How to remove specific elements in a numpy array


Question

How can I remove some specific elements from a numpy array? Say I have

import numpy as np

a = np.array([1,2,3,4,5,6,7,8,9])

I then want to remove 3,4,7 from a. All I know is the index of the values (index=[2,3,6]).

1
173
6/12/2012 11:54:37 AM

Accepted Answer

Use numpy.delete() - returns a new array with sub-arrays along an axis deleted

numpy.delete(a, index)

For your specific question:

import numpy as np

a = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9])
index = [2, 3, 6]

new_a = np.delete(a, index)

print(new_a) #Prints `[1, 2, 5, 6, 8, 9]`

Note that numpy.delete() returns a new array since array scalars are immutable, similar to strings in Python, so each time a change is made to it, a new object is created. I.e., to quote the delete() docs:

"A copy of arr with the elements specified by obj removed. Note that delete does not occur in-place..."

If the code I post has output, it is the result of running the code.

230
6/16/2016 8:40:07 PM

There is a numpy built-in function to help with that.

import numpy as np
>>> a = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> b = np.array([3,4,7])
>>> c = np.setdiff1d(a,b)
>>> c
array([1, 2, 5, 6, 8, 9])

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