How to zip each individual element from two lists into one list using OCaml

Question

If I have an input of a tuple containing two lists of integers of the same length, and I want my output to be a list of these two lists zipped, after extracting these two lists from the tuple how do I zip each individual element into one list? For example, if my input is twolists= ([1;2;3], [4;5;6]), then I want my output to be [(1,4); (2,5); (3,6)]. How do I zip each element and add it to my output? The function name and type is as follows:

let rec pairlists twolists = ...

val pairlists : 'a list * 'b list -> ('a * 'b) list = fun

So far I have:

let rec pairlists twolists = 
  let (l1, l2) = twolists in
  let rec zip (l1,l2) =
    match l1 with 
    |[] -> l2
    |x :: xs -> x :: zip(l2, xs) in
  twolists ;;

but this is clearly not doing what I want.

Answer 1

Are you looking for List.combine ?

val combine : 'a list -> 'b list -> ('a * 'b) list

Transform a pair of lists into a list of pairs: combine [a1; ...; an] [b1; ...; bn] is [(a1,b1); ...; (an,bn)].

Raises Invalid_argument if the two lists have different lengths. Not tail-recursive.