Swift: second occurrence with indexOf

Question

let numbers = [1,3,4,5,5,9,0,1]

To find the first 5, use:

numbers.indexOf(5)

How do I find the secon

Answer 1

Here's a general use extension of Array that will work for finding the nth element of a kind in any array:

extension Array where Element: Equatable {
    // returns nil if there is no nth occurence
    // or the index of the nth occurence if there is
    func findNthIndexOf(n: Int, thing: Element) -> Int? {
        guard n > 0 else { return nil }
        var count = 0
        for (index, item) in enumerate() where item == thing {
            count += 1
            if count == n {
                return index
            }
        }
        return nil
    }
}

let numbers = [1,3,4,5,5,9,0]

numbers.findNthIndexOf(2, thing: 5) // returns 4