任何在swift中停止块文字的方法

Question

如下所示,我想通过类似的东西来阻止字面意思 break

var numbers = [1,2,3,4]
numbers.forEach {
    if $0 % 2 == 0 {
        break
    }
}

Answer 1

forEach不是循环(它是传递给循环的块,而不是循环本身),或者更确切地说,forEach不是Swift 控制流的一部分.这就是为什么你不能使用break或continue.

只需使用for-in循环.

示例:

var numbers = [ 1,2,3,4]

func firstEvenNumber(inArray array: [Int]) -> Int? {

    var firstMatch : Int?

    for number in numbers {
        if (number % 2) == 0 {
            firstMatch = number
            break
        }
    }
    return firstMatch
}

firstEvenNumber(inArray: numbers) // 2

你可以return在forEach闭包内部使用,但它不会破坏循环,它只返回当前传递中的块.

var numbers = [ 1,2,3,4]

func evenNumbers(inArray: [Int]) -> [Int] {

    var matches : [Int] = []

    numbers.forEach{
        if $0 % 2 == 0 {
            matches.append($0)

            // early return on even numbers
            // does not stop forEach
            // comparable to a continue in a for in loop
            return
        }

        // only reached on uneven numbers
    }
    return matches
}

evenNumbers(numbers) // [2,4]

Answer 2

由于您想在第一场比赛中停止，您应该使用first而不是forEach：

numbers.first { (number) -> Bool in
    return number % 2 == 0
}

对结果做一些事情（列表中的第一个偶数）：

if let firstEvenNumber = numbers.first(where: { $0 % 2 == 0 }) {
    print("The first even number on the list is \(firstEvenNumber)")
}

奖励：使用以下方法创建所有偶数的列表filter：

let allEvenNumbers = numbers.filter { $0 % 2 == 0 }

Answer 3

该文件已经解释过.

    /// - Note: You cannot use the `break` or `continue` statement to exit the
    ///   current call of the `body` closure or skip subsequent calls.
    /// - Note: Using the `return` statement in the `body` closure will only
    ///   exit from the current call to `body`, not any outer scope, and won't
    ///   skip subsequent calls.

但是你想尝试一下.

extension CollectionType {

    /// Use as
    ///
    ///      let num = [1,2,3,4,5]
    ///      num.forEach{
    ///        if $1 > 3 { $2 = true }
    ///        print($0,$1)
    ///      }
    func forEach(body: ((Self.Generator.Element, Int, inout Bool) -> Void)) {
        var stop = false
        let enumerate = self.enumerate()
        for (index,value) in enumerate {
            if stop { break }
            body(value,index,&stop)
        }
    }

    /// Use as
    ///
    ///      let num = [1,2,3,4,5]
    ///      num.forEach{
    ///        if $0 > 3 { $1 = true }
    ///        print($0)
    ///      }
    func forEach(body: ((Self.Generator.Element, inout Bool) -> Void)) {
        var stop = false
        for value in self {
            if stop { break }
            body(value,&stop)
        }
    }
}