Question

I written the following code to randomly select items n items from a list and place the selections in a new list
The code is below

let random_extract list n =
    let l =  ((List.length list)- 1) 
    and acc = []  in
    for i = 1 to n do 
        (List.nth list (Random.int l) ) :: acc  (* Line 447 *)
    done;
    acc
;;

When I load the file that contains this code I get the following error

File "all_code.ml", line 447, characters 2-40:
Warning 10: this expression should have type unit.
val random_extract : 'a list -> int -> 'b list = <fun>

Two questions ,
Question 1: what does this warning mean.

Secondly when I run this function I dont get the list expected but an empty List.

Question 2: how do I get the value of acc from inside the for loop

Answer 1

What Jeffrey Scofield said, and also:

let random_extract list n =
let l =  ((List.length list)- 1) 
and acc = ref []  in
for i = 1 to n do 
    acc := (List.nth list (Random.int l) ) :: !acc  (* Line 447 *)
done;
!acc

If you are going to use a for loop for something like that, then acc must be a reference, i.e., a mutable value. In Ocaml and ML in general let x = ... defines an immutable value. Next, when you wrote (List.nth list (Random.int l)) :: acc, that just meant “look, I can make a list”. Nowhere did you say that the newly constructed list has to be assigned to anything, in particular it was not assigned to acc (which it couldn’t, because acc is immutable). Ocaml issued a warning because it saw that the body of your for loop produces a list, but Ocaml knows that the body of a for loop ought to produce a unit (“void” in C/C++/Java mis-terminology).

Using a for loop like that is ugly. The Right Way is to do it like this:

let random_extract lst =
  let l = List.length lst - 1 in
  let rec extract = function
   | 0 -> []
   | n -> List.nth lst (Random.int l) :: extract (n - 1)
  in
    extract

If you are new to Ocaml you really should invest time to understand the above code completely. After that, you should study the following code, and make sure you understand why it is more efficient that the first version (look up “tail recursion”):

let random_extract lst =
  let l = List.length lst - 1 in
  let rec extract acc = function
    | 0 -> acc
    | n -> extract (List.nth lst (Random.int l) :: acc) (n - 1)
  in
    extract []