Splitting a list of items into two lists of odd and even indexed items

Question

I would like to make a function that accepts a list and returns two lists: the first contains every odd item, and the second contains every even item.

For example, g

Answer 1

Here is a straightforward non-recursive solution:

let splitList ll =
    ll
    |> List.mapi (fun i x -> (i % 2 = 0, x))
    |> List.partition fst
    |> fun (odd,even) -> [List.map snd odd, List.map snd even];;

val splitList : 'a list -> 'a list list

Being applied to your sample it yields exactly what you wanted:

splitList [1;2;4;6;7;9];;

val it : int list list = [[1; 4; 7]; [2; 6; 9]]

Answer 2

My 2¢, in OCaml, since there still is a bounty open.

Maybe you could give us a hint what you want. Elegance? FP? Tail recursion? Performance?

Edit:

I removed the longer solution. For List.partition to work, the predicate was missing. Here it is:

let so_split lst = 
  let flag = ref false in
  List.partition (fun e -> flag := not !flag; !flag) lst

Improvements any? Testing the solution:

# so_split [1;2;4;6;7;9];;
- : int list * int list = ([1; 4; 7], [2; 6; 9])