快速表达动态范围的循环

使用辅助函数（最初在将使用除法的C样式for循环转换为Swift3时定义

public func sequence<T>(first: T, while condition: @escaping (T)-> Bool, next: @escaping (T) -> T) -> UnfoldSequence<T, T> {    let nextState = { (state: inout T) -> T? in        // Return `nil` if condition is no longer satisfied:        guard condition(state) else { return nil }        // Update current value _after_ returning from this call:        defer { state = next(state) }        // Return current value:        return state    }    return sequence(state: first, next: nextState)}

您可以将循环写为

let num = 1000for i in sequence(first: 5, while: { num/$0 > 0 }, next: { $0 * 5 }) {    print(i)}

一个更简单的解决方案是while循环：

var i = 5while num/i > 0 {    print(i)    i *= 5}

但是第一种解决方案的优点是，循环变量的范围仅限于循环主体，并且循环变量是常量。

Swift 3.1 将提供一个

prefix(while:)

let num = 1000for i in sequence(first: 5, next: { $0 * 5 }).prefix(while: { num/$0 > 0 }) {    print(i)}

以上所有解决方案均与给定的C循环“等效”。 但是， 如果它们

num

Int.max

$0 * 5

$0 * 5

实际上，这使循环更简单–至少如果我们假设

num >= 5

for i in sequence(first: 5, next: { $0 <= num/5  ? $0 * 5 : nil }) {    print(i)}