F#: Filter items found in one list from another list

Question

Say I have two lists:

let a = [1 .. 1000]
let b = [250 .. 500]

How do I get a new list that contains the values {1-249, 501-1000}?

Answer 1

Since your list is sorted, you can solve this in linear time using this (non-tail recursive) function:

let rec except a b =
    match (a, b) with
    | [], x | x, [] -> x
    | x::xs, y::ys ->
        if x < y then x :: except xs (y::ys)
        elif x > y then y :: except (x::xs) ys
        else except xs ys

Tail-recursive version:

let rec except_tail_recursive a b =
    let rec loop acc a b =
        match (a, b) with
        | [], x | x, [] -> (List.rev acc) @ x
        | x::xs, y::ys ->
            if x < y then loop (x::acc) xs (y::ys)
            elif x > y then loop (y::acc) (x::xs) ys
            else loop acc xs ys
    loop [] a b