f# - I have two list (list a and list b) , with different lengths, how do i get a new list with all the unique elements of list a?

Question

I have this issue where i need a new list with, all the unique elements from list a, but the issue is that the list have different lengths.

Answer 1

[ 1; 2; 4; 5 ] |> List.except [ 2; 3; 5 ] // [ 1; 4 ]

That is, the following should do what you want

let c = a |> List.except b

Update

As @rcoy pointed out, List.except returns distinct elements only, i.e.

[ 1; 1 ] |> List.except [] = [ 1 ] // true

To keep duplicates, a straightforward way is

let b = [ 2 ]
let toRemove x = not(List.contains x b)
[ 1; 1; 2 ] |> List.filter toRemove // [ 1; 1 ]

Depending on the size of the b list and equivalency function (structural vs. referential), i.e. the cost of traversing and comparing, changing the datastructures might be beneficial. E.g.

open System.Collections.Generic
let b = [ 2 ]
let toRemove = HashSet(b)
[ 1; 1; 2 ] |> List.filter (fun x -> not(toRemove.Contains(x))) // [ 1; 1 ]

which is similar to what List.except does internally.