如果我在Swift中有一个数组,并尝试访问一个越界的索引,有一个不足为奇的运行时错误:

var str = ["Apple", "Banana", "Coconut"]

str[0] // "Apple"
str[3] // EXC_BAD_INSTRUCTION

然而,我本以为有了Swift带来的所有可选的链接和安全性,做这样的事情是微不足道的:

let theIndex = 3
if let nonexistent = str[theIndex] { // Bounds check + Lookup
    print(nonexistent)
    ...do other things with nonexistent...
}

而不是:

let theIndex = 3
if (theIndex < str.count) {         // Bounds check
    let nonexistent = str[theIndex] // Lookup
    print(nonexistent)   
    ...do other things with nonexistent... 
}

但事实并非如此——我必须使用ol' if语句来检查并确保索引小于str.count。

我尝试添加我自己的下标()实现,但我不确定如何将调用传递给原始实现,或者访问项目(基于索引)而不使用下标符号:

extension Array {
    subscript(var index: Int) -> AnyObject? {
        if index >= self.count {
            NSLog("Womp!")
            return nil
        }
        return ... // What?
    }
}

当前回答

适用于Swift 2

尽管这个问题已经被回答过很多次了,但我想给出一个更符合Swift编程时尚走向的答案,用Crusty的话来说就是:“先考虑协议”。

• What do we want to do? - Get an Element of an Array given an Index only when it's safe, and nil otherwise • What should this functionality base it's implementation on? - Array subscripting • Where does it get this feature from? - Its definition of struct Array in the Swift module has it • Nothing more generic/abstract? - It adopts protocol CollectionType which ensures it as well • Nothing more generic/abstract? - It adopts protocol Indexable as well... • Yup, sounds like the best we can do. Can we then extend it to have this feature we want? - But we have very limited types (no Int) and properties (no count) to work with now! • It will be enough. Swift's stdlib is done pretty well ;)

extension Indexable {
    public subscript(safe safeIndex: Index) -> _Element? {
        return safeIndex.distanceTo(endIndex) > 0 ? self[safeIndex] : nil
    }
}

不正确,但它给出了一个概念

其他回答

extension Array {
    subscript (safe index: Index) -> Element? {
        0 <= index && index < count ? self[index] : nil
    }
}

O(1)的性能 类型安全 正确处理[MyType?)(返回MyType??,可以在两个级别上展开) 不会导致set的问题吗 简洁的代码

这是我为你做的一些测试:

let itms: [Int?] = [0, nil]
let a = itms[safe: 0] // 0 : Int??
a ?? 5 // 0 : Int?
let b = itms[safe: 1] // nil : Int??
b ?? 5 // nil : Int? (`b` contains a value and that value is `nil`)
let c = itms[safe: 2] // nil : Int??
c ?? 5 // 5 : Int?

因为数组可以存储nil值,如果数组[index]调用越界,返回nil是没有意义的。 因为我们不知道用户想如何处理越界问题,所以使用自定义操作符没有意义。 相反,使用传统的控制流来展开对象并确保类型安全。

if let index = array.checkIndexForSafety(index:Int) let item = array[safeIndex: index] if let index = array.checkIndexForSafety(index:Int) array[safeIndex: safeIndex] = myObject

extension Array {

    @warn_unused_result public func checkIndexForSafety(index: Int) -> SafeIndex? {

        if indices.contains(index) {

            // wrap index number in object, so can ensure type safety
            return SafeIndex(indexNumber: index)

        } else {
            return nil
        }
    }

    subscript(index:SafeIndex) -> Element {

        get {
            return self[index.indexNumber]
        }

        set {
            self[index.indexNumber] = newValue
        }
    }

    // second version of same subscript, but with different method signature, allowing user to highlight using safe index
    subscript(safeIndex index:SafeIndex) -> Element {

        get {
            return self[index.indexNumber]
        }

        set {
            self[index.indexNumber] = newValue
        }
    }

}

public class SafeIndex {

    var indexNumber:Int

    init(indexNumber:Int){
        self.indexNumber = indexNumber
    }
}

我知道这是个老问题了。我现在用的是Swift5.1, OP是swift1还是swift2 ?

今天我需要这样的东西,但是我不想仅仅为一个地方添加一个完整的扩展,而想要一些更实用的扩展(更线程安全?)我也不需要防止负标,只需要保护那些可能超过数组末尾的负标:

let fruit = ["Apple", "Banana", "Coconut"]

let a = fruit.dropFirst(2).first // -> "Coconut"
let b = fruit.dropFirst(0).first // -> "Apple"
let c = fruit.dropFirst(10).first // -> nil

对于那些争论具有nil的序列的人,对于空集合返回nil的第一个和最后一个属性该怎么办?

我喜欢这个,因为我可以抓住现有的东西,用它来得到我想要的结果。我也知道dropFirst(n)不是一个完整的集合副本,只是一个切片。然后已经存在的第一个行为接管了我。

我对数组做了一个简单的扩展

extension Array where Iterator.Element : AnyObject {
    func iof (_ i : Int ) -> Iterator.Element? {
        if self.count > i {
            return self[i] as Iterator.Element
        }
        else {
            return nil
        }
    }

}

它的设计完美无缺

例子

   if let firstElemntToLoad = roots.iof(0)?.children?.iof(0)?.cNode, 

适用于Swift 2

尽管这个问题已经被回答过很多次了,但我想给出一个更符合Swift编程时尚走向的答案,用Crusty的话来说就是:“先考虑协议”。

• What do we want to do? - Get an Element of an Array given an Index only when it's safe, and nil otherwise • What should this functionality base it's implementation on? - Array subscripting • Where does it get this feature from? - Its definition of struct Array in the Swift module has it • Nothing more generic/abstract? - It adopts protocol CollectionType which ensures it as well • Nothing more generic/abstract? - It adopts protocol Indexable as well... • Yup, sounds like the best we can do. Can we then extend it to have this feature we want? - But we have very limited types (no Int) and properties (no count) to work with now! • It will be enough. Swift's stdlib is done pretty well ;)

extension Indexable {
    public subscript(safe safeIndex: Index) -> _Element? {
        return safeIndex.distanceTo(endIndex) > 0 ? self[safeIndex] : nil
    }
}

不正确,但它给出了一个概念