how to step over a list in sml?

Question

i have a list , and a tuple (start,end,interval) i am trying to iterate over the list and return the elements that are in list from start to end with interval steps . for example:

cutt [1,2,3,4,77,8,7] (1,5,2);
val it = [2,4,8] : int list;

the problem is that i can't use recursion , i was thinking to use foldl but the problem is how to skip over elements i don't want to use list.nth? i would appriciate little hints ! thank you

Answer 1

One solution to correctly skip elements you don't want in your resulting list, is to add a parameter to your function used to fold your list. The meaning of this parameter is basically "how much items you still want to skip".

As an example, below is a solution that uses List.foldl. The function used to fold items of the list takes two parameters: the first one is the one discussed above and the second one is the resulting list being constructed.

fun cutt lst (idx_start, idx_end, interval) = let
    val (_, result) =
        List.foldl
            (fn (x, (0, result)) =>
                (* no more item to skip => add x to the resulting list *)
                (interval - 1, x :: result)
            |   (_, (n, result)) =>
                (* there are still items to skip *)
                (n - 1, result))
            (idx_start, [])  (* initially we skip idx_start items *)
            lst
in
    (* items are prepended in the resulting list in reverse order =>
    reverse the list *)
    List.rev result
end;
                         
val x = cutt [1, 2, 3, 4, 77, 8, 7] (1, 5, 2);

Of course the function needs to be adapted as it does not take care of the idx_end parameter.