OCaml error in basic loop

Question

I'm new with OCaml. I'm making a function working this way : I have "tab" in the scope which represents a 2D map, and three parameters, x, y and u.

x and y represent the position of the player, and u the direction of the bomb (right, top left etc.). I want the function to update tab so that every cell which are not in the given direction are updated to 0.

Here is my code so far :

let test = fun x y u ->
(for i = 0 to (w-1) do
    for j = 0 to (h-1) do
        if i > x then
            if j > y then
                tab.(i).(j) = (if u = "DR" then tab.(i).(j) else 0)
            else
                if j = y then
                    tab.(i).(j) = (if u = "R" then tab.(i).(j) else 0)
                else
                    tab.(i).(j) = (if u = "UR" then tab.(i).(j) else 0)
        else
            if i = x then
                if j > y then 
                    tab.(i).(j) = (if u = "D" then tab.(i).(j) else 0)
                else
                    tab.(i).(j) = (if u = "U" then tab.(i).(j) else 0)
            else
                if j > y then
                    tab.(i).(j) = (if u = "DL" then tab.(i).(j) else 0)
                else
                    if j = y then
                        tab.(i).(j) = (if u = "L" then tab.(i).(j) else 0)
                    else
                        tab.(i).(j) = (if u = "UL" then tab.(i).(j) else 0)
    done
done)

On the line 6, I get the following error : "characters 20-71: Warning 10: this expression should have type unit." and I don't know why.

Could someone explain my error please ?

Have a good day !

Answer 1

The = symbol is here to check an equality when not preceded by a let. If you want to change the value of one element of an array, you have to use <- instead.

let test = fun x y u ->
  for i = 0 to (w-1) do
    for j = 0 to (h-1) do
      if i > x then
        if j > y then
          tab.(i).(j) <- (if u = "DR" then tab.(i).(j) else 0)
        else
          if j = y then
            tab.(i).(j) <- (if u = "R" then tab.(i).(j) else 0)
          else
            tab.(i).(j) <- (if u = "UR" then tab.(i).(j) else 0)
      else
        if i = x then
          if j > y then 
            tab.(i).(j) <- (if u = "D" then tab.(i).(j) else 0)
          else
            tab.(i).(j) <- (if u = "U" then tab.(i).(j) else 0)
        else
          if j > y then
            tab.(i).(j) <- (if u = "DL" then tab.(i).(j) else 0)
          else
            if j = y then
              tab.(i).(j) <- (if u = "L" then tab.(i).(j) else 0)
            else
              tab.(i).(j) <- (if u = "UL" then tab.(i).(j) else 0)
    done
  done

The error you got was that you returned booleans where unit was expected.