You can use enumerate to convert a sequence (Array, String, etc.) to a sequence of tuples with the index and element paired together. That is:

EnumerateWhenMap.swift

let numbers = [7, 8, 9, 10]
let indexAndNum: [String] = map(enumerate(numbers)) { (index, element) in
        return "\(index): \(element)"
    }
println(indexAndNum)
// [0: 7, 1: 8, 2: 9, 3: 10]