Get complement of numpy array

Question

I have the following array and a list of indices

my_array = np.array([ [1,2], [3,4], [5,6], [7,8] ])
indices = np.array([0,2])

I can get the values of the array corresponding to my indices by just doing my_array[indices], which gives me the expected result

array([[1, 2],
       [5, 6]])

Now I want to get the complement of it. As mentioned in one of the answers, doing

my_array[~indices]

Will not give the expected result [[3,4],[7,8]].

I was hoping this could be done in a 1-liner way, without having to define additional masks.

Riccardo Bucco · Accepted Answer · 2022-11-15 09:19:22Z

2

You can use numpy.delete. It returns a new array with sub-arrays along an axis deleted.

complement = np.delete(my_array, indices, axis=0)

>>> np.delete(my_array, indices, axis=0)
array([[3, 4],
       [7, 8]])

answered Nov 15, 2022 at 9:19

Riccardo Bucco

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4 Comments

Could you explain why the intuitive ~ returns unexpected result?

Why on earth would ~ (bitwise inverse) give the expected result?

@JonasPalačionis A simple reason is that arr.shape == (~arr).shape, which is clearly not what we need here.

Thank you Riccardo for answering in a kind and explicative way!