Filter elements in a list by length - Ocaml

Question

I have the following list:

["A";"AA";"ABC";"BCD";"B";"C"]

I am randomly extracting an element from the list. But the element I extract should be of size 3 only not lesser than 3.

I am trying to do this as follows:

let randomnum = (Random.int(List.length (list)));;
let rec code c =
    if (String.length c) = 3 then c
    else (code ((List.nth (list) (randomnum)))) ;;
print_string (code ( (List.nth (list) (randomnum)))) ;;

This works fine if randomly a string of length 3 is picked out from the list.

But the program does not terminate if a string of length < 3 is picked up. I am trying to do a recursive call so that new code keeps getting picked up till we get one of length = 3.

I am unable to figure out why this is does not terminate. Nothing gets output by the print statement.

Answer 1

What you probably want to write is

 let rec code list =
   let n = Random.int (List.length list) in
   let s = List.nth list in
   if String.length s < 3 then code list else s

Note that, depending on the size of the list and the number of strings of size greater than 3, you might want to work directly on a list with only strings greater than 3:

let code list =
  let list = List.filter (fun s -> String.length s >= 3) list in
  match list with
  | [] -> raise Not_found
  | _  -> List.nth list (Random.int (List.length list)) 

This second function is better, as it always terminate, especially when there are no strings greater than 3.

Answer 2

You only pick a random number once. Say you pick 5. You just keep recursing with 5 over and over and over. You need to get a new random number.