【Swift】Arrayの便利な変換関数たち

func map<T>(_ transform: (Element) throws -> T) rethrows -> [T]

// 数字を書式化してくれるクラス
let formatter = NumberFormatter()
formatter.locale = Locale(identifier: "ja_JP")
formatter.numberStyle = .currency

// 値段を表すInt配列
let prices = [2000, 19800, 548000]

// 変換
let mapped = prices.map { value in
    formatter.string(from: NSNumber(value: value))!
}

print(mapped)

// 数字を書式化してくれるクラス
let formatter = NumberFormatter()
formatter.locale = Locale(identifier: "ja_JP")
formatter.numberStyle = .currency

// 値段を表すInt配列
let prices = [2000, 19800, 548000]

// 変換
var mapped = [String]()

for value in prices {
    mapped.append(formatter.string(from: NSNumber(value: value))!)
}

print(mapped)

["¥2,000", "¥19,800", "¥548,000"]

func compactMap<ElementOfResult>(_ transform: (Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]

// String配列
let array = ["1", "2", "3", "ダー！"]

// compactMapで変換
let compactMapped = array.compactMap { value in
    Int(value)
}

print("compactMapped: \(compactMapped)")

// 比較のため、mapでも変換してみる
let mapped = array.map { value in
    Int(value)
}

print("mapped: \(mapped)")

compactMapped: [1, 2, 3]
mapped: [Optional(1), Optional(2), Optional(3), nil]

let array = ["1", "2", "3", "ダー！"]

var compactMapped = [Int]()

for value in array {
    if let result = Int(value) {
        compactMapped.append(result)
    }
}

print("compactMapped: \(compactMapped)")

// 型もバラバラ、nilも混在なAny?配列
let array: [Any?] = [1, nil, 2, 3, nil, "ダー！"]

// compactMapで要素をそのまま返してみる
let compactMapped = array.compactMap { value in
    value
}

print("compactMap: \(compactMapped)")

compactMap: [1, 2, 3, "ダー！"]

func flatMap<SegmentOfResult>(_ transform: (Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence

// カンマ区切りのStringが入った配列
let array = ["a", "b,c,d", "e,f"]

// 最初にmapで変換してみる
let mapped = array.map { value in
    // 文字列をカンマで分割
    value.components(separatedBy: ",")
}

print("map: \(mapped)")

// 次に本命のflatMapで変換
let flatMapped = array.flatMap { value in
    value.components(separatedBy: ",")
}

print("flatMap: \(flatMapped)")

map: [["a"], ["b", "c", "d"], ["e", "f"]]
flatMap: ["a", "b", "c", "d", "e", "f"]

let array = ["a", "b,c,d", "e,f"]

var flatMapped = [String]()

// 変換
for value in array {
    let subarray = value.components(separatedBy: ",")

    for subvalue in subarray {
        flatMapped.append(subvalue)
    }
}

print("flatMap: \(flatMapped)")

// Intの2次元配列
let array = [[1,2,3], [4,5], [6]]

// flatMapでそのまま返す
let flatMapped = array.flatMap { value in
    value
}

print("flatMap: \(flatMapped)")

// joinedでも結果は一緒（でもArrayへの変換が必要）
let joined = Array(array.joined())

print("joined: \(joined)")

flatMap: [1, 2, 3, 4, 5, 6]
joined: [1, 2, 3, 4, 5, 6]

// 初期値を渡すバージョン
func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Element) throws -> Result) rethrows -> Result

// 変数を渡すバージョン
func reduce<Result>(into initialResult: Result, _ updateAccumulatingResult: (inout Result, Element) throws -> ()) rethrows -> Result

// Intの配列
let array = [1, 2, 3, 4, 5]

// 初期値0に対し、各要素を足し合わせていく
let reduced1 = array.reduce(0) { (result, value) in
    result + value
}

print("reduced1: \(reduced1)")

// 合計だけなら第2引数に「+」だけ書いてもOK
// これはIntで定義されているstaticな関数↓を使用している
//   public static func + (lhs: Int, rhs: Int) -> Int
let reduced2 = array.reduce(0, +)

print("reduced2: \(reduced2)")

reduced1: 15
reduced2: 15

let array = [1, 2, 3, 4, 5]

var result = 0

for value in array {
    result = result + value
}

let reduced1 = result

print("reduced1: \(reduced1)")

// 年齢を表すInt配列
let ages = [27, 18, 41, 7, 65, 33]

// [String: [Int]]のDictionary変数を渡して、20以上と未満に分けていく
let reduced = ages.reduce(into: [String: [Int]]()) { (result, age) in
    if age < 20 {
        result["under", default: []].append(age)
    }
    else {
        result["over", default: []].append(age)
    }
}

print("reduced: \(reduced)")

reduced: ["over": [27, 41, 65, 33], "under": [18, 7]]

let ages = [27, 18, 41, 7, 65, 33]

var result = [String: [Int]]()

for age in ages {
    if age < 20 {
        result["under", default: []].append(age)
    }
    else {
        result["over", default: []].append(age)
    }
}

let reduced = result

print("reduced: \(reduced)")

// エリア名とポイント（要素数は同じ前提）
let areas = ["Tokyo", "Osaka", "Hokkaido"]
let points = [111, 222, 333]

var result = [String]()

for (index, value) in areas.enumerated() {
    result.append("\(value): \(points[index])")
}

print(result)

// エリア名とポイント（要素数は同じ前提）
let areas = ["Tokyo", "Osaka", "Hokkaido"]
let points = [111, 222, 333]

let result = areas.enumerated().map { value in
    "\(value.element): \(points[value.offset])"
}

print(result)

["Tokyo: 111", "Osaka: 222", "Hokkaido: 333"]

// 身長を表すDoubleの配列に対し
let heights = [168.1, 154.2, 177.6, 149.0, 162.3]

// NSExpressionを使って平均を出してみる
let average = NSExpression(format: "average(%@)", heights)

if let result = average.expressionValue(with: nil, context: nil) as? Double {
    print("average: \(result)")
}
else {
    print("failed")
}

// 中央値も出してみる
let median = NSExpression(format: "median(%@)", heights)

if let result = median.expressionValue(with: nil, context: nil) as? Double {
    print("median: \(result)")
}
else {
    print("failed")
}

average: 162.24
median: 162.3