arrays - Swift - 如何获取数组过滤项的索引

Question

let items: [String] = ["A", "B", "A", "C", "A", "D"]

items.whatFunction("A") // -> [0, 2, 4]
items.whatFunction("B") // -> [1]

Swift 3 是否支持像 whatFunction(_: Element) 这样的函数？

如果不是，最有效的逻辑是什么？

Answer 1

可以直接过滤数组的indices，避免额外的mapping。

let items = ["A", "B", "A", "C", "A", "D"]
let filteredIndices = items.indices.filter {items[$0] == "A"}

或作为数组扩展:

extension Array where Element: Equatable {

    func whatFunction(_ value :  Element) -> [Int] {
        return self.indices.filter {self[$0] == value}
    }

}

items.whatFunction("A") // -> [0, 2, 4]
items.whatFunction("B") // -> [1]

或者更通用

extension Collection where Element: Equatable {

    func whatFunction(_ value :  Element) -> [Index] {
        return self.indices.filter {self[$0] == value}
    }

}