Delete array of values from numpy array

Question

This post is an extension of this question.

I would like to delete multiple elements from a numpy array that have certain values. That is for

import numpy as np
a = np.array([1, 1, 2, 5, 6, 8, 8, 8, 9])

How do I delete one instance of each value of [1,5,8], such that the output is [1,2,6,8,8,9]. All I have found in the documentation for an array removal is the use of np.setdiff1d, but this removes all instances of each number. How can this be updated?

Possible duplicate of Delete rows from numpy array depending on a condition on a single cell — luis.parravicini
– luis.parravicini, Commented Oct 17, 2019 at 14:48
In the example above, I want to remove the values 1,5, and 8. These integers refer to the values I want to remove, not the index of the array — Sid
– Sid, Commented Oct 17, 2019 at 14:55

user3483203 · Accepted Answer · 2019-10-17 15:21:54Z

1

Using outer comparison and argmax to only remove once. For large arrays this will be memory intensive, since the created mask has a.shape * r.shape elements.

r = np.array([1, 5, 8])
m = (a == r[:, None]).argmax(1)
np.delete(a, m)

array([1, 2, 6, 8, 8, 9])

This does assume that each value in r appears in a at least once, otherwise the value at index 0 will get deleted since argmax will not find a match, and will return 0.

edited Oct 17, 2019 at 15:21

answered Oct 17, 2019 at 14:56

user3483203

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6 Comments

Sid Over a year ago

I don't understand the notation used to define m. Could you explain that?

user3483203 Over a year ago

It's just broadcasted comparison to compare row-wise each value in r with all of array a.

Sid Over a year ago

Is it more efficient to use this approach as opposed to LampToasts approach?

user3483203 Over a year ago

Time it on your data, but it should be much faster.

LampToast Over a year ago

There's no performance difference. They're functionally identical

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LampToast · Accepted Answer · 2019-10-17 14:58:27Z

1

delNums = [np.where(a == x)[0][0] for x in [1,5,8]]
a = np.delete(a, delNums)

here, delNums contains the indexes of the values 1,5,8 and np.delete() will delete the values at those specified indexes

OUTPUT:

[1 2 6 8 8 9]

answered Oct 17, 2019 at 14:58

LampToast

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