How to get the arrays not contained in another array?

Question

I have two Python arrays with arrays inside:

first = [[0,10],[0,11],[0,12],[0,13]]
second = [[0,10],[0,11]]

and I want to get as a result the array that is in the first one, but not in the other:

difference(first,second)
#returns [[0,12],[0,13]]

right now they are numpy arrays, but a solution with regular ones is also valid.

I tried using np.setdiff1d but it returned an array with the exclusive ints, not exclusive arrays. And I tried to iterate through the second array deleting its elements from the first one:

diff = first.view()
for equal in second:
     diff = np.delete(diff, np.where(diff == equal))

but it returned similar not useful results

score 1 · Accepted Answer · answered May 28 '23 at 17:45

1

You could do this with regular old Python lists (not numpy arrays) and a simple list comprehension:

>>> diff = [lst for lst in first if lst not in second]
>>> diff
[[0, 12], [0, 13]]

answered May 28 '23 at 17:45

Michael M.

10,486
9
18
34

score 0 · Answer 2 · answered May 28 '23 at 18:15

Using numpy, you can broadcast to compare the elements and use np.all/np.any to consolidate the comparison results into a mask:

import numpy as np

first  = np.array([[0,10],[0,11],[0,12],[0,13]])
second = np.array([[0,10],[0,11],[1,7]])

mask = ~np.any(np.all(first[:,None]==second,axis=-1),axis=-1)

print(first[mask])
[[ 0 12]
 [ 0 13]]

Dmitriy · Answer 3 · 2023-05-28T22:21:58.617

-1

If I understand correctly, you need a function, try this:

first = [[0,10],[0,11],[0,12],[0,13]]
second = [[0,10],[0,11]]

def difference(first,second):             #declare function
    lst = []                              #declare list
    for i in first:                       #run loop
        if i not in second: lst.append(i) #just remember method
    return(lst)                           #output of this function 'lst'
difference(first,second)

I'm not sure if this is the best option, but looking by the way you ask, this method is for you

edited May 28 '23 at 22:21

answered May 28 '23 at 18:42

Dmitriy

17
4

It would be helpful to explain how this code works (and not with obvious comments and dark magic expected to be "just remember[ed]") and how it differs from the other answers. – pppery May 29 '23 at 01:51
@pppery i think you are wrong, many 'beginner' tutorials are written like this, you never know which line is not clear for someone(its about 'obvious'), and you contradict yourself, why should i write obvious explain 'for'? you allways can find million tutorials, and if i do, i need expl. 'def' and many more, it will be a dead end – Dmitriy May 29 '23 at 09:20

How to get the arrays not contained in another array?

3 Answers3