Swift：indexOf第二次出现

let numbers = [1,3,4,5,9,1]

要找到前5个,请使用：

numbers.indexOf(5)

我如何找到第二次出现？

您可以在剩余的数组切片上执行另一个元素索引搜索,如下所示：

编辑/更新：Xcode 9•Swift 4

extension Collection where Element: Equatable {
    func secondIndex(of element: Element) -> Index? {
        guard let index = index(of: element) else { return nil }
        return self[self.index(after: index)...].index(of: element)
    }
    func indexes(of element: Element) -> [Index] {
        var indexes: [Index] = []
        var position = self.startIndex
        while let index = self[position...].index(of: element) {
            indexes.append(index)
            position = self.index(after: index)
        }
        return indexes
    }
}

测试：

let numbers = [1,1]
numbers.secondIndex(of: 5)         // 4
numbers.indexes(of: 5)             // [3,4]