Filter list by another list (list subtraction)

Question

The vimscript allows list union via the + operator, but the list subtraction doesn't come as nice as the union. Is there an easy way to filter a list by another list?

:echo [1, 2, 3] + [1, 10]
" [1, 2, 3, 1, 10]


:echo [1, 2, 3] - [1, 10]
" E730: using List as a String

" What I want: [2, 3]

I tried to write a call with filter() and map() but could not go very far.

Answer 1

It is a little bit faster to use a string expression than a lambda (and arguably easier to understand in this case),

call filter(a, 'index(b, v:val)<0')

This naive implementation is O(N^2) but is still fairly fast in practice as index() is implemented in C. If you have a very large number of elements in the RHS, it may be faster to use a pre-computed hash table (Dict). However, building the table (especially using a for loop) is likely to destroy any benefit.

" [1, 2, 3] - [1, 10]

let a = [1, 2, 3]
let b = [1, 10]

let hash = {}
for bv in b
    let hash[bv] = 1
endfor
" a more obtuse way is..
" call mapnew(b, 'extend(hash, {v:val: 1})')

call filter(a, '!has_key(hash, v:val)')

Answer 2

To keep it simple, I assume all elements are different (i.e. it's "set difference"):

:call filter(list1, {_, v -> index(list2, v) < 0})