create a histogram OCaml -

my task create histogram output number of times element in list.

input:[2;2;2;3;4;4;1] output[(2, 3); (2, 2); (2, 1); (3, 1); (4, 2); (4, 1); (1, 1)]   expected output : [(2, 3); (3, 1); (4, 2); (1, 1)]  code:  let rec count ls = match ls   |[]              -> 0   |x::xs  when x=a -> 1 + count xs   |_::xs           -> count xs  let rec count = function   |[]              -> 0   |x::xs  when x=a -> 1 + count xs   |_::xs           -> count xs  let rec histo l = match l |[] -> [] |x :: xs ->  [(x, count x l)] @ histo xs ;; 

what have done wrong?

the issue xs contains potentially elements equal x. see in ouput : (2,3) means there 3 times 2 in list; xs equals [2;2;3;4;4;1]... , on.

also (not impacting conclusion): have 2 definitions of count, identical.

to implement histogram, use hashtbl :

let h = hashtbl.create 1000;;     list.iter (fun x -> let c = try hashtbl.find h x not_found -> 0 in hashtbl.replace h x (c+1)) your_list;; hashtbl.fold (fun x y acc ->  (x,y)::acc) h [];;