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Hey guys! Welcome back to a 100 Days of Code. Today

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we're going to look at debugging,

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the process of removing bugs from your code.

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If you manage to get to this point,

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you've probably written quite a bit of code already,

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and I will bet that you've probably made some mistakes, some typos,

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some errors and some bugs. So in this lesson,

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I want to talk about some techniques and tips for how to find bugs and how to

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get rid of them from your code. Now,

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the first documented bug was actually found by this lady, Grace Hopper.

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She was probably one of the first programmers and one of the pioneers of the job

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that we're undertaking right now. Now in her notes,

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you'll find a moth that's been taped to the notebook,

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and this was found in a relay that was preventing her code from running

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properly. And this was pretty much the first

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actual case of a bug that's been found in a computer.

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Even though we're not dealing with moths these days anymore,

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we're still going to end up finding bits of things in our code that prevent the

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code from doing the things that we want it to.

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The important thing to remember is don't feel down when you've created a bug

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because everyone gets bugs. And when I say gets bugs,

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I mean, we create them, right? So once you've gotten over this, next,

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I want to talk about some of the tips and techniques that you should follow on

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how to quickly be able to identify and remove these bugs.

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So the first step is to describe the problem.

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If the problem is messy and it's not well understood in your head,

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then it's almost impossible to debug it.

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So untangled the problem and try to make sense of what's going on. To begin,

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go ahead and fork the starting repl for today's code.

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Now, the first thing to notice here is that I've written a bunch of code in the

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starting repl

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and we're going to be uncommenting these blocks one by one and eventually

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solving all of the debugging problems.

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So the first thing you're going to do is to uncomment the first block of code.

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Now in this block of code,

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we're going to practice describing the problem to help us solve this debugging

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problem. Now if you take a look at this function

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you can see that at some point in the function,

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we're supposed to print this line out into the console.

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But if we go ahead and hit run, you can see that nothing gets printed.

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So what's going on here?

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I want you to take a look at this function and describe the actual problem.

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What is the for-loop doing? When is the function meant to print "You got it"?

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What assumptions are you making about the value of i? So pause the video,

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read the function and have a go at describing the problem.

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All right.

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So what we've got here is we've got a function that loops through all the

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numbers between 1 and 20,

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and then once that number reaches 20, it's supposed to print "You got it".

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But the problem is that when i reaches 20,

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it doesn't print this line into the console.

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So now that we've described the problem and we understand what's going on,

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let's see if we can solve it. If we think about this problem,

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when i reaches 20, it doesn't print out this line.

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There's an assumption in there, right?

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The assumption is that i will definitely reach 20.

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But if you think back to how the range function works,

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the stop or the upper bound is actually omitted.

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So when you write range(4),

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it actually produces a number from 0 up to 3.

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And in our case, when we write (1, 20),

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it actually goes from 1 all the way up to 19

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but not including 20.

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The problem here is that i actually never reaches 20.

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So this assumption is completely false. Pause the video

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and see if you can fix the code so that this line actually gets printed.

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All right. So all we need to do is change this to 21 instead of 20

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and now when we hit run, you can see it actually works

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and we managed to debug this problem. Always when you come across a problem

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in your code,

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try to describe it so that you really understand what the issue is,

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and then test your assumptions and see which of those assumptions is actually

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false.