Python for Beginners, Part 9: Loops

In this one, we explore the concept of looping constructs, visually, in computer programming.

In case you missed it, here are the previous parts of the series

• Python for Beginners, Part 1: How to Download and Install Python
• Python for Beginners, Part 2: Hello World Exercise
• Python for Beginners, Part 3: The Turtle Module
• Python for Beginners, Part 4: Interactive Shell vs. Script
• Python for Beginners, Part 5: Variables
• Python for Beginners, Part 7: Fun Fun Functions
• Python for Beginners, Part 8: Function Arguments

Transcript

0:00 Okay, so now that we've talked a little bit

0:03 about functions with their arguments

0:05 and also a little bit about functions

0:07 with multiple arguments, this naturally leads us

0:11 to the next thing. What if I want to make that one square

0:16 but I want to make it multiple times?

0:19 Sure, I can copy it and paste this line

0:23 and paste it again, right,

0:26 and then paste it again,

0:28 so and and so forth. So, for example,

0:30 let's try having it pasted twice.

0:33 I'm gonna remove these comments here,

0:36 and just so you remember, this 50 goes in for the length,

0:40 it replaces every part of the length with 50.

0:43 This 90 goes for the angle, it replaces every angle with 90.

0:47 That's how it works, okay.

0:49 So let's go back real quick and simply run this now.

0:54 Okay, so I'm gonna hit the shortcut key to run it.

0:59 And you can see I said draw two squares and it did.

1:03 But, here is the most amazing concept in programming,

1:09 which is the concept of doing things over and over again,

1:12 also known as loops.

1:14 So if I said I want you to do 10 pushups,

1:19 that's like a loop.

1:22 And every time you do a pushup

1:23 I count one, two, three, four.

1:25 And then once you hit 10 I stop automatically.

1:28 So a way to say that here is you can say

1:32 for pushup in range 10.

1:35 So for each pushup for 10 times, do this.

1:40 And this pushup part, I can call it whatever I want.

1:42 I can call it this,

1:44 I can call it I,

1:46 we'll just keep it simple and leave it at one letter I.

1:50 And what we're gonna do is,

1:52 all this part says is run something 10 times, okay.

1:58 That's what that part essentially says.

2:01 If I run that code on the left hand side,

2:03 if I show you what a range of 10 gives me,

2:05 it gives me this.

2:07 So when I do range 10 it won't really show you,

2:10 it kinda hides what it gives you,

2:12 but in reality it gives you a list of 10 numbers, okay.

2:17 So we're gonna talk about more things like lists

2:20 and integers and floating points in our next video.

2:23 But for now, I just want you to think of it

2:26 like it gives us 10 things and so it runs 10 times.

2:30 Okay, so if I did for I in range 10,

2:35 and I said print I,

2:39 what actually happens is the first time in this list,

2:42 the first time you run through in this loop,

2:45 I is zero.

2:46 Then the next time,

2:48 this range 10 is really this list here,

2:52 so I is zero the first time then it's one,

2:55 then it's two, then it's three, then it's four.

2:57 And each time what happens in the loop

2:59 is the part that's indented inside of the loop,

3:02 that's the part that runs over and over and over again.

3:06 So what happens the first time I is zero,

3:08 we say print zero and we get the zero right here.

3:11 Then we say I is one, so I is one over here,

3:14 and then it says print one, we get one over here.

3:18 Then in the last one, I turns out to be nine,

3:21 so we print nine and then we get this nine over here.

3:25 Okay, that's the basic idea of how the loop works.

3:29 So the part that we want to run over and over again,

3:34 we're going to put it inside of this loop.

3:36 Notice the colon at the end, very important.

3:39 And I'm gonna put this part inside of the loop.

3:42 And I don't even need to put that,

3:44 I can just do this

3:48 and I'm basically saying,

3:49 do this thing here, draw a square, four times.

3:53 That's all I'm saying, okay.

3:55 And I will replace this code in that regular way

3:59 without a loop and I'll prove it to you.

4:00 So first let's run it like this.

4:08 Dang, that was cool, right?

4:11 That was really frickin cool.

4:13 It just ran that four times in a row.

4:15 Now, I can show you what I mean, okay,

4:21 and I'm gonna comment out these lines.

4:26 And you can see it's gonna draw that

4:28 same four squares that we just drew, okay, just like that.

4:32 But by using a loop, your code gets a lot more clear.

4:37 What if you didn't want to run it four times?

4:38 What if you wanted to run it one million times, yeah?

4:42 It would be a lot harder to write

4:44 that out one million times,

4:45 but it's really easy to write one million in here.

4:48 I think that's one million.

4:50 That's 10, 100, 1000,

4:54 10,000, 100,000,

4:56 1,000,000. That's actually 10 million.

4:59 So there you go, 10 million.

5:02 So this thing is gonna run 10 million times.

5:05 If I ran it, and I'm gonna close it really quickly

5:08 cause this is gonna keep running forever,

5:10 but you'll notice that it just keeps overlapping

5:13 on the same square and it's gonna do that

5:15 10 million times so if I went to sleep

5:17 and woke up this will probably still be happening

5:20 and then it'll still be happening after that.

5:23 So let's exit that before something crazy happens.

5:29 Okay, so that's loops for you guys.

5:33 That is it for this video,

5:35 I will see you in the next video.