Flatten an Array of Arrays in Swift

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 >= 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())

Swift 4.x

This usage of flatMap isn't deprecated and it's make for this. https://developer.apple.com/documentation/swift/sequence/2905332-flatmap

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

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: