Question

I am new to OCaml, and I am now trying to implement a function that returns a list of elements of a given list x at indexes in list y.

For example, that function should perform the following computation: [5,6,7,8], [0, 3] => [5, 8]

I am not sure how to store temp variables in ML and don’t have a clear idea how it works. I do know how to find a element from a list given specified index, though.

Any idea will be appreciated, but I’d like to use recursive functions and avoid the List module.

Answer 1

No need for temporary variables, just use recursion!

# let rec indices xs = function
    | i :: is -> (List.nth xs i) :: indices xs is
    | [] -> []
  ;;
val indices : 'a list -> int list -> 'a list = <fun>

# indices [5;6;7;8] [0;3] ;;
- int list = [5; 8]

It builds up the list by going through each of the indexes provided and then consing that onto the list returned by the next step.

Hopefully this is also optimised into a tail recursive form, but I’m not so sure about that. You may want to change it to be properly tail-recursive, but I’ll leave that up to you.