euler_1.ml

(* If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
*
* Find the sum of all the multiples of 3 or 5 below 1000. 
*) 

let rec find_multiples counter accum =
  let x = 3 * counter in
  let y = 5 * counter in
  if ((List.exists (fun a -> x == a) accum)) then 
    match (y < 1000) with 
      | true -> find_multiples (counter+1) ( y :: accum)
      | false -> find_multiples (counter+1) accum 
  else
    match (x < 1000, y < 1000) with 
      |  (true, true) -> find_multiples (counter+1) (x :: y :: accum) 
      |  (true,false) -> find_multiples (counter+1) (x :: accum) 
      |  (false,true) -> find_multiples (counter+1) (y :: accum)
      |  (false,false) -> accum 
;;

let result = find_multiples 1 [];;
List.map print_int result ;; 
Printf.printf "\n";; 
print_int (List.fold_left (+) (0) result );;