OCaml insert an element in list

Question

What is the standard way of inserting an element to a specific position in a list in OCaml. Only recursion is allowed. No assignment operation is permitted.

My goal is to compress a graph in ocaml by removing vertexes with in_degree=out_degree=1. For this reason I need to remove the adjacent edges to make a single edge. Now the edges are in a list [(6,7);(1,2);(2,3);(5,4)]. So I need to remove those edges from the list and add a single edge. so the above list will now look like [(6,7);(1,3);(5,4)]. Here we see (1,2);(2,3) is removed and (1,3) is inserted in the second position. I have devised an algorithm for this. But to do this I need to know how can I remove the edges (1,2);(2,3) from position 2,3 and insert (1,3) in position 2 without any explicit variable and in a recursive manner.

Answer 1

OCaml list is immutable so there's no such thing like removing and inserting elements in list operations.

What you can do is creating a new list by reusing certain part of the old list. For example, to create a list (1, 3)::xs' from (1, 2)::(2, 3)::xs' you actually reuse xs' and make the new list using cons constructor.

And pattern matching is very handy to use:

let rec transform xs =                                             
  match xs with
  | [] | [_] -> xs
  | (x, y1)::(y2, z)::xs' when y1 = y2 -> (x, z)::transform xs'
  | (x, y1)::(y2, z)::xs' -> (x, y1)::transform ((y2, z)::xs')