当你只需要从一个数组中获取值，并且你不介意一个小的性能损失(即如果你的集合不是很大)，有一个基于dictionary的替代方案，它不涉及(对我来说太通用了)集合扩展:

// Assuming you have a collection named array:
let safeArray = Dictionary(uniqueKeysWithValues: zip(0..., array))
let value = safeArray[index] ?? defaultValue;

extension Array {
  subscript (safe index: UInt) -> Element? {
    return Int(index) < count ? self[Int(index)] : nil
  }
}

使用上述提到的扩展返回nil，如果任何时候索引超出界限。

let fruits = ["apple","banana"]
print("result-\(fruits[safe : 2])")

结果-无

我发现安全数组获取，设置，插入，删除非常有用。我更倾向于记录并忽略错误，因为所有其他错误很快就会变得难以管理。完整代码如下

/**
 Safe array get, set, insert and delete.
 All action that would cause an error are ignored.
 */
extension Array {

    /**
     Removes element at index.
     Action that would cause an error are ignored.
     */
    mutating func remove(safeAt index: Index) {
        guard index >= 0 && index < count else {
            print("Index out of bounds while deleting item at index \(index) in \(self). This action is ignored.")
            return
        }

        remove(at: index)
    }

    /**
     Inserts element at index.
     Action that would cause an error are ignored.
     */
    mutating func insert(_ element: Element, safeAt index: Index) {
        guard index >= 0 && index <= count else {
            print("Index out of bounds while inserting item at index \(index) in \(self). This action is ignored")
            return
        }

        insert(element, at: index)
    }

    /**
     Safe get set subscript.
     Action that would cause an error are ignored.
     */
    subscript (safe index: Index) -> Element? {
        get {
            return indices.contains(index) ? self[index] : nil
        }
        set {
            remove(safeAt: index)

            if let element = newValue {
                insert(element, safeAt: index)
            }
        }
    }
}

测试

import XCTest

class SafeArrayTest: XCTestCase {
    func testRemove_Successful() {
        var array = [1, 2, 3]

        array.remove(safeAt: 1)

        XCTAssert(array == [1, 3])
    }

    func testRemove_Failure() {
        var array = [1, 2, 3]

        array.remove(safeAt: 3)

        XCTAssert(array == [1, 2, 3])
    }

    func testInsert_Successful() {
        var array = [1, 2, 3]

        array.insert(4, safeAt: 1)

        XCTAssert(array == [1, 4, 2, 3])
    }

    func testInsert_Successful_AtEnd() {
        var array = [1, 2, 3]

        array.insert(4, safeAt: 3)

        XCTAssert(array == [1, 2, 3, 4])
    }

    func testInsert_Failure() {
        var array = [1, 2, 3]

        array.insert(4, safeAt: 5)

        XCTAssert(array == [1, 2, 3])
    }

    func testGet_Successful() {
        var array = [1, 2, 3]

        let element = array[safe: 1]

        XCTAssert(element == 2)
    }

    func testGet_Failure() {
        var array = [1, 2, 3]

        let element = array[safe: 4]

        XCTAssert(element == nil)
    }

    func testSet_Successful() {
        var array = [1, 2, 3]

        array[safe: 1] = 4

        XCTAssert(array == [1, 4, 3])
    }

    func testSet_Successful_AtEnd() {
        var array = [1, 2, 3]

        array[safe: 3] = 4

        XCTAssert(array == [1, 2, 3, 4])
    }

    func testSet_Failure() {
        var array = [1, 2, 3]

        array[safe: 4] = 4

        XCTAssert(array == [1, 2, 3])
    }
}

我认为这不是一个好主意。似乎更可取的做法是构建稳定的代码，不导致试图应用越界索引。

请考虑让这样的错误通过返回nil无声地失败(正如上面的代码所建议的那样)，很容易产生更复杂、更难以处理的错误。

你可以用类似的方式重写，用你自己的方式写下标。唯一的缺点是现有的代码将不兼容。我认为找到一个钩子来覆盖通用的x[I](也没有C中的文本预处理器)将是具有挑战性的。

我能想到的最接近的就是

// compile error:
if theIndex < str.count && let existing = str[theIndex]

编辑:这实际上是可行的。一行程序! !

func ifInBounds(array: [AnyObject], idx: Int) -> AnyObject? {
    return idx < array.count ? array[idx] : nil
}

if let x: AnyObject = ifInBounds(swiftarray, 3) {
    println(x)
}
else {
    println("Out of bounds")
}

基于Nikita Kukushkin的回答，有时候你需要安全地赋值给数组下标，也需要从它们中读取。

myArray[safe: badIndex] = newValue

因此，这里是对Nikita的答案(Swift 3.2)的更新，通过添加safe:参数名，也允许安全写入可变数组索引。

extension Collection {
    /// Returns the element at the specified index if it is within bounds, otherwise nil.
    subscript(safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

extension MutableCollection {
    subscript(safe index: Index) -> Element? {
        get {
            return indices.contains(index) ? self[index] : nil
        }

        set(newValue) {
            if let newValue = newValue, indices.contains(index) {
                self[index] = newValue
            }
        }
    }
}

Alex的回答对这个问题有很好的建议和解决方案，然而，我碰巧发现了一个更好的实现这个功能的方法:

extension Collection {
    /// Returns the element at the specified index if it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

例子

let array = [1, 2, 3]

for index in -20...20 {
    if let item = array[safe: index] {
        print(item)
    }
}