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Hello and welcome to Chapter 5,
loops and iteration.

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As always, this lecture is copyright
Creative Commons Attribution, including

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the audio and video and, and the
slides, and the book even.

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So, now we're getting to our
fourth basic pattern.

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We've talked about sequential, where
steps happen one after another.

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We've talked about conditional, where
steps may or may not happen.

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In Chapter Four we talked about
the store and retrieve pattern.

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And now we're going to talk
about the looping pattern.

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And the looping pattern is the last
of our really foundational ones, and

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it potentially is the most important one,
because it's the thing that allows us

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to get computers to do lots of
things that, say,

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humans might get tired of,
but computers don't tire of.

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And so after this we'll start
sort of becoming a little more

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skilled in the basic language
capabilities.

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We'll talk about strings and, and
then start to talk about files.

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Add start doing some real work after we
get done with this, so bear with us.

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It's going to be a while,
but we'll get there.

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We'll get there.
So, welcome to repeated steps.

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This is the example that I had in the
first, first lecture, Chapter 1.

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And the basic idea, just to review,

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is that you have this while keyword.

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The while keyword sort of
functions like an if in that

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it implicitly has a decision that
it's going to make.

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And it's either going to do
the code in the

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indented block or not do it,
or skip it basically.

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Alright?
So you either do it or you skip it.

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The difference between the while
and the if is that it's going

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to do it many times, as long as this
question that we have remains true.

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Okay?

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And so, in this case, n is 5, while n
greater than 0 functions like an if.

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So yes, it's going to run it.
Prints out 5, subtracts 1, so it's 4.

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Goes back up.

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Goes back up and asks the question again,
is n still greater than 0?

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Well, since it's 4, yes,
we'll continue on.

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Out comes 4 then n gets subtracted.
3, 2, 3, 2, and then

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we come through, we print 1.
Print 1.

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We subtract n to 0. We go up,
we go back up.

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n is now not greater than 0, so we come

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up and we execute outside the loop,
we leave the loop.

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And that really means in the
Python code we

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skip to whatever is lined with
the while statement.

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The same indent level as
the while statement.

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And so that's how it works, and

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I just print n at the end here to remind
ourselves that n ended up at 0, not at 1,

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the last thing we printed out
in the loop. The last thing we

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printed out in the loop was the 1,
but n ended up at 0 because

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it was, this loop was going to run
as long as n was greater than 0,

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so n had to sort of be not greater
than 0 to get out of the loop.

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Okay?

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So that's basically a
review of what we've done.

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Now, oh, wait, wait, wait, wait,

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wait, something else.
Iteration variables.

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Okay, so the key to this is
these loops can't run forever.

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We don't want them to run forever.

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We want them to run until,
as long as we want them to run.

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They may run a very long time,
but not forever.

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There's got to be a way to get
out of them, otherwise we

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call them infinite loops, which
we'll talk about in the next slide.

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And so the iteration variable
is generally some variable

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that is changing each

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time through the loop, and we're changing
it by subtracting 1 to it, from it.

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And so this thing is going to keep
running, and we

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can pretty much see that Oh, this is
going to exit, right?

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Whatever n is, it could be a large number,
but eventually it's going to get to 0.

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Right?

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So the iteration variable controls how
many times the loop runs.

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And it also allows us to do something
different inside the loop.

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And, of course, this is like a trivial

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loop, where we're just printing the
iteration variable.

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But it just means that this
loop is going to run

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five times and it's going to do something
potentially different each time.

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If you just ran the loop that
did the same thing over

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and over and over again, with no data
changing, that's kind of dull and pointless.

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So just because you have an
iteration variable

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doesn't mean that you've properly
constructed your loop.

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It's a, it's a common problem or
something we want to avoid,

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is an infinite loop. And here is a, a
carefully constructed loop.

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We start n at 5 at the beginning.

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We have a good question at the end,
while n greater than 0.

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It's going to run this

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as long as n is greater than 0.

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But the problem is, is we don't
change in the little block.

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We don't change the n.

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Which means it's going to come back
and n is going to be 5.

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And it's going to run this and
n is going to be 5.

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And it's going to run this and
n is going to be 5.

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And so this is an infinite loop, which
means this loop will never exit.

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It will never get out,

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it's just going to run forever in here,
because n is not changing.

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Neither of these statements change n.

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So part of the iteration variable is there
needs to be something that changes

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so that the loop will ultimately
make progress to

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accomplish what it is and
know when to stop.

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So this is an infinite loop
and, of course,

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Lather, Rinse, Repeat is commonly put on
shampoo and conditioner.

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And so you know, you can,
next time you are

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in the shower, take a look at
your shampoo and

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conditioner and find the
infinite loop that's,

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that's on most bottles of
shampoo and conditioner.

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Now here is another loop,

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just to emphasize that it's
possible to structure

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these loops in a way that they never run.

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So this function is, is an if.

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The while functions as an if.

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And so when n is set to 0 and
we ask the question,

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it is literally going to make the
decision based on n greater than 0.

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Well it is not greater than 0, so it's
going to go right by it, over

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here it's going to come in here and go
right to there and never run these

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lines of code. And that's, we
call this a zero trip loop.

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And that's okay.
I mean, this is a silly one, of course.

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It just shows that the test, the
question that's being asked is

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above the lines that make up
the body of the loop.

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And if it's false, the loop never runs.

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So it's possible that these loops
have zero trips.

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Okay?

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So, that's a loop.

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Now, there are more than one way
to sort of control the flow

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of a loop.

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The basic flow of the loop is
when it gets to the

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bottom, it goes back up to the while,
and does the check.

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This is a different way of getting out of

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a loop or controlling the execution
of a loop.

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There is a keyword or a part of
the Python language called...

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What color do I got?

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No, green's over here.
Called break.

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If you look back at the reserved words,
break was one of the reserved words.

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Break says,

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hey, if I'm in a loop, stop the loop.
All right?

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Get out of this loop.

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I'm done with this loop.

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And so here's this loop.

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Now the interesting thing we've
done is I've

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just got done talking to you about
infinite loops.

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We have just constructed an
"infinite loop", because the

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question is right there, and
the answer is yes.

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True.

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True.

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And that's a way to construct
an infinite loop.

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We've done this because we have a
different

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way of getting out of the loop, so we've

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constructed a loop that,
just on the face of it,

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just looking at that line, looks
like an infinite loop.

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So what this loop does is it
reads a line of input,

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checks to see if the string that
we've entered is done.

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And if it is, we're going to
skip out with this break

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and get out of the loop, otherwise
we're going to print it.

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So at a high level, what this loop is

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going to do is prompt for
strings of characters

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until we enter done.

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And that's exactly what it does.

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It prompts, we say hello there,
it prints that out.

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We say, we say finished, it
prints that out.

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We say done, and it's done.

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So it, when we say done, it comes out and

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finishes the loop, and, and that's
the end of the program.

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Okay?

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So, to look at this in some more detail.

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The first time, it comes in,
does the raw input.

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Because True is true, so it's
going to run it.

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And then we enter hello there.

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It checks to see if what we entered is
equal to the string done.

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It is not, so it skips and it
does the print.

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And we do this one more time,
we type finished.

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And then the line is not done,
that variable

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line does not have the value done in it.

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So we print that. We come in

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one more time, but this time
this is true, so it

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goes in and executes the break
and then it escapes the loop.

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And so you can think of...

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Right?

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Here is the body of this loop,

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because that's where the indentation
starts and ends.

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The break says, break me out of the
current loop that I am in

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and get to that next line that has the
same indent as the while.

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So, whatever it is, break says, we are
done with this loop.

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When that statement executes, we are
done with the loop.

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We're finished with the loop.

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It'll run until that executes
because we got this set to be while True.

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Okay, so there's a simpler, I mean this is
sort of a simple way to draw this.

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Break is sort of a jump to the statement
immediately following the loop.

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If you really want to picture this, I think

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of this as a kind of like a Star Trek
transporter.

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Where you kind of come into break and
then [SOUND] your molecules are sent

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to the four corners of the universe, and
you reassemble outside of the loop.

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And so if we look at this in sort of
my little road map

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version of these things, right?

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The while loop is going to run for
a while, yada, yada.

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There can actually be more
than one break

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as long as they only get this. But the
moment that somehow, some if or

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whatever hits the break, then it gets out
completely and so it escapes the loop.

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And so it sort of like you, you,
you're zoom, zoom,

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zoom, zoom, zoom, zoom, you
come in here and then you are,

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you are re-materialized outside the loop.
That's what the break does, okay?

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So, break is one way to control the
execution of loops.

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Now, another way to control
the execution of

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loops, that doesn't actually exit the
loop, is called continue.

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Continue basically says, hey, I'm done
with this iteration of the loop.

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Now each time through the loop, we
call that an iteration.

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Continue says, I don't want to
stop the loop, but I

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want to stop this iteration and
advance to the next iteration.

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And so what we have here is
we have the same basic loop,

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a while True which kind of
makes us an infinite loop.

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We're going to read a line prompting
with a less than sign.

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And if it's done, we are going to
break, that code is

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down here, and we are going to
print it if we fall through.

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So normally we'll be reading and

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printing, and if the line is done,
we are going to break out.

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That's we just got done doing.
But the new part is right here.

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And this is, we'll learn this
in the next chapter.

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If line sub 0, if the first character of

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the line, is a pound sign, we're
going to continue.

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And what continue says is it doesn't
actually get us out of the loop.

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It jumps back up to the top of the loop.

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Which means that it ignores,
for that iteration,

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the rest of the loop. Right?

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So if execution comes in here.

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I'm going to clear that.

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If execution comes in here and hits this
line, it goes back up to the while.

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Okay?

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Which means it, whatever this is, it's not
coming out of this if.

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It's going back up to the while.

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Okay?

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So continue ends the current
iteration and jumps to

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the top of the loop, and starts
the next iteration.

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And so if we look at how the code runs,

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hello there prints, pound sign with the
first character

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doesn't print, so there is
no printout right here.

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Print this is not done. And we enter done,
and then the loop ends.

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Now another way to sort of draw this is

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the continued jumps to the top
of the loop.

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00:11:44,440 --> 00:11:49,600
It, it does run the question, right?
It does check the question.

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So here is another way to, to draw

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that picture. And so here again we have a
loop and it's happily running.

234
00:11:55,830 --> 00:11:57,503
And there can be breaks in there
and there could

235
00:11:57,503 --> 00:12:00,110
be continues in there, and as
long as we don't

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00:12:00,110 --> 00:12:01,760
hit a break or continue, the
loop just sort of

237
00:12:01,760 --> 00:12:04,962
runs and goes up to the top.
And at some point, some if,

238
00:12:04,962 --> 00:12:09,020
we hit the continue. And
like a transporter, instead of

239
00:12:09,020 --> 00:12:11,320
going out of the loop, we go to
the top of the loop.

240
00:12:11,320 --> 00:12:15,600
But it's important that we go and
we check the question, right?

241
00:12:15,600 --> 00:12:16,505
So the continue is

242
00:12:16,505 --> 00:12:20,020
not likely to exit the loop
unless the question has become false.

243
00:12:20,020 --> 00:12:22,670
So the continue is likely to
come up here,

244
00:12:22,670 --> 00:12:25,540
run some more, then hit the continue.
It comes up here.

245
00:12:25,540 --> 00:12:26,640
Oops!
Oops, it did that backwards.

246
00:12:26,640 --> 00:12:27,830
Run some more.

247
00:12:27,830 --> 00:12:29,470
Clear this out.

248
00:12:29,470 --> 00:12:32,100
So, the continue could run
many times, right?

249
00:12:32,100 --> 00:12:33,110
So we have the loop.

250
00:12:33,110 --> 00:12:36,618
Loop runs a bunch of times, then
finally we hit the continue.

251
00:12:36,618 --> 00:12:37,900
Continue goes up to the top.

252
00:12:37,900 --> 00:12:40,690
If it's still true, we'll run the
loop some more.

253
00:12:40,690 --> 00:12:42,136
Then you might hit the continue.

254
00:12:42,136 --> 00:12:43,606
Then you might go up to the top, come

255
00:12:43,606 --> 00:12:46,930
down, round and round and round and round,
hit the continue again.

256
00:12:46,930 --> 00:12:49,240
Go up to the top, yada, yada.

257
00:12:49,240 --> 00:12:52,610
Now in this, in this particular loop,
this break eventually

258
00:12:52,610 --> 00:12:55,240
is down here and that's how we
get out, okay?

259
00:12:55,240 --> 00:12:58,040
So the continue goes back up
to the top of the loop.

260
00:13:00,020 --> 00:13:01,935
So these loops that we construct with the

261
00:13:01,935 --> 00:13:05,340
while keyword are what we call
indefinite loops.

262
00:13:05,340 --> 00:13:07,419
I mean, looking at the ones that we've

263
00:13:07,419 --> 00:13:09,630
written, which are two lines or six lines,

264
00:13:09,630 --> 00:13:12,800
we can kind of inspect them and
understand when they are going

265
00:13:12,800 --> 00:13:16,780
to stop, and we are going to know that
they're possible to stop them.

266
00:13:16,780 --> 00:13:20,130
A loop that won't stop is an infinite loop.

267
00:13:21,500 --> 00:13:24,864
Sometimes these loops can be rather [COUGH]
complex and you may not actually be able to

268
00:13:24,864 --> 00:13:29,910
look at them, because there are many lines
and, and so we don't know.

269
00:13:29,910 --> 00:13:32,460
And so, so if you, you have to be
real careful when you

270
00:13:32,460 --> 00:13:37,200
construct these to make sure that they
stop as, as things get more complicated.

271
00:13:37,200 --> 00:13:42,380
Now the cousin to indefinite loops are
definite loops.

272
00:13:42,380 --> 00:13:46,710
And definite loops is something where we
have a list of things or a set

273
00:13:46,710 --> 00:13:49,900
of things that are kind of a known set of
things, a finite set of things.

274
00:13:51,050 --> 00:13:53,270
And we are going to write a loop that's
going to go through that set of things

275
00:13:53,270 --> 00:13:57,460
and do something to each thing
in that set of things.

276
00:13:57,460 --> 00:13:59,630
And the keyword that we use
for this is the for.

277
00:13:59,630 --> 00:14:05,130
So we use the Python for keyword that
says we are going to write a loop, but

278
00:14:05,130 --> 00:14:07,640
instead of it just running until some
condition

279
00:14:07,640 --> 00:14:09,320
becomes true or false or we hit a break,

280
00:14:10,320 --> 00:14:13,720
we're actually going to know how many
times this is going to run.

281
00:14:13,720 --> 00:14:16,640
Now you can actually use break and
continue in for loops.

282
00:14:16,640 --> 00:14:19,820
We call these definite loops because
the, how long

283
00:14:19,820 --> 00:14:23,120
they're going to run is kind of
well known, basically.

284
00:14:23,120 --> 00:14:26,630
So here's a simple definite loop, and it's
kind of like that while loop

285
00:14:26,630 --> 00:14:28,340
that we just got done looking at

286
00:14:28,340 --> 00:14:31,400
where it's counting down and then
saying blastoff.

287
00:14:31,400 --> 00:14:35,290
And so the way we construct this loop is
we have the for keyword,

288
00:14:35,290 --> 00:14:37,580
it's part of Python language,

289
00:14:37,580 --> 00:14:40,950
the in keyword, and then we
have an iteration variable.

290
00:14:40,950 --> 00:14:43,700
I've chosen i as my iteration variable.

291
00:14:43,700 --> 00:14:46,590
And basically what we're saying is,
dear Python,

292
00:14:48,560 --> 00:14:52,970
run this indented block, and there's only
one line in the indented block.

293
00:14:52,970 --> 00:14:56,920
Run it once for each of the values
in this little list.

294
00:14:56,920 --> 00:15:00,590
This is a Python list, square brackets
make Python lists.

295
00:15:00,590 --> 00:15:02,250
Comma-separated values.

296
00:15:02,250 --> 00:15:05,210
So it says, I would like i to be 5,
then run this code.

297
00:15:05,210 --> 00:15:07,230
Then I would like i to be 4,
then run this code.

298
00:15:07,230 --> 00:15:09,430
Then I would like i to be 3,
then run this code.

299
00:15:09,430 --> 00:15:13,540
i should be 2, then run this code.
And i should be 1, then run this code.

300
00:15:13,540 --> 00:15:16,450
And so this is pretty clear,
and I like this word in.

301
00:15:17,810 --> 00:15:22,570
It says you know, doop, doop, doop,
doop, doop, and then run this each time.

302
00:15:22,570 --> 00:15:27,810
And so out of that comes 5, 4, 3, 2, 1,
and then the loop is done.

303
00:15:27,810 --> 00:15:29,920
Python is doing all the tricky bits

304
00:15:29,920 --> 00:15:32,640
here, Python's figuring all these
things out for us

305
00:15:34,880 --> 00:15:38,570
and handling all this, and then we're done.
And so it's, it's, if you look at it,

306
00:15:38,570 --> 00:15:41,320
we have an iteration variable, but we
didn't have to increment it,

307
00:15:41,320 --> 00:15:44,920
we didn't have to do anything. Python took
care of a lot of things for us.

308
00:15:44,920 --> 00:15:47,850
And so when we're looping through
a known list of things,

309
00:15:47,850 --> 00:15:49,712
or later when we read a file,

310
00:15:49,712 --> 00:15:51,480
we're going to loop through
the lines in the file.

311
00:15:51,480 --> 00:15:54,460
And so the for loop is a really nice
powerful.

312
00:15:54,460 --> 00:15:58,100
And it's syntactically cleaner, it's
really quite nice.

313
00:15:58,100 --> 00:15:59,920
Now it's important to realize that you

314
00:15:59,920 --> 00:16:03,740
don't have to just loop through numbers.
I did that one with the set of

315
00:16:03,740 --> 00:16:05,330
descending numbers, so that
it was equivalent to the

316
00:16:05,330 --> 00:16:07,680
while loop that I started at the
beginning.

317
00:16:07,680 --> 00:16:11,510
But this is a loop where what it's
going to loop through is a list.

318
00:16:11,510 --> 00:16:14,970
Closed square brackets are a list in Python.

319
00:16:14,970 --> 00:16:18,990
This is a list of three strings, Joseph,
Glenn, and Sally.

320
00:16:18,990 --> 00:16:23,110
They're string constants, and then commas
are how we make lists.

321
00:16:24,370 --> 00:16:25,820
And so friends

322
00:16:25,820 --> 00:16:27,300
is a mnemonic variable.

323
00:16:27,300 --> 00:16:29,550
Python doesn't know anything about friends
in particular,

324
00:16:29,550 --> 00:16:31,960
but I've chosen this variable name
to be friends.

325
00:16:31,960 --> 00:16:35,110
And it's a list of three people, Joseph,
Glenn, and Sally.

326
00:16:35,110 --> 00:16:37,900
And so I have an iteration variable
called friend.

327
00:16:37,900 --> 00:16:40,150
And I'm going to loop through the
set of friends.

328
00:16:40,150 --> 00:16:42,410
Now Python doesn't know anything about
singular.

329
00:16:42,410 --> 00:16:44,260
Python doesn't know anything about plural.

330
00:16:44,260 --> 00:16:47,320
I'm just choosing these variable names
because it makes a lot of sense.

331
00:16:48,350 --> 00:16:50,894
This is a set of friends.
Because it has three of them in it.

332
00:16:50,894 --> 00:16:54,030
And this is a single friend.

333
00:16:54,030 --> 00:16:57,620
What it's really going to do is friend is
going to take on the successive

334
00:16:57,620 --> 00:17:00,600
of values Joseph, Glenn, and Sally,
and this little block of code

335
00:17:00,600 --> 00:17:05,160
is going to once for each of
those three items in the set, and

336
00:17:05,160 --> 00:17:10,200
the variable friend is going to take on
the successive values of that set.

337
00:17:10,200 --> 00:17:13,360
So out of this comes
three lines of printout.

338
00:17:13,360 --> 00:17:15,980
Happy New Year Joseph, Happy New Year
Glenn, Happy New Year Sally.

339
00:17:15,980 --> 00:17:19,940
Of course this is the i bit
right down here, but we just

340
00:17:19,940 --> 00:17:23,330
made it so, hey Python, look, how ever
many friends they are, run this

341
00:17:23,330 --> 00:17:26,820
code one time for each one, change this
variable friend to be each

342
00:17:26,820 --> 00:17:30,400
of the successive ones in order,
and then we print that we're done.

343
00:17:30,400 --> 00:17:30,888
Okay?

344
00:17:30,888 --> 00:17:32,860
So the for loop.

345
00:17:32,860 --> 00:17:34,860
Sort of if we go and try to make a
picture of the for loop.

346
00:17:34,860 --> 00:17:39,420
The for loop is kind of a powerful thing.
It's does, it does two things.

347
00:17:39,420 --> 00:17:41,080
It decides if we're done or not.

348
00:17:42,470 --> 00:17:44,060
Do we keep going in the loop?

349
00:17:44,060 --> 00:17:46,210
Or, well I mean as long as we keep

350
00:17:46,210 --> 00:17:49,280
going, we're going to advance
the i value, the iteration variable.

351
00:17:49,280 --> 00:17:52,660
It takes care of the responsibility of
changing the iteration variable.

352
00:17:52,660 --> 00:17:57,050
We do not have to add lines of code in
that change the iteration variable, okay?

353
00:17:57,050 --> 00:18:00,910
And so if we take a look, you know,
we come in.

354
00:18:00,910 --> 00:18:02,280
Are we done? We're not done.

355
00:18:02,280 --> 00:18:04,310
Set i to the right thing, then print it.

356
00:18:04,310 --> 00:18:07,190
Out comes 5. Advance i,

357
00:18:07,190 --> 00:18:09,370
advance i, print it. Advance it.

358
00:18:09,370 --> 00:18:10,170
Print it. Advance it. Print it.

359
00:18:10,170 --> 00:18:12,500
Oh, now we're done.

360
00:18:12,500 --> 00:18:13,350
Right?

361
00:18:13,350 --> 00:18:16,510
i was not the thing that decided
when we were done.

362
00:18:16,510 --> 00:18:19,660
The for loop just keeps track
internally as i moves

363
00:18:19,660 --> 00:18:22,790
through these things and it
goes like oh, I'm all done.

364
00:18:22,790 --> 00:18:25,260
I'll take care of that, you finished.

365
00:18:25,260 --> 00:18:29,150
So it doesn't, there's no if in here,
so it's not like like if i equals 1, stop.

366
00:18:29,150 --> 00:18:31,780
No, no, no, it just says you told me
to do five things.

367
00:18:31,780 --> 00:18:32,360
I'm going to do five things

368
00:18:32,360 --> 00:18:33,710
and then we're going to stop.

369
00:18:33,710 --> 00:18:40,220
And so again the for loop, the for loop
here has got sort of two functions.

370
00:18:40,220 --> 00:18:43,910
It decides how long the loop is going to
run, and changes the

371
00:18:43,910 --> 00:18:47,570
iteration variable based on what you've
told it to, in this in clause.

372
00:18:48,690 --> 00:18:49,190
Okay?

373
00:18:50,310 --> 00:18:53,580
So I think in is a real elegant
construct.

374
00:18:53,580 --> 00:18:58,290
It's just a keyword, but it's
sort of, if you think about math,

375
00:18:58,290 --> 00:18:59,680
if you're familiar with sets,

376
00:18:59,680 --> 00:19:02,120
it's like something inside of
a set of something.

377
00:19:02,120 --> 00:19:03,980
I think it's a real pretty way
to think about it.

378
00:19:05,080 --> 00:19:07,350
And you can kind of think of it
a little more abstractly.

379
00:19:07,350 --> 00:19:10,640
That you say, well here's a little
indented block of code.

380
00:19:10,640 --> 00:19:11,080
Right?

381
00:19:11,080 --> 00:19:14,990
And I want it to run some number
of times for each

382
00:19:14,990 --> 00:19:18,660
of the i values in the set 5, 4, 3, 2, 1.

383
00:19:18,660 --> 00:19:19,950
That's how I kind of think of it.

384
00:19:19,950 --> 00:19:23,170
So I think that this is a
real pretty syntax.

385
00:19:23,170 --> 00:19:25,490
Different languages have different
looping syntax.

386
00:19:25,490 --> 00:19:28,930
I think this is really a very
expressive, very pretty one.

387
00:19:32,720 --> 00:19:33,920
Yeah.

388
00:19:33,920 --> 00:19:36,870
So another way to, so, so, so one way to

389
00:19:36,870 --> 00:19:40,440
think about this picture is that, you
know, the for loop

390
00:19:40,440 --> 00:19:42,760
causes sort of repeated execution
in that we're

391
00:19:42,760 --> 00:19:45,410
driving in the circle and then we
stop, right?

392
00:19:45,410 --> 00:19:47,890
The other way to think about is to, to

393
00:19:47,890 --> 00:19:50,588
not, to think about a little more
abstractly, right?

394
00:19:50,588 --> 00:19:53,440
To say, hmm, you know, at the
end of the day, all I'm

395
00:19:53,440 --> 00:19:58,130
really telling Python is, I want to
execute this block of code five times,

396
00:19:58,130 --> 00:20:02,190
then I want the variable i to
change from, to these three values.

397
00:20:02,190 --> 00:20:04,030
So in a way, you could think of
this as expanded

398
00:20:04,030 --> 00:20:06,960
is the for loop sets it to 5,
then runs your code.

399
00:20:06,960 --> 00:20:09,290
The for loop then sets it to 4,
runs your code.

400
00:20:09,290 --> 00:20:11,525
The for loop sets it to 3, runs your code.

401
00:20:11,525 --> 00:20:15,830
For loop sets it to 2, runs your code.
Sets it to 1, runs your code.

402
00:20:15,830 --> 00:20:18,740
These two ways of looking at it
are the same

403
00:20:19,910 --> 00:20:23,390
from your perspective, because
you're just asking Python

404
00:20:23,390 --> 00:20:24,590
to do something.

405
00:20:24,590 --> 00:20:28,030
Whether it does it this way, or
whether it does it this way,

406
00:20:28,030 --> 00:20:32,290
you hardly can tell the difference.
It's probably going to do it this way.

407
00:20:32,290 --> 00:20:37,150
But logically it's not that different.
It's not different from doing it this way.

408
00:20:37,150 --> 00:20:41,440
You're saying, run this block of code,
change i in the following way.

409
00:20:44,860 --> 00:20:48,600
Cool.
It's like, we don't have to worry.

410
00:20:48,600 --> 00:20:51,640
I mean, we can use, mentally,
either model of what's

411
00:20:51,640 --> 00:20:54,520
going on inside, because it doesn't
matter, because they're the same.

412
00:20:57,375 --> 00:21:00,330
Okay, so these definite loops
are really cool.

413
00:21:00,330 --> 00:21:02,960
Starting in a couple of chapters we'll
mostly use definite

414
00:21:02,960 --> 00:21:07,200
loops to go through lists or
dictionaries or tuples or files.

415
00:21:08,290 --> 00:21:09,970
And so that's a finite set of things.

416
00:21:09,970 --> 00:21:12,100
It can be a large set of things, but
it's a finite set of things.