While Loops in Python | Python for Beginners

INSTRUCTOR: Hello, everybody.

Today we're going to be taking
a look at While loops in Python.

The While loop in Python is used
to iterate over a block of code

as long as the test
condition is true.

Now, the difference between
a For loop and a While loop

is that a For loop
is going to iterate

over the entire sequence,
regardless of a condition,

but the While loop is only going
to iterate over that sequence as

long as a specific
condition is met.

Once that condition is not
met, the code is going to stop.

And it's not going to
iterate through the rest

of the sequence.

So if we take a look at
this flowchart right here,

we're going to enter
this While loop.

And we have a test
condition right here.

The first time that this test
condition comes back false,

it's going to exit
the While loop.

So let's start actually
writing out the code

and see how this
While loop works.

So let's create a variable.

We're just going to say,
number is equal to 1.

And then we'll say While.

And now we need to
write our condition that

needs to be met in order
for our block of code

beneath this to run.

So we're going to say,
While number is less than 5,

and then we'll do colon, Enter.

And now this is
our block of code.

We're going to say Print.

And then we'll say, number.

Now what we need to do is
basically create a counter.

We're going to say, number
equals number plus 1.

If you've never done something
like this, it's like a counter.

Most people start it at 0.

In fact, let's start it at 0.

And then each time it runs
through this While loop,

it's going to add 1 to
this number up here.

And then it's going to
become a 1, a 2, a 3 each

time it iterates
through this While loop.

Now, once this number is
no longer less than 5,

it'll break out
of the While loop.

And it will no longer run.

So let's run this really
quick by hitting Shift-Enter.

So it starts at 0.

And it's going to say, While
the number is less than 5,

Print number.

So the first time that
it runs through, it is 0.

And so it prints 0.

And then it adds 1 to Number.

And then it continues that
While loop right here.

And it keeps looping
through this portion.

It never goes back up
here to this line of code.

This is just our variable
that we start with.

And then once this condition is
no longer met, once it is false,

then it's going to
break out of that code.

Now that we basically know
how a While loop works,

let's look at something
called a Break statement.

So let's copy this
right down here.

And what we're going to say
is, if number is equal to 3,

we're going to Break.

Now, with the
Break statement, we

can basically stop the loop even
if the While condition is true.

So while this number
is less than 5,

it's going to continue
to loop through.

But now we have this
Break statement.

So it's going to say,
if the Number equals 3,

we're going to break
out of this While loop.

But if this is
false, we're going

to continue adding to that
number just like normal.

So let's execute this.

So as you can see, it only
went to 3 instead of 4

like before because
each time it was

running through this While
loop, it was checking

if the number was equal to 3.

And once it got to
3, this became true.

And then we broke out
of this While loop.

The next thing that I want to
look at-- and we'll copy this

right down here--

is an Else statement,
much like an If statement.

But we can use the Else
statement with a While loop,

which runs the block of code.

And when that condition
is no longer true,

then it activates
the Else statement.

So we'll go right down here.

And we'll say, Else.

And we'll do a colon and Enter.

And then, we'll say Print.

And we'll say, no
longer less than 5.

Now because this If statement is
still in there, it will break.

So let's say, 6.

And then we'll run this.

And so it's going to iterate
through this block of code.

And once this statement
is no longer true,

once we break out
of it, we're going

to go to our Else statement.

Now, as long as this
statement is true,

it's going to continue
to iterate through.

But once this
condition is not met,

then it will go to
our Else statement.

And we'll run that line of code.

Now the Else statement is
only going to trigger if the

While loop no longer is true.

If we have something
like this If statement

that causes it to break
out of the While loop,

the Else statement
will no longer work.

So let's say, if the number
is 3, and we run this,

the Else statement is no
longer going to trigger.

So this body of code
will not be run.

Now, the next thing
that I want to look at

is the Continue statement.

If the Continue
statement is triggered,

it basically rejects
all remaining statements

in the current
iteration of the loop.

And then we'll go to
the next iteration.

Now to demonstrate
this, I'm going

to change this Break
into a Continue.

So before when we had the Break,
if the number was equal to 3,

it would stop all
the code completely.

But when we change this
to Continue, which we'll

do right now, what
it's going to do

is, it's no longer
going to run through any

of the subsequent code
in this block of code,

it's just going to go
straight up to the beginning

and restart our While loop.

So what's going
to happen when we

run this is, it's going to come
to 3, it's going to become 3.

And it's going to continue
back into the While loop.

But it's never going to
have that number changed

to be add it to 1 to
continue with the While loop.

This will basically
create an infinite loop.

Let's try this really quickly.

And as you can see, it's
going to stay 3 forever.

Eventually, this would time
out, but I'm just going

to stop the code really quick.

So if we just change up
the order of which we're

doing things, we're
going to say, there,

and we're going to
put this down here.

So what it's going
to do now, instead

of printing the number
immediately and then

adding the number
later, we're going

to add the number right away.

And then we're going
to say, if it is 3,

we're going to continue.

And it's going to
print the number.

So let's try executing
this and see what happens.

So as you can see, we no longer
have the 3 in our output.

What it did was, when
we got to the number 3,

it continued and didn't
execute this right here,

which prints off that number.

So that really is the
basics of the While loop.

I hope that this was helpful.

I hope that you learned
something in this video.

If you did, be sure to
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And I'll see you
in the next video.