How to reshape a list in OCaml-CodePudding

So consider getting a list of [1; 2; 3; 4; 5; 6; 7; 8; 9] and reshape it into [[1; 2; 3]; [4; 5; 6]; [7; 8; 9]]. How would you do that in OCaml? I want a simple function or something from the standard library.

CodePudding user response：

Turns out, it can be easily done with 3 lines of code, considering that the length of the list is divisible by 3.

let rec re_shape = function
| x :: xs :: xz :: xt -> [x; xs; xz] :: re_shape xt
| _ -> []

How this works is that for each iteration it cons a list of 3 to the rest of the function, till it reaches the end. The last line is added for safety.

CodePudding user response：

As you have shown an effort to solve this, for your consideration, see a strategy below for generalizing this to allow for any length.

The partition function will allow us to get the first nelements from the list and the remainder, raisingInvalid_argumentif there aren'tn` elements in the list.

The chunks function applies this recursively to the remainder to build a list of lists.

let partition n lst =
  let rec partition' n (first, rest) =
    match n, rest with  
    | 0, _ -> (List.rev first, rest)
    | _, [] -> raise (Invalid_argument "List not long enough")
    | _, x::xs -> partition' (n-1) (x :: first, xs)
  in
  partition' n ([], lst)
  
let rec chunks n lst =
  match partition n lst with  
  | first, [] -> [first]
  | first, rest -> first :: chunks n rest
  | exception (Invalid_argument _) ->
    raise (Invalid_argument (Format.sprintf "List length not evenly divisible by %d" n))

This second function is not tail-recursive, though that can readily be addressed in OCaml 4.14 and later with:

let[@tail_mod_cons] rec chunks n lst =
  ...