List reversing in Ocaml

Question

How to reverse even sublists of a list if we assume that we count elements from 0. I want the solution to be \"manually-coded\". I\'ve got a big problem with this task.

Answer 1

For fun, I've done a stuff like this,

let rev_at_even_idx list = 
  let s0, s1 = ([], 0), [] in
  let aux0 (a, i) x = 
    (x, i mod 2) :: a, succ i 
  in
  let aux1 a = function 
    | l, 0 -> List.rev l :: a 
    | l, _ -> l :: a 
  in 
  List.fold_left aux1 s1 
  @@ fst @@ 
  List.fold_left aux0 s0 list
;;

rev_at_even_idx [[1;2;3] ; [2;3] ; [1;2;3] ; [5;6;7]];;  
- : int list list = [[3; 2; 1]; [2; 3]; [3; 2; 1]; [5; 6; 7]]