Im having an issue with my separate function. Separate returns a list that inserts element x after each k elements of list l (counting from the end of the list). For example, separate (1, 0, [1,2,3,4]) should return [1,0,2,0,3,0,4] and separate (3, 0, [1,2,3,4]) should return [1,0,2,3,4]. Whenever I run any tests on it i get the error:
! Unbound value identifier: separate
This is the code i'm using:
(*Function returns length of lst *)
fun length(lst: int list): int =
case lst of
[] => 0
| h::t => 1 + length(t)
(*Insert element x at the kth position in the list
and return the new list*)
fun kinsert [] x k = [x]
| kinsert ls x 0 = x::ls
| kinsert (l::ls) x k = l::(kinsert ls x (k - 1))
(* c: keeps track of where we are in the list
n: determines if we insert element at given position
z: holds length of the list *)
fun sep_help k x l c n z=
if c = z then l
else if n = k then (sep_help k x (kinsert l x c) (c+2) 0 z )
else (sep_help k x l (c+2) (n+1) z) ;
(*Returns list l with x inserted after each k element *)
fun separate (k: int, x: 'a, l: 'a list) : 'a list =
| separate k x l = (sep_help k x l 0 0 (length l));
Anyone know what might be causing the error?
Your separate looks like its a merge of two different definitions - first an uncurried version that has no definition, and then a curried version that has one.
You probably meant
fun separate (k: int, x: 'a, l: 'a list) : 'a list = sep_help k x l 0 0 (length l);
But working backwards through a list complicates things quite a bit.
It's much easier to work from the head towards the tail, and reverse the list before and after processing.
Then "all" you need is a helper that inserts an element in every k:th place in a list.
Something like this, perhaps:
(*Returns list l with x inserted after each k element *)
fun separate (k: int, x: 'a, l: 'a list) : 'a list =
let
fun kinsert [] _ = []
| kinsert ls 0 = x::(kinsert ls k)
| kinsert (l::ls) i = l::(kinsert ls (i-1))
in
List.rev (kinsert (List.rev l) k)
end