How can I flatten an array swiftily in Swift?

Question

I want to turn this:

let x = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

into this:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

very gracefully.

The most straightforward way, of course, is

var y = [Int]()
x.forEach { y.appendContentsOf($0) }

But that makes the resulting array mutable, which is unnecessary. I don't like this way.

I tried using reduce:

let y = x.reduce([Int]()) { (array, ints) -> [Int] in
    array.appendContentsOf(ints)
    return array
}

But the compiler complains that array is immutable, so I can't call the mutating method appendContentsOf.

Hence, I added some stuff:

let y = x.reduce([Int]()) { (array, ints) -> [Int] in
    var newArr = array
    newArr.appendContentsOf(ints)
    return newArr
}

This is just plain bad. I have an instinct that this is not swifty.

How can I flatten an array more swiftily than the above methods? A one-liner would be good.

Answer 1

There's a built-in function for this called joined:

[[1, 2, 3], [4, 5, 6], [7, 8, 9]].joined()

(Note that this doesn't actually return another Array, it returns a FlattenSequence, but that usually doesn't matter because it's still a sequence that you can use with for loops and whatnot. If you really care, you can use Array(arrayOfArrays.joined()).)

---

The flatMap function can also help you out. Its signature is, roughly,

flatMap<S: SequenceType>(fn: (Generator.Element) -> S) -> [S.Generator.Element]

This means that you can pass a fn which for any element returns a sequence, and it'll combine/concatenate those sequences.

Since your array's elements are themselves sequences, you can use a function which just returns the element itself ({ x in return x } or equivalently just {$0}):

[[1, 2, 3], [4, 5, 6], [7, 8, 9]].flatMap{ $0 }