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Let's talk about while loops in this
video. Python provides two main ways to

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loop around a block of code; for loops
and while loops. We've seen how a for loop

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can be used to iterate through an
iterable. In our examples we use the

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items in a sequence and the numbers in a
range. Sometimes we need to keep looping

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as long as some condition is true and
stop when it becomes False. We do this

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using a while loop. So the basic form is
while condition : execute block of code.

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The condition can be anything that can
evaluate to True or False, and so long as

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the condition is True, the code in the
loop will be executed. When the condition

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becomes False, the loop terminates.
Alright, so I think a few examples will help here.

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Let's create a new Python
file. We'll call this whileloop.py.

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Alright so I'm going to start here by
using a while loop to do the same as the

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for loop that we've seen in previous
videos. Firstly, I'm gonna type in the

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original loop, so it's for i in range
parentheses 10 colon print parentheses

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double quotes i is now, left and right
curly braces dot format

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and i in parentheses. Then we've got two closing
right parentheses there. Remember that

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the starting value of a range is
optional and will default to zero if not

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specified. You should be able to work out
what this code will do before running it.

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So when I do run it now, you can see the
output down the bottom - we've got the

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values zero through nine. Alright so I'm
going to comment that code out,

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and we're gonna start instead on line four
and we're going to use a while loop to do

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the same thing. So I'm gonna type i
equals zero on line four. Next line while

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i is less than 10 colon, then print
indented parentheses double quotes i is now.

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We've got left and right curly braces,
format then i in parentheses, then our two

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closing right parentheses. Then
we'll put i plus equals 1. Okay.

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So there's our code. So line seven just adds
one to the current value of i,

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and basically, that line is identical to
this line, i equals i plus 1. Now we're

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going to talk about this later in this
section. The code on line seven is

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preferable to the code on line eight, and
I want you to see things written in the

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best way whenever possible. Alright so
I'm going to delete line eight.

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What I'm also going to do now is set a breakpoint
on line five before running the code in

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the debugger. Now i is bound to the value
zero on line four and the debugger is

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going to be stopped on line five. Let's
do that. We'll run the debugger, and you

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can see we stopped on line five there.
If we click over to the console tab,

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we'll see the output as we step through
the code. So we run line five ready to

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test if the value of i is less than 10.
We can see that it is, so if we step over,

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it takes us to line 6. We're gonna step over
again to print the value of i. The value

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of i is now printed in the console. So
we're now on line seven, as you can see.

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So this is that strange way of increasing
the value of i that I just mentioned.

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Watch the value of i that's shown on
line four as I step over again.

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1 is being added to i. It now has the value of
1, 0 plus 1 is 1. So we're still less than

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10, so if we step over again, it takes us
back to line 6. Step over once more to

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print the value and we move to line 7.
Alright so we're now on line 7. That's

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going to add 1 to i again which should
give us the value 2. We should also go

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back to line 5 to loop again. Notice that
I'm not mindlessly clicking the step

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over button. When you're using the
debugger like this to understand your

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code, make sure you stop on each line and
work out what's going to happen. As I

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said earlier, we're learning to become
programmers, not mindless button clickers.

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Alright so let's see if we were right.
Step over, and i does have the

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value 2 and we have gone back to line 5.
Next time we go around the loop we

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should get the message i is now 2 printed
out, and i will be increased to 3 and

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after that happens we'll get back to
line 5.

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I'm going to use resume program over
here this time. If you prefer to step

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over a few more times then go ahead and
do that. You don't need to watch me

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clicking the button repeatedly. Pause the
video frequently if you do need to catch

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up, and if I click on machine program now, we
have got i is now 2 printed and we can

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see that I has been incremented to 3. Resume
program again, i is now 4. I'm going

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to click on the resume program 5 more
times until i has the value 9 and we're

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back on line 5. Now things are going to get
interesting, so I'm going to come over here and

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use step over once more to print
the value of i. So I'm going to step over

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here, i is now 9, and when I step over one
more time to execute line 7, we're back

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on line 5 with i having the value of 10.
10 isn't less than 10. That means the

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condition on line 5 will be False now
and we'll exit the loop. Because there's

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no more code in the program that means
the program should terminate. So let's

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see if we were right. I'm going to click
step over one more time to execute

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line 5, and you can see that has in fact
happened. The program has now terminated.

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And we were right - there's no more
output from our code and we get the

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message: process finished with exit code
zero produced by the IDE. Notice that our

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programs output appears in black and
messages from the IDE normally appear in

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blue. It sometimes uses red for errors
and other important information.

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Alright so that's a while loop. As we saw, a
while loop will continue to loop, as long

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as the condition evaluates to True. When
the condition evaluates to False, the

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loop terminates and execution continues
with the code after the loop. We don't

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have any more code, so the program
terminated as well. Alright I'll stop

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the video here and in the next one we'll
look at a few more things about while loops.

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See you in the next video.