slicenumerate.swift

let array = "abcdefghijklmnop".characters.map({String($0)})
let range = 5..<array.count
extension ArraySlice {
    func slicenumerate() -> Zip2Sequence<Range<Int>, ArraySlice<Element>> {
        // slice.indices gleefully stolen from Mike Ash, originally startIndex..<endIndex
        return zip(self.indices, self)
    }
    func forEachWithIndex(@noescape closure: (Int, Generator.Element) -> Void) {
        // thanks, https://twitter.com/publicfarley/status/630091086971699202
        // (via http://twitter.com/oisdk/status/638809052332617728)
        self.slicenumerate().forEach{ closure($0.0, $0.1) }
    }
}

let slice = array[range]
for (index, value) in slice.slicenumerate() {
    print(index, value)
}

slice.forEachWithIndex {
    index, value in
    print(index, value)
}

// Or just do this:
// https://gist.github.com/natecook1000/b6be8929451bb6f35ad4
// Thanks, Nate Cook
extension CollectionType {
    /// Return a lazy SequenceType containing pairs *(i, x)*,
    /// where *i*s are the sequential indices and *x*s are the elements of `base`.
    func enumerateWithIndex() -> AnySequence<(Index, Generator.Element)> {
        return AnySequence(zip(indices, self))
    }
}