Using arrays with other arrays in Python

Question

Trying to find an efficient way to extract all instances of items in an array out of another.

For example

array1 = ["abc", "def", "ghi", "jkl"]

array2 = ["abc", "ghi", "456", "789"]

Array 1 is an array of items that need to be extracted out of array 2. Thus, array 2 should be modified to ["456", "789"]

I know how to do this, but no in an efficient manner.

Answer 1

These are lists, not arrays. (The word "array" means different things to different people, but in python the objects call themselves lists, and that's that; there are other modules that provide objects that call themselves arrays, such as array and numpy)

To answer your question, the easiest way is to not modify array2 at all. Use a list comprehension:

set1 = set(array1)
array2 = [e for e in array2 if e not in set1]

(the set makes this O(n) instead of O(n^2))

If you absolutely must mutate array2 (because it exists elsewhere), you can use slice assignment:

array2[:] =  [e for e in array2 if e not in set1]

It's just as efficient, but kind of nasty.

edit: as Mark Byers points out, this only works if array1 only contains hashable elements (such as strings, numbers, etc.).