seq.reduce(_:_:)

📘 手冊

1. Swift ⟩ Sequence ⟩

   • .reduce()

   • .reduce(into:_:)

📗 參考

1. Advanced Swift, p.37

👥 相關

1. seq.reduce(into:_:)

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

3. seq.stateMap(_:_:)

// declaration
func reduce<Result>(
    _ initialResult    : Result, 
    // ⭐️ combine function (with return value)
    _ nextPartialResult: (Result, Element) throws -> Result
) rethrows -> Result

• 只要設定一個初始值 (initialResult)，此方法就會使用 nextPartialResult 這個 closure 來進行累算，並將最後的累算結果回傳。👉 比較：seq.reduce(into:_:)

on every execution of the combine function, a brand-new array is being created by appending the transformed or included element to the previous one. This means that both these implementations are O(n²), not O(n) ...

👉 Advanced Swift, p.37

.reduce(into:_:) 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

let numbers = [1, 2, 3, 4]
let sum = numbers.reduce(0, +)        // 10

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