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DOUG LLOYD: All right, so
let's talk about loops.

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So loops are kind of cool
because they allow your program

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to execute lines of code
over and over and over.

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Repeatedly, without having to copy
and paste or otherwise repeat them.

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There are three major
kinds of loops you'll see.

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You'll probably have occasion to use
each one as you progress through CS50.

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Some of these loops are pretty
familiar to you from Scratch,

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so again, as we did with conditionals,
we'll put them up side by side

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if there is an analogy we can draw.

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First is forever from Scratch,
which compares to while (true).

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This is what we call an infinite loop.

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The lines of code between the curly
braces, much like the puzzle pieces

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that fit inside the C
of the forever block,

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will execute repeatedly from top
to bottom, over and over and over,

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forever.

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After all, if you recall our
discussion of Boolean expressions,

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true is always true.

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So if we're doing
something so long as true

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is true-- it seems a little silly-- true
is always true, so it'll always run.

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It'll run forever and
ever and ever until we

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find a way to break out of it with
a break statement, which again,

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we saw a little while ago we
were talking about switch.

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Or otherwise just kill our
program, which incidentally,

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if you ever find yourself in a
situation with an infinite loop

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and you don't know how to stop
your program from running.

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Just hit control and C and
that'll kill your program for you.

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But this, again, is
called an infinite loop.

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It is a loop that will run forever.

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Now, while loops are not
always infinite, because also

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we can replace the Boolean
expression with something

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a little more useful than just true.

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So here's another example.

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While and some Boolean
expression and then curly braces.

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That's pretty much analogous
to repeat until, from scratch.

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Although in a second, I'll
explain the difference.

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So in C, if the Boolean expression
inside of the while loop

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evaluates to true, the lines of
code between the curly braces

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will execute repeatedly over and over
and over until Boolean expression

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evaluates to false.

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So for example, you might
have some sort of counter.

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So say you at the beginning of
this loop say int x equals zero.

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While x is less than 100, do a couple
things inside that curly braces,

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then in the end of your loop, you say
x plus plus, you're incrementing x.

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Eventually x will get
to 100 and you will

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stop because the condition
x is less than 100

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will no longer be true as
soon as x is equal to 100.

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100 is not less than 100.

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Now somewhat confusingly, the behavior
of the scratch block is reversed.

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So be really careful if this is
your first foray into using loops.

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Basically, the repeat
until block in Scratch

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will repeat something
over and over and over,

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until the expression, the Boolean
expression in the hexagon is true.

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So it will keep doing
it until it is true.

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Meanwhile, the while loop will
continue to do it until it is false.

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So they're quite similar, but
there's that little distinction just

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to be careful of especially as you make
your first foray from transitioning

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to Scratch into c.

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The next kind of loop is
actually pretty similar.

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It's called the do while loop.

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This loop will execute all the lines
of code between the curly braces once,

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and then it will check
the Boolean expression.

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If the Boolean expression
evaluates to true,

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it'll go back and repeat
that process over and over

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and over until the Boolean
expression evaluates to false.

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So this loop, unlike a while loop, is
guaranteed to run at least one time.

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This can be pretty useful.

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Lastly, we have a for loop.

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For loops are kind of
syntactically unattractive

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and there's a lot going
on here, so bear with me

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as we try and break these pieces apart.

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Generally, the use case
of a for loop is you

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want to repeat something a
specific number of times.

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In this example here,
I have the repeat block

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from Scratch, which is
analogous to the for loop

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in C, repeating something 10 times.

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And the for loop on
the left there, which

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is a four loop that
would do the same thing.

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This would go 10 times.

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Counting from zero, we
increment each pass of the loop.

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And we keep doing that
until i is less than 10.

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So what happens?

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There's a whole lot of code there.

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Let's break it down to
what is happening step

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by step as we dig through a for loop.

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The first thing that happened is
the counter variable is center.

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Everything to the left of that first
semicolon inside the parentheses

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is executed.

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In this case, we are
saying int i equals 0.

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Declaring a new variable called i,
that variables is a type integer

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and we're saying the value inside
that variable is going to be zero.

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The second thing we do is
we then immediately evaluate

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the Boolean expression.

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Boolean expression is in the
middle of the two semicolons.

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I is less than 10.

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That's true here, right?

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We just said i is equal to
zero, and so 0 is less than 10,

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and so that condition
is true and we will now

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proceed to execute the body of the loop.

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If it was false.

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If, for example, I said instead of
i equals 0 there I said i equals 15,

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int i equals 15.

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Well 15 is not less than 10, so we
would never enter the body of the loop

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because the Boolean expression
there would evaluate to be false.

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After we go through from top to bottom,
we encounter that closing curly brace,

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a third thing happens.

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The counter variable is
incremented, or rather, the lines

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of code of a statement after the
second semicolon inside of the

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for loops parentheses is executed.

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So we start out by
saying int i equals 0.

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Then we check whether or not
the Boolean expression is true.

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0 is less than, 10 so it's true.

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So we're going to proceed
into the body the loop.

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Things will happen inside the loop, and
when we encounter that closing curly

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brace, the next thing we
do is we say i plus, plus.

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I was zero now i is one.

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Then, we again check the value of
the Boolean expression in the middle.

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One is less than 10.

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So we'll go through this
process again and again.

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We'll get to the closing
curly brace again.

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We'll increment i from
1 to 2 and from 2 to 3.

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And so on and so on and so on, until
eventually i's value becomes 10.

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Is 10 less than 10?

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No.

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Counting from 0, we've gone
through 10 iterations of loop.

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We've repeated 10 times, just
as we did in the Scratch block.

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And so that's basically the
process for a four loop.

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Now taking away the
actual code and putting it

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in just some basic general statements.

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Here's what happens.

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All the statements in
start are executed first.

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You might have more than one.

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Then, the Boolean expression is checked.

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If the expression evaluates to true,
execute the body the loop one time.

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If the expression evaluates
to false, we're done.

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We don't execute the
body of the loop at all.

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After we've executed the
body the loop one time,

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we then do what is in
the increment part there.

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Which is usually going to be
something like i plus, plus

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or something like that, that
modifies a counter variable.

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Then, after we increment, we
check the expression again

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and repeat those steps over and over
and over until the expression is

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no longer true.

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So what are the use cases for a loop?

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Use you use a while loop when
you want a loop to repeat

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an unknown number of times, but
it possibly might not run all.

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A really common example of
a while looping being used

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is to run the control flow for a game.

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You might not know how long the
user is going to be playing,

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but you want to keep
doing the same thing.

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Updating the positions of
various sprites on the board.

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You want to keep things
moving at all times,

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but you don't know when the
user is going to stop playing

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or when they're going
to run out of lives.

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So a while loop is really
good for something like that.

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That's a good use case.

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For a do while loop,
it's pretty similar.

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You want a loop to repeat an unknown
number of times, but at least once.

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You might use this for a game
too, but a really common use case

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is prompting the user for input.

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Generally, you might say something
like, give me a positive integer,

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and you might put that
inside of a do while loop.

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So it will always ask them at
least once for a positive integer.

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If they give you a positive integer,
you might break out of the loop.

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You might be done.

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The Boolean expression in
the do while might be false.

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And you won't ask them again.

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If they give you a negative number where
they type some word or something that's

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not really useful to
you, you might use the

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do while loop to go back and do
it again and again and again.

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You definitely want to ask
them to give you a number once,

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but you don't know how many times
after that they might be pests.

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And so a do while it's a great use
case for prompting the user for input.

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And a for loops use
case typically is when

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you want to repeat a loop
a discrete number of times,

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though you might not
know the number of times

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the moment the program is compiled.

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So for example, maybe you have a program
where you prompt the user for a number.

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And they enter 100.

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And then your for loop will
in that case run 100 times.

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Or maybe they enter
1,000 in your program

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and then run the loop 1,000 times.

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It's a specific number of times.

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They don't necessarily
know what that number

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is the moment program is compiled.

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It's not like a while loop
where it could be infinite.

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It's rather a number
that you just don't know.

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Now even though I've
outlined all these use cases,

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you should know that in pretty
much every circumstance,

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you can interchange all
three of these loops.

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You can use a for loop where you
would otherwise use a while loop.

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You can use a for loop wherever
you'd use a do while loop and so on.

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It can be a little tricky sometimes,
so generally it's a good practice

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to adhere to a typical use case.

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Use a do while loop if you
wanted something once, at least.

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Use a for loop if you want to do
something a specific number of times.

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After all, that's why there are
three different kinds of loops.

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So they can be used
in the right context.

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But you can interchange
them if you want.

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If you feel comfortable with while loops
and you always want to use while loops.

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It is something that
you can do, although it

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might be a little syntactically
tricky, where a for loop

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would be a lot more straightforward.

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My name is Doug Lloyd and this is CS50.

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