Swift Array - 检查索引是否存在

Question

在Swift中,有没有办法检查数组中是否存在索引而没有抛出致命错误？

我希望我能做到这样的事情:

let arr: [String] = ["foo", "bar"]
let str: String? = arr[1]
if let str2 = arr[2] as String? {
    // this wouldn't run
    println(str2)
} else {
    // this would be run
}

但我明白了

致命错误:数组索引超出范围

Answer 1

Swift的优雅方式:

let isIndexValid = array.indices.contains(index)

Answer 2

Swift 3&4扩展:

extension Collection {

    subscript(optional i: Index) -> Iterator.Element? {
        return self.indices.contains(i) ? self[i] : nil
    }

}

使用此选项可以在向索引添加关键字optional时获得可选值,这意味着即使索引超出范围,程序也不会崩溃.在你的例子中:

let arr = ["foo", "bar"]
let str1 = arr[optional: 1] // --> str1 is now Optional("bar")
if let str2 = arr[optional: 2] {
    print(str2) // --> this still wouldn't run
} else {
    print("No string found at that index") // --> this would be printed
}

Answer 3

只需检查索引是否小于数组大小:

if 2 < arr.count {
    ...
} else {
    ...
}

Answer 4

最好的办法。

  let reqIndex = array.indices.contains(index)
  print(reqIndex)

Answer 5

添加一些延长糖:

extension Collection {
  subscript(safe index: Index) -> Iterator.Element? {
    guard indices.contains(index) else { return nil }
    return self[index]
  }
}

if let item = ["a","b","c","d"][safe:3] {print(item)}//Output: "c"
//or with guard:
guard let anotherItem = ["a","b","c","d"][safe:3] else {return}
print(anotherItem)//"c"

if let与数组一起进行样式编码时增强了可读性

Answer 6

你可以用更安全的方式重写这个来检查数组的大小,并使用三元条件:

if let str2 = (arr.count > 2 ? arr[2] : nil) as String?

Answer 7

Swift 4扩展名：

对我来说，我更喜欢方法。

// MARK: - Extension Collection

extension Collection {

    /// Get at index object
    ///
    /// - Parameter index: Index of object
    /// - Returns: Element at index or nil
    func get(at index: Index) -> Iterator.Element? {
        return self.indices.contains(index) ? self[index] : nil
    }
}

感谢@Benno Kress