seq.reduce(into:_:)

📘 手冊

• Swift ⟩ Sequence ⟩

  • .reduce()

  • .reduce(into:_:)

📗 參考

1. Advanced Swift, p.37

👥 相關

1. seq.reduce(_:_:)

2. reduce() vs. reduce(into:)

3. seq.stateMap(_:_:)

// declaration
func reduce<Result>(
    into initialResult: Result, 
    _ updateAccumulatingResult: (
        _ partialResult: inout Result,    // ⭐️ inout parameter
        Self.Element    
    ) throws -> ()                        // ⭐️ no return value
) rethrows -> Result

使用此方法時，一定要在 updateAccumulatingResult 這個 closure 的 function body 裡面更新 partialResult 這個 inout 參數，reduce(into:_:) 就是透過更新此參數，最後再將此參數的累積更新總結果 (accumulating result) 當成 return value 傳回。這也是為什麼這個 closure 稱為 updateAccumulatingResult 了，因為你必須在它的 body 裡面 "update" accumulating result (partialResult)。👉 比較：seq.reduce(_:_:)

註：我認為 Apple 在這個方法的參數名稱上，有「命名不一致」的問題。既然 closure 稱為 updateAccumulatingResult，那麼 partialResult 這個 inout 參數就應該命名為 accumulatingResult 才比較一致。

This method is preferred over reduce(_:_:) for efficiency when the result is a copy-on-write type, for example an Array or a Dictionary.

👉 .reduce(into:_:)

1

👉 stackoverflow

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

// ⭐️ filter adjacent equal entries: [1, 2, 3, 4, 5, 4, 3]
//                                  ╭──1──╮    ╭─2──╮
let filtered = numbers.reduce(into: [Int]()) { result, n in
    if n != result.last { result.append(n) }
//                        ╰───── 3 ──────╯
}
// ⭐️ 1. intial result: empty array
// ⭐️ 2. accumulating result (inout parameter)
// ⭐️ 3. update accumulating result in closure body

2

let letters = "abracadabra"

// ⭐️ letter frequencies of a string
// ["a": 5, "b": 2, "r": 2, "c": 1, "d": 1]
//                   ╭1╮    ╭─2──╮
letters.reduce(into: [:]) { counts, letter in
    counts[letter, default: 0] += 1
//  ╰───────────── 3 ─────────────╯
}
// ⭐️ 1. intial result: empty dictionary
// ⭐️ 2. accumulating result (inout parameter)
// ⭐️ 3. update accumulating result in closure body