Function Return Values (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/

We're back with our program
that uses a function

to draw Winston four times.

And I've changed it
to actually show what age he is

at each point of life,
and you'll see why soon.

But first I need to tell you
something about Winston:

Winston has an addiction to donuts.

He eats three of them a day.

It's probably why his face
is so big and yellow.

So to warn Winston about
how bad donuts are for him,

I wanna modify this program
to show how many donuts

he's eaten total
at each point in his life.

For example, when he's two years old,

that means he's
eaten 3 times 365 times 2,

so three in a day,
times 365 days, times 2 years.

And we'll just display that
underneath that header, so wow!

Two thousand donuts, that
is a lot of donuts for a two-year-old.

Now when he's 14 years old, let's see,
he's had 3 times 365 times 14

And that is 15,000 donuts. All right.

So I could keep doing this math,
but I'm beginning to notice a pattern.

I'm repeating my calculation here,

and I'm just changing one thing about it:
the number of years.

Whenever I see repeated code like this,

I think to myself, "Hmm,
can I make that into a function?"

Yeah, definitely, we can.
So let's do that now.

I'll define my function up here

and call it calcTotalDonuts = function

And it's going to take one parameter,
the number of years,

because that's the only thing
that we're changing

each time we do this calculation.

And then inside, we'll do the calculation,

and save it into a variable,
so it'll be 3 times 365 times numYears.

All right, so now that we
have that function,

I'm going to replace this expression here
with calcTotalDonuts,

the call to the function,
and passing in "2".

Okay, um, well now we
don't see any total at all.

Hmm, okay, what happened?

Well, our function
did calculate the total, here,

but it didn't tell anybody
about that total.

It's like if a teacher calls on you
in class to answer a question

and you answer it in your head

but you're too shy to say the answer.

You've done the work, but your teacher's
never gonna know about it.

If we want the function
to communicate a value

to whoever called it, the function
has to explicitly return the value.

So for a function to return a value,
we need to type return,

and then whatever it wants to return.

Maybe it's a variable
or a value or an expression,

so here we'll say
return totalDonuts, okay?

So whoever's calling that function's
going to get this response.

And now our value displays, yay!

And actually we
can shorten our function,

we don't even have to
store it into a variable,

we can just take
this whole expression here

put it in the return,
and then it's just a one-liner. Nice.

All right, so now we can go through

and calculate the total donuts
at each point in life

by just calling this function
and passing in the number of years.

Um, let's see, it's calcTotalDonuts(25),

position is correctly, calcTotalDonuts(65)

Okay! Wow, so if he makes it to 65,

he will eat 70,000 donuts.
That is a lot of donuts.

I don't think Winston's
gonna make it. (laugh)

But now that we've
made it into a function,

it's really easy for us
to change parts of it.

Like if Winston sees this and says,

"Whoa, whoa, that's a lot.
What if I just ate one a day?"

Okay, well we can just go here,
change this one number,

and see everything change.

So that's 23,000. Still a lot.
So maybe Winston's like,

"All right, all right, what if
I just had one a week?"

Yeah, that's reasonable, okay.

So then we could just change 365 to 50,

because there's like
50 weeks in a year, right?

Okay, that's a lot better, right?
3,000 donuts, that seems reasonable.

All right, so if you see,
with functions and return values,

we can save code and save lives.