SML how to get while on char list to return int?

Question

I'm having a bit difficulty figuring out, how to get each of the processed chars back to an int value.

The function should work like: val caesar = fn : int * int -> int

So if k = 2466 and n = 2, then the output should be 4688

Hope the code isn't too weird (I'm a SML newbie).

 (* Load Libs *)
    load "Int";
    load "Real";
    load "String";
    load "Char";
    load "List";


    fun caesar (k, n) =
      let
        fun k_string (i) = Int.toString(i)
        fun item_k_char (x, y) = Char.ord (List.nth (x, y))

        val val_k_string = k_string(k)
        val k_explode = String.explode(val_k_string)
        val counter = ref 0
        val counter_end = (String.size(val_k_string) - 1)

      in 
        while (!counter >= counter_end) do (
          item_k_char(k_explode, !counter) + n;
          counter := !counter + 1
        )
      end;

Answer 1

A while loop isn't the best tool here. Instead you can use map which executes a given function for each item in a given list and returns a new list containing the result of each call to the function.

In other words: map (fn c => (Char.ord c) + 2) [#"2", #"4", #"6", #"6"] will return [52,54,56,56]. You can the convert this back to char and use String.implode to get the string "4688".

You probably also want to add some logic so that the numbers "wrap around", i.e. caesar (7,7) becomes 4.

So all in all your code becomes:

fun caesar (k, n) =
  let
    val val_k_string = Int.toString k
    val k_explode = String.explode val_k_string
    val ints = map (fn c => (Char.ord c) + n) k_explode
    val wrappedAroundInts = map (fn i => (i - 48) mod 10 + 48) ints
    val chars = map Char.chr wrappedAroundInts
    val string = String.implode chars
  in
    Option.valOf (Int.fromString string)
  end