Booleans (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/

In the English language,
we have different parts of speech,

like noun, adjective, preposition, verb.

And then there are a bunch of rules

that tell us how to put
these different parts of speech together.

So if I said something like,
"Dog books my eats,"

you'd be like,
"What the heck does that mean?"

And if you didn't realize this before,

apparently, you can't just stick
two nouns

in front of an adjective,
in front of a verb.

Doesn't work.

But if I'd switched those and said,
"My dog eats books,"

then you would totally know what I meant.

I could even replace this verb "eats"

with another verb like,
I don't know, "throws",

and it would still make grammatical sense,

even if you can't imagine
my dog throwing a book.

So in programming,
instead of parts of speech,

we have these things called types.

You've already seen one of these: numbers.

We use numbers all the time
in our drawing code.

And just like in English,

there are times it makes sense to use
a number, and times when it doesn't.

If I started typing in
this background function, "100 minus",

then whatever comes next
better be a number,

or at least something that evaluates
to a number like "14 + 15."

On the other hand,
if I'd just typed "100 space",

well, I can't really put
a number after that

because "100-space-10"
doesn't mean anything.

So there's another type in programming,
called the Boolean type.

And it's called Boolean

because some dude
named George Boole invented it.

And unlike a number
which has a ton of possible values,

a Boolean can only be
one of two values: true or false.

And you can see
when I type them they turn blue,

which means they're
super special awesome words.

And you've already seen one place
where we use booleans,

though you may not have realized it:
if statements!

Let's get a quick refresh
on how those work.

I'm just going to make a variable
called 'number, ' give it a number, 40.

And write an If statement that says,

"If number is less than 50,
then I will draw this first ellipse."

I'm just going to copy this
into the If statement

and indent it by selecting everything
and pressing tab.

So now this statement says,

"If number is less than 50," which it is,
"then we'll draw the top ellipse."

And if I make number greater than 50,

you can see
that the top ellipse disappears.

Alright, so this thing
inside the parentheses

is actually a Boolean expression.

Remember, a math expression
is anything that evaluates to a number:

like 3 plus 2 plus 4 times 8.

So a Boolean expression is anything
that evaluates to a Boolean.

A good way to check
if an expression evaluates to a Boolean,

is to stick the word "is" in front of it,
and ask it like a question.

If it sounds like a yes or no question,
then you know it's a Boolean expression.

So here we can say,
"Is number less than 50?"

Yes, yes it is, and yes,
that is a Boolean expression.

On the other hand,
if I had something like, "4 + 4"

and I tried to ask, "is 4 + 4?"
No. not a Boolean.

So back to our If statement.

I can actually put anything
inside these parentheses,

as long as it's a Boolean
or Boolean expression.

So I could say, "If true,"
and that ellipse would always be drawn.

Or I could say, "If false,"
and the ellipse would never be drawn.

I could also do something like
"If 3 is less than 4,"

which is a Boolean expression
that will always evaluate to true,

which is kinda pointless,
the ellipse will always be drawn,

or "3 greater than 4,"
and that's always going to be false.

And I can also assign Booleans
to variables, like this:

so I'm going to make a new variable,
call it WinstonIsCool, and assign it

a Boolean value, so true or false.

Say true because Winston
is definitely cool.

And now that this variable
has a Boolean value,

I can copy it and stick it
inside this If statement

and now you can see the ellipse is drawn,

because the value
of WinstonIsCool is true.

I could also replace this
with a Boolean expression,

so could be "2 less than 4."

Now if you're making a variable
that's meant for a Boolean value,

you should give it a name

that describes the condition
when the variable is true.

A good way to check if you've picked
a good name for your variable

is to put it in an If statement
and see if it makes sense as a condition.

So, forget WinstonIsCool,
we already know that's true.

Let's say I had a variable
called "muffins."

All right, "If muffins." Hmm.

Well, you know what?

That doesn't tell me anything,
so that's a pretty bad variable name,

but if I had "If muffinsAreBaking",
then that would tell me

that when this variable is true,
then the muffins are baking.

And don't ask me what muffins,
it's not important.

So for now let's go back
to "If number is less than 50."

Cool.

Now let's look
at some other Boolean expressions.

You've already seen
"less than" and "greater than",

but you can also check
if something is "less than or equal to."

So let's try, "If number is
less than or equal to 48."

And we could also say, "If number
is greater than or equal to 48."

If it is, we will draw
this top-right ellipse.

Indent that.

And if you'd like to check if two things
are exactly equal to each other

or you could say: "If number"

and then three equals signs,
or "triple equals 48."

So that's a lot more like the equals sign
you're used to in math,

except this time
you have three of them in a row.

It's kind of overkill, right?

And then finally, we have
if you want to check

if two things are not equal to,

so strictly not equal to, you can say,

"If number" and then an exclamation point,
and then 2 equals signs, "48".

And then we will draw that last ellipse.

So if we go back to the top,
we can see that number is 48,

so it is less than or equal to 48,

which is why
the top-left ellipse is drawn.

It's also greater than or equal to 48,
it's also equal to 48,

but it is not not equal to 48,

which is why we're missing
that bottom-right ellipse.

And if we play around with number

you can see it changes
which ellipses are drawn.

So now you guys know about Booleans.

And just like math expressions,

Boolean expressions
can get really complicated.

But we will talk about those another time.