Flatten an Array of Arrays in Swift

Is there a counterpart in Swift to flatten in Scala, Xtend, Groovy, Ruby and co?

var aofa = [[1,2,3],[4],[5,6,7,8,9]]
aofa.flatten() // shall deliver [1,2,3,4,5,6,7,8,9]

of course i could use reduce for that but that kinda sucks

var flattened = aofa.reduce(Int[]()){
    a,i in var b : Int[] = a
    b.extend(i)
    return b
}

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())

enter image description here

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: enter image description here

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]

Edit: Use joined() instead:

https://developer.apple.com/documentation/swift/sequence/2431985-joined

Original reply:

let numbers = [[1, 2, 3], [4, 5, 6]]
let flattenNumbers = numbers.reduce([], combine: +)

Flatten an Array of Arrays in Swift

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