Adding two list in OCaml

September 28, 2022

let rec add_lists (xs : float list) (ys : float list): float list option =
  match xs ys with
  | ([],[]) -> Some[]
  | (x::xs', []) -> None
  | ([],y::ys') -> None 
  | hd::tl ->
      (match xs ys with 
       |[] -> None
       |hd2 :: tl2 ->(hd+hd2):: (add_lists tl tl2) 
      )

I wanted to add 2 list together. To add the elements of two lists together,
if they are different lengths return None.

Examples:
add_lists [1.; 2.] [3.; 4.] = Some([4.; 6.])
add_lists [] [1.2] = None

>Solution :

let add_lists (xs : float list) (ys : float list): float list option =
  let rec aux acc first second =
    match first, second with
    | [], [] -> Some(acc)
    | [], hd::tl -> None
    | hd::tl, [] -> None
    | hd::tl, hd'::tl' -> aux ((hd +. hd')::acc) (List.tl first) (List.tl second)
  in
  let res = aux [] xs ys in
  match res with
  | Some(x) -> print_string "Returned"; Some(List.rev x)
  | None -> print_string "Lists not same length"; None

Works with the good output.

Edit: Tail Recursive so you get good grades 🙂