substract items from array / numpy

Question

org = [1,1,1,1,2,2,4]
remove = [1,1,2]

result = foo(org, remove)
# result = [1,1,2,4]
# two 1 are removed and one 2

I want to remove items from org, but not all with the same value - only one remove each item in the remove-array

Is there a numpy function to do so?

Turn the array into a count table? That would be more generally useful anyway — CJR
– CJR, Commented Nov 28, 2021 at 21:52
you arent actualy using numpy, for a list you can just do [org.remove(i) for i in remove]. Note that this will throw ValueError if i is not in org, so you need to handle that. Also @CJR has a point, I dont know what is the use case but consider it. — Dima Maligin
– Dima Maligin, Commented Nov 28, 2021 at 21:54

Matthew Strawbridge · Accepted Answer · 2021-11-28 22:11:09Z

2

Following on from CJR's comment, it turns out that the built-in Counter understands subtraction and "does the right thing". It silently ignores any elements that aren't present in the first counter.

So you can do something like this:

from collections import Counter

c1 = Counter(org)
c2 = Counter(remove)
result = list((c1 - c2).elements())

To give result = [1,1,2,4].

Edit: Of course, this won't necessarily preserve the order. And if you know both collections are already sorted at the start anyway, there will be a more efficient approach.

answered Nov 28, 2021 at 22:11

Matthew Strawbridge

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