swift - How to enumerate a slice using the original indices? -

if want enumerate array (say map() function need use index of element value), use enumerate() function. e.g.:

import foundation  let array: [double] = [1, 2, 3, 4]  let powersarray = array.enumerate().map() {     pow($0.element, double($0.index)) }  print("array == \(array)") print("powersarray == \(powersarray)")  // array == [1.0, 2.0, 3.0, 4.0] // powersarray == [1.0, 2.0, 9.0, 64.0] <- expected 

now, if want use sub-sequence array, use slice, , allow me use same indices use in original array (which want in case if use subscript accessor in for loop). e.g.:

let range = 1..<(array.count - 1) let slice = array[range] var powersslice = [double]()  index in slice.indices {     powersslice.append(pow(slice[index], double(index))) }  print("powersslice == \(powersslice)")  // powersslice == [2.0, 9.0] <- expected    

however, should try use enumerate().map() approach did original array, totally different behaviour. instead of slice's range of indices new, 0-based range:

let powerssliceenumerate = slice.enumerate().map() {     pow($0.element, double($0.index)) }  print("powerssliceenumerate == \(powerssliceenumerate)")  // powerssliceenumerate == [1.0, 3.0] <- not expected 

question whether there decent way (i.e. without manual adjustments using offsets, or something) enumerate slice using own indices , not auto-generated 0-based ones?

enumerate() returns sequence of (n, elem) pairs, ns consecutive ints starting @ zero. makes sense because protocol extension method sequencetype, , arbitrary sequence not have index associated elements.

you expected result with

let powersslice = slice.indices.map { pow(slice[$0], double($0)) } 

or

let powersslice = zip(slice.indices, slice).map { pow($1, double($0)) } 

the latter approach generalized protocol extension method arbitrary collections:

extension collectiontype {     func indexenumerate() -> anysequence<(index: index, element: generator.element)> {         return anysequence(zip(indices, self))     } } 

this returns sequence of (index, elem) pairs index index of collection , elem corresponding element. anysequence used "hide" specific type zip2sequence<rangegenerator<self.index>, self> returned zip() caller.

example:

let powerssliceenumerate = slice.indexenumerate().map() { pow($0.element, double($0.index)) } print("powerssliceenumerate == \(powerssliceenumerate)") // powerssliceenumerate == [2.0, 9.0] 

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update swift 3:

extension collection {     func indexenumerate() -> anysequence<(indices.iterator.element, iterator.element)> {         return anysequence(zip(indices, self))     } }