How to store truth with Booleans

Paul Hudson    @twostraws    October 25th 2021

Updated for Xcode 15

So far we’ve looked at strings, integers, and decimals, but there’s a fourth type of data that snuck in at the same time: a very simple type called a Boolean, which stores either true or false. If you were curious, Booleans were named after George Boole, an English mathematician who spent a great deal of time researching and writing about logic.

I say that Booleans snuck in because you’ve seen them a couple of times already:

let filename = "paris.jpg"
print(filename.hasSuffix(".jpg"))

let number = 120
print(number.isMultiple(of: 3))

Both hasSuffix() and isMultiple(of:) return a new value based on their check: either the string has the suffix or it doesn’t, and either 120 is a multiple of 3 or it isn’t. In both places there’s always a simple true or false answer, which is where Booleans come in – they store just that, and nothing else.

Making a Boolean is just like making the other data types, except you should assign an initial value of either true or false, like this:

let goodDogs = true
let gameOver = false

You can also assign a Boolean’s initial value from some other code, as long as ultimately it’s either true or false:

let isMultiple = 120.isMultiple(of: 3)

Unlike the other types of data, Booleans don’t have arithmetic operators such as + and - – after all, what would true + true equal? However, Booleans do have one special operator, !, which means “not”. This flips a Boolean’s value from true to false, or false to true.

For example, we could flip a Boolean’s value like this:

var isAuthenticated = false
isAuthenticated = !isAuthenticated
print(isAuthenticated)
isAuthenticated = !isAuthenticated
print(isAuthenticated)

That will print “true” then “false” when it runs, because isAuthenticated started as false, and we set it to not false, which is true, then flip it again so it’s back to false.

Booleans do have a little extra functionality that can be useful. In particular, if you call toggle() on a Boolean it will flip a true value to false, and a false value to true. To try this out, try making gameOver a variable and modifying it like this:

var gameOver = false
print(gameOver)

gameOver.toggle()
print(gameOver)

That will print false first, then after calling toggle() will print true. Yes, that’s the same as using ! just in slightly less code, but it’s surprisingly useful when you’re dealing with complex code!

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