490 --> 00:13:26,440
And I didn't implement either of those.

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00:13:26,440 --> 00:13:30,550
And allow me to propose that the
people who invented Python probably

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00:13:30,550 --> 00:13:32,800
implemented input and int correctly.

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00:13:32,800 --> 00:13:36,010
So there's really no point in
my stepping into their functions

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or even trying to.

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00:13:37,120 --> 00:13:39,640
I really should only be
stepping into my own functions.

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00:13:39,640 --> 00:13:41,440
But I do want line two to execute.

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00:13:41,440 --> 00:13:43,540
But let me call your attention to this.

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00:13:43,540 --> 00:13:48,700
At the top left of VS Code now, there
is, under the Run and Debug panel,

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00:13:48,700 --> 00:13:52,000
some mention of variables,
both locals and globals.

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00:13:52,000 --> 00:13:54,100
And there's nothing under
locals at the moment

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00:13:54,100 --> 00:13:58,480
because at this moment, before
line two has been executed,

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00:13:58,480 --> 00:14:01,330
no variables actually yet exist.

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00:14:01,330 --> 00:14:05,620
In a moment, once I do execute
this line of code and step over it,

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00:14:05,620 --> 00:14:08,110
well, a variable called
height should exist.

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00:14:08,110 --> 00:14:09,220
So let's try that.

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00:14:09,220 --> 00:14:10,690
I'm not going to click Step Into.

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00:14:10,690 --> 00:14:12,580
I'm going to click Step Over.

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00:14:12,580 --> 00:14:14,740
But that will not
ignore the line of code.

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00:14:14,740 --> 00:14:17,950
It will execute it but not
dive into it, if you will.

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00:14:17,950 --> 00:14:20,380
Once I click Step Over,
notice that the line

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00:14:20,380 --> 00:14:24,220
is no longer highlighted because
execution hasn't broken or paused yet.

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00:14:24,220 --> 00:14:27,460
Notice in my terminal window,
despite the output from earlier,

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00:14:27,460 --> 00:14:29,980
I'm being prompted for the
actual height of the pyramid.

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00:14:29,980 --> 00:14:31,630
So this part is still interactive.

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00:14:31,630 --> 00:14:33,640
Let me go ahead and
type 3 and hit Enter.

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00:14:33,640 --> 00:14:37,090
And now we'll see what's happened
at the top of the screen.

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00:14:37,090 --> 00:14:38,710
Line two has just executed.

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00:14:38,710 --> 00:14:40,420
The height variable now exists.

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00:14:40,420 --> 00:14:43,240
And indeed, if you look up
here, you'll see in the debugger

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00:14:43,240 --> 00:14:48,760
a graphical summary of all, or in this
case, the one variable that now exists.

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00:14:48,760 --> 00:14:51,730
And better still, you see
the value of that variable.

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00:14:51,730 --> 00:14:53,710
Now, that's not that
helpful at the moment

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00:14:53,710 --> 00:14:56,800
because I'm pretty sure the
height is as the user typed in.

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00:14:56,800 --> 00:14:59,650
So there's no off by one
error or any bug there.

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00:14:59,650 --> 00:15:03,460
I think then the bug is probably
inside of my pyramid function.

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00:15:03,460 --> 00:15:06,460
So here, I don't want to
click Step Over because that's

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00:15:06,460 --> 00:15:08,770
going to execute pyramid
and not allow me a chance

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00:15:08,770 --> 00:15:10,450
to step through each of its lines.

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00:15:10,450 --> 00:15:15,190
I'm going to, again, this time, because
it's my function, Step Into the pyramid

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00:15:15,190 --> 00:15:15,850
function.

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00:15:15,850 --> 00:15:18,610
Once I do that, now
it's line seven that's

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00:15:18,610 --> 00:15:21,640
highlighted because I've stepped
into the pyramid function.

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00:15:21,640 --> 00:15:24,470
Notice that the panel
up here has changed.

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00:15:24,470 --> 00:15:27,560
The local variable, that
is the variables that

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00:15:27,560 --> 00:15:31,010
exist local to the pyramid
function, are indeed

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00:15:31,010 --> 00:15:35,690
n because n was the argument that
was passed in with a value of 3.

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00:15:35,690 --> 00:15:39,620
i does not yet exist because
line seven has not yet executed.

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00:15:39,620 --> 00:15:41,300
But here comes my for loop.

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00:15:41,300 --> 00:15:44,120
And here's where it's going to be
interesting to step over and step

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00:15:44,120 --> 00:15:45,680
over a few times.

327
00:15:45,680 --> 00:15:48,590
I don't want to step into because
the range function, again,

328
00:15:48,590 --> 00:15:50,240
was invented by someone other than me.

329
00:15:50,240 --> 00:15:51,890
And let's just trust that it's correct.

330
00:15:51,890 --> 00:15:54,800
So let's step over this
line, thereby executing it.

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00:15:54,800 --> 00:15:57,740
But watch the top left-hand
corner of VS Code.

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00:15:57,740 --> 00:15:59,570
Let's step over it once.

333
00:15:59,570 --> 00:16:03,350
And now notice not only
does n equal 3, now there's

334
00:16:03,350 --> 00:16:06,350
another local variable
called i that equals 0.

335
00:16:06,350 --> 00:16:08,240
And what am I about to do on line eight?

336
00:16:08,240 --> 00:16:10,448
Well, in a moment, I'm going
to click Step Over again

337
00:16:10,448 --> 00:16:12,490
because I'm going to trust
that print is correct.

338
00:16:12,490 --> 00:16:14,060
So I don't need to step into print.

339
00:16:14,060 --> 00:16:17,630
But watch what happens now in
my terminal window down here.

340
00:16:17,630 --> 00:16:20,030
No pyramid, no bricks have been printed.

341
00:16:20,030 --> 00:16:23,990
But as soon as I step over line
eight, thereby executing it,

342
00:16:23,990 --> 00:16:25,730
we see the first of my--

343
00:16:25,730 --> 00:16:27,020
wait a minute.

344
00:16:27,020 --> 00:16:28,730
I'm not seeing an actual brick.

345
00:16:28,730 --> 00:16:31,100
If you notice, the cursor did move down.

346
00:16:31,100 --> 00:16:33,410
And effectively, a
blank line was printed.

347
00:16:33,410 --> 00:16:36,560
And now there's an opportunity for
that light bulb again to go off.

348
00:16:36,560 --> 00:16:37,220
Wait a minute.

349
00:16:37,220 --> 00:16:38,360
I saw the cursor move.

350
00:16:38,360 --> 00:16:40,730
It printed something,
but it's really nothing.

351
00:16:40,730 --> 00:16:42,050
It's no bricks.

352
00:16:42,050 --> 00:16:42,650
Oh.

353
00:16:42,650 --> 00:16:46,370
This is where now I might realize,
this is why my program is flawed.

354
00:16:46,370 --> 00:16:51,110
Because when i equals 0, I'm
apparently printing zero bricks.

355
00:16:51,110 --> 00:16:51,830
Aha.

356
00:16:51,830 --> 00:16:53,960
Now hopefully I understand.

357
00:16:53,960 --> 00:16:55,310
If not, no big deal.

358
00:16:55,310 --> 00:16:58,760
I can continue stepping over or
stepping into each of my lines of code.

359
00:16:58,760 --> 00:17:02,900
I can click the big Stop icon, or
I can restart the whole process.

360
00:17:02,900 --> 00:17:05,480
Sometimes it might not be
obvious the first time around.

361
00:17:05,480 --> 00:17:09,470
And sometimes you might want to
unset or reset a different breakpoint

362
00:17:09,470 --> 00:17:12,650
so as to figure out
what's actually going on.

363
00:17:12,650 --> 00:17:15,043
It's just part of the
process of debugging.

364
00:17:15,043 --> 00:17:17,960
For now that I figured it out, I'm
going to go ahead and just continue

365
00:17:17,960 --> 00:17:19,752
my program because I
know that it's flawed.

366
00:17:19,752 --> 00:17:21,619
But I now know what the problem is.

367
00:17:21,619 --> 00:17:23,842
Let me go ahead and close
this panel over here.

368
00:17:23,842 --> 00:17:27,050
I'm going to get rid of the breakpoint
because I've now, in my mind at least,

369
00:17:27,050 --> 00:17:28,130
solved this problem.

370
00:17:28,130 --> 00:17:29,750
I'm going to clear my terminal window.

371
00:17:29,750 --> 00:17:31,910
I'm going to go in
and just logically add

372
00:17:31,910 --> 00:17:36,380
what I think is the solution to this
problem by doing i + 1 in parentheses

373
00:17:36,380 --> 00:17:37,010
again.

374
00:17:37,010 --> 00:17:41,375
I'm going to now run one final
time mario.py with Python.

375
00:17:41,375 --> 00:17:44,240
I'm going to again type in height of 3.

376
00:17:44,240 --> 00:17:46,970
And there it is, my
pyramid of height three.

377
00:17:46,970 --> 00:17:49,610
So ultimately, what are
the tools in your toolkit?

378
00:17:49,610 --> 00:17:52,400
Well, certainly Print is something
that you can always reach for,

379
00:17:52,400 --> 00:17:55,670
not just in Python, but so many other
languages as well or some equivalent

380
00:17:55,670 --> 00:17:56,240
thereof.

381
00:17:56,240 --> 00:18:00,410
But sometimes once your programs get
sufficiently complicated, sufficiently

382
00:18:00,410 --> 00:18:03,350
sophisticated, or, heck,
maybe it's someone else's code

383
00:18:03,350 --> 00:18:06,380
that you yourself didn't write and,
therefore, you would especially

384
00:18:06,380 --> 00:18:11,120
benefit from stepping through it
line by line, stepping into functions

385
00:18:11,120 --> 00:18:14,360
of interest, a debugger, like
that that comes with VS Code,

386
00:18:14,360 --> 00:18:17,860
is going to be your new best friend.

387
00:18:17,860 --> 00:18:19,000