Swift Language Arrays Flattening the result of an Array transformation with flatMap(_:)
Example
As well as being able to create an array by filtering out nil from the transformed elements of a sequence, there is also a version of flatMap(_:) that expects the transformation closure to return a sequence S.
extension SequenceType {
public func flatMap<S : SequenceType>(transform: (Self.Generator.Element) throws -> S) rethrows -> [S.Generator.Element]
}
Each sequence from the transformation will be concatenated, resulting in an array containing the combined elements of each sequence – [S.Generator.Element].
Combining the characters in an array of strings
For example, we can use it to take an array of prime strings and combine their characters into a single array:
let primes = ["2", "3", "5", "7", "11", "13", "17", "19"]
let allCharacters = primes.flatMap { $0.characters }
// => "["2", "3", "5", "7", "1", "1", "1", "3", "1", "7", "1", "9"]"
Breaking the above example down:
primesis a[String](As an array is a sequence, we can callflatMap(_:)on it).- The transformation closure takes in one of the elements of
primes, aString(Array<String>.Generator.Element). - The closure then returns a sequence of type
String.CharacterView. - The result is then an array containing the combined elements of all the sequences from each of the transformation closure calls –
[String.CharacterView.Generator.Element].
Flattening a multidimensional array
As flatMap(_:) will concatenate the sequences returned from the transformation closure calls, it can be used to flatten a multidimensional array – such as a 2D array into a 1D array, a 3D array into a 2D array etc.
This can simply be done by returning the given element $0 (a nested array) in the closure:
// A 2D array of type [[Int]]
let array2D = [[1, 3], [4], [6, 8, 10], [11]]
// A 1D array of type [Int]
let flattenedArray = array2D.flatMap { $0 }
print(flattenedArray) // [1, 3, 4, 6, 8, 10, 11]
