Haskell opposite of intersect (List)

Question

I'm trying to get the "opposite" of intersect of two list: like:

let all  = [1..5]
let mask = [2,3]
let res  = ???
-- let res = all `intersect` mask <-- reverse/opposite ?
-- I want to get [1,4,5] ?

I think that's called a relative complement. – Pubby Dec 20 '12 at 07:34 — Pubby, Dec 20 '12 at 07:34

score 19 · Accepted Answer · answered Dec 20 '12 at 07:36

19

You're looking for set difference, which is the \\ operator from Data.List:

Prelude> import Data.List ((\\))
Prelude Data.List> let all  = [1..5]
Prelude Data.List> let mask = [2,3]
Prelude Data.List> all \\ mask
[1,4,5]

answered Dec 20 '12 at 07:36

Thomas

3

@user914584 But be aware that `(\\)` removes only one element from the first list per element of the second, e.g. `[1,2,3,2] \\ [2] = [1,3,2]`. If that's not good for your use case, ``filter (`notElem` second) first`` is an alternative. – Daniel Fischer Dec 20 '12 at 13:30

1 Answers1