If/Else Part 1 (Video Version)

This is just a screen grab of our interactive coding talk-through, prepared to make captioning and translation easier. It is better to watch our talk-throughs here:
https://www.khanacademy.org/cs/programming/

Here's a function
you might not know about: random.

It takes in two parameters:
a lower bound and an upper bound,

and it gives you back a random number
somewhere between those two bounds.

So here, this variable number will be
somewhere between zero and one.

Then we're going to draw that number
to the canvas using this text function.

Those last two parameters are
for the x and y coordinates of the text

and we use textSize and fill
to set the fill and the color of the text.

So if I press Restart a couple of times,

you can see the random numbers
being generated,

and you'll see the precision of
these numbers is to three decimal places.

So here's a question--

what if I only wanted to generate
a number that's either zero or one?

Well, we can use
this other function called round,

and this takes in a number that can
have as many decimals as you want,

and it rounds it to the nearest integer.

So I'm just going to go ahead
and make a new variable called integer

and assign it whatever round gives us.

We can also draw that integer
to the screen

with our handy-dandy text function.

So text(integer

--we'll put it maybe at 160
and 350. Nice.

So this shows us that round of 0.2314
rounds down to zero,

and if I put in something like 4.6,
that would round me up to 5. Neat.

So if I wanted to randomly generate
a zero or a one,

I can take this random decimal
that we're generating,

that falls between zero and one,

and stick it right into
that round function.

So just like this:
I'm going to grab this number

and plop it down here.

And now you can see

that whenever we generate
a number that's less than 0.5,

it gets rounded down to zero,

and whenever we generate a number
that is greater than or equal to 0.5,

it gets rounded up to one.

Maybe you can start to see the beginning
of some sort of coin-flip game here

where if you flip a zero,
your friend gives you a dollar,

and if you flip a one,
your friend gives you ten dollars.

Great game, right?

In fact, let's go ahead
and illustrate this coin-flipping game

with some super realistic coins

that just happen to look
like really boring ellipses.

Just like this: I'm going to draw
an ellipse in the middle of our canvas

and that's going to be our coin.

Ooh! It's covering the text.
Let's scoot that up a bit.

Great, and I have this idea
where, if I flip a zero,

I'm going to show
the purple side of the coin,

so to make the coin purple,
I can just fill it with some purple.

If I flip a 1,
I'll show the yellow side of the coin

so it'll be a purple
and yellow-sided coin.

And luckily, with our impressive
knowledge of if-statements

this is super easy.

We can just say if integer equals zero,

(remember we use
three equals signs to check for equality),

then we will fill the ellipse purple.

Then, if integer is equal to one,

we have a different fill function

and we'll make that one yellow.

Great. And it works! Woo hoo!

But let's think about this for a second--

integer here will only ever
be zero or one, right?

We designed it that way

so that means
that either this statement will be true,

or this statement will be true

Always.

We've covered every possible case here, which means

we can start thinking about
our decision-making a little differently.

That is, if integer is equal to zero,
we will fill it purple,

otherwise, we'll fill yellow.

So, do you see
how we don't have to say anything

about integer being one
in that second case?

All we have to say is,

"If integer is zero, do this;
otherwise, do that",

and, in programming,
the way we say "otherwise" is "else."

So watch this:

I'm just going to replace this second
if-condition with the word "else"

and what this means is,

if the stuff inside these parentheses
is true,

then run the code in these brackets.

Otherwise, run the code in these brackets.

Sometimes we'll even put the "else"
on the same line as that closing bracket

just to remind ourselves

that these two blocks of code
are very, very connected.

You can't have an else block
unless you've just had an if block.

Got it?

This will also help you remember

to not put something in between
the two blocks, like var y equals zero,

and that would just break everything!
So don't do that.

Great. So now we know about if/else
which is really good

when we are deciding
between two possible things to do.

But what if we have more?

What if I generated
an integer between zero and two

and then I had three possibilities:
zero, one, or two? What then?

Duh duh duh! To be continued!