Delete an element of certain value in numpy array once

Question

I would like to delete an element from a numpy array that has a certain value. However, in the case there are multiple elements of the same value, I only want to delete one occurrence (doesn't matter which one). That is:

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

How do I delete one instance of 8? Specifically

a_new = np.delete(a, np.where(a == 8))
print(a_new)

removes all 8's.

Sounds like an xy problem. Why are you trying to delete anything from a numpy array to begin with? — Mad Physicist
– Mad Physicist, Commented Oct 17, 2019 at 10:39
It's a pattern matching step. Then I continue with the use of numpy arrays. — Sid
– Sid, Commented Oct 17, 2019 at 11:44
You reallocate the entire array to delete one element. Sounds like masking might be a better way potentially. — Mad Physicist
– Mad Physicist, Commented Oct 17, 2019 at 12:10

Kasravnd · Accepted Answer · 2019-10-17 10:27:42Z

4

You can simply choose one of the indices:

In [3]: np.delete(a, np.where(a == 8)[0][0])
Out[3]: array([1, 1, 2, 6, 8, 8, 9])

answered Oct 17, 2019 at 10:27

Kasravnd

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Paul Panzer · Accepted Answer · 2019-10-17 10:36:17Z

1

If you know there is at least one 8 you can use argmax:

np.delete(a,(a==8).argmax())
# array([1, 1, 2, 6, 8, 8, 9])

If not you can still use this method but you have to do one check:

idx = (a==8).argmax()
if a[idx] == 8:
    result = np.delete(a,idx)
else: # no 8 in a
    # complain

answered Oct 17, 2019 at 10:36

Paul Panzer

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