How can I flatten an array swiftily in Swift?

With Swift 5, you can choose one of the three following ways in order to flatten an array.

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#1. Flatten an array with Array's flatMap(_:) method

With Swift, types that conform to Sequence protocol (including Array) have a flatMap(_:) method. Array's flatMap(_:) has the following declaration:

func flatMap<SegmentOfResult>(_ transform: (Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence

Returns an array containing the concatenated results of calling the given transformation with each element of this sequence.

The Playground sample code below shows how to flatten an Array of type [[Int]] to type [Int] using flatMap(_:):

let array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let flattenArray = array.flatMap({ (element: [Int]) -> [Int] in
    return element
})
print(flattenArray) // prints [1, 2, 3, 4, 5, 6, 7, 8, 9]

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#2. Flatten an array with Array's joined() method

Array's has a method called joined(). joined() has the following declaration:

func joined() -> FlattenSequence<Array<Element>>

Returns the elements of this sequence of sequences, concatenated.

The following Playground sample code shows how to flatten an Array of type [[Int]] to type [Int] using joined():

let array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let flattenCollection = array.joined()
let flattenArray = Array(flattenCollection)
print(flattenArray) // prints [1, 2, 3, 4, 5, 6, 7, 8, 9]

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#3. Flatten an array with Array's reduce(_:_:) method

Swift Array has a reduce(_:_:) method with the following declaration:

func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) throws -> Result) rethrows -> Result

Returns the result of combining the elements of the sequence using the given closure.

The following Playground sample code shows how to flatten an Array of type [[Int]] to type [Int] using reduce(_:_:):

let array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let flattenArray = array.reduce([], { (result: [Int], element: [Int]) -> [Int] in
    return result + element
})
print(flattenArray) // prints [1, 2, 3, 4, 5, 6, 7, 8, 9]

As an alternative to reduce(_:_:), you can use reduce(into:_:):

let array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
let flattenArray = array.reduce(into: [Int]()) { (result: inout [Int], element: [Int]) in
    result += element
}
print(flattenArray) // prints [1, 2, 3, 4, 5, 6, 7, 8, 9]

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()).)

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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 }