map, compactMap, flatMapについて

mapの挙動

配列の全ての要素を参照して新しい配列を返す.

let intNumberList: [Int] = [1, 2, 3, 4, 5]

let stringNumberList: [String] = intNumberList.map { "\($0)" }

print(stringNumberList) // ["1", "2", "3", "4", "5"]

compactMapの挙動

nil以外の要素を参照して新しい配列を返す.

let optionalStringNumberList: [Int?] = [1, 2, nil, 4, 5]

let intNumberList: [Int] = optionalStringNumberList.compactMap { $0 }

print(intNumberList) // [1, 2, 4, 5]

flatMapの挙動

多次元配列を参照し, 一次元配列に変換する.

let intNumberList: [[Int]] = [[1, 2, 3], [1, 2, 3]]

let intNumberListWithFlatMap: [Int] = intNumberList.flatMap { $0 }

print(intNumberListWithFlatMap) // [1, 2, 3, 1, 2, 3]

併用

flatMapとmapを併用できる.

let intNumberList: [[Int]] = [[1, 2, 3], [1, 2, 3]]

let stringNumberListWithFlatMap: [String] = intNumberList.flatMap {  $0.map { "\($0)" }

print(stringNumberListWithFlatMap) // ["1", "2", "3", "1", "2", "3"]