Flatten an Array of Arrays in Swift

Swift >= 3.0

reduce:

let numbers = [[1,2,3],[4],[5,6,7,8,9]]let reduced = numbers.reduce([], +)

flatMap:

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

joined:

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

In Swift standard library there is joined function implemented for all types conforming to Sequence protocol (or flatten on SequenceType before Swift 3), which includes Array:

let numbers = [[1,2,3],[4],[5,6,7,8,9]]let flattened = Array(numbers.joined())

In certain cases use of joined() can be beneficial as it returns a lazy collection instead of a new array, but can always be converted to an array when passed to Array() initialiser like in the example above.

Swift 4.x/5.x

Just to add a bit more complexity in the array, if there is an array that contains array of arrays, then flatMap will actually fail.

Suppose the array is

var array:[Any] = [1,2,[[3,4],[5,6,[7]]],8]

What flatMap or compactMap returns is:

array.compactMap({$0})//Output[1, 2, [[3, 4], [5, 6, [7]]], 8]

In order to solve this problem, we can use our simple for loop logic + recursion

func flattenedArray(array:[Any]) -> [Int] {    var myArray = [Int]()    for element in array {        if let element = element as? Int {            myArray.append(element)        }        if let element = element as? [Any] {            let result = flattenedArray(array: element)            for i in result {                myArray.append(i)            }        }    }    return myArray}

So call this function with the given array

flattenedArray(array: array)

The Result is:

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

This function will help to flatten any kind of array, considering the case of Int here

Playground Output: