excluding element from numpy array

Question

I want to get the c array as result, but I don't know how:

import numpy as np
a = xrange(10)
b = np.array([3,2,1,9])

c is made of elements of a that are not in b:

c = np.array([0,4,5,6,7,8])

What is np.xrange? You mean np.arange?

eumiro
– eumiro

2011-12-31 13:05:35 +00:00
Commented Dec 31, 2011 at 13:05 — eumiro
– eumiro, Commented Dec 31, 2011 at 13:05

JoshAdel · Accepted Answer · 2011-12-31 16:12:56Z

10

Perhaps a more straightforward solution is the following:

import numpy as np
a = xrange(10)
b = np.array([3,2,1,9])

c = np.setdiff1d(a,b)

Which results in:

In [7]: c
Out[7]: array([0, 4, 5, 6, 7, 8])

You can find all of the set-like operations for numpy arrays in the documentation: http://docs.scipy.org/doc/numpy/reference/routines.set.html

answered Dec 31, 2011 at 16:12

JoshAdel

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eumiro · Accepted Answer · 2011-12-31 13:08:43Z

3

import numpy as np
a = np.arange(10)
b = np.array([3,2,1,9])

np.array(sorted(set(a) - set(b)))
# array([0, 4, 5, 6, 7, 8])

UPDATE: works with a = xrange(10) too.

answered Dec 31, 2011 at 13:08

eumiro

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2 Comments

jterrace Over a year ago

This is inefficient for large arrays. @JoshAdel's answer is a better way.

JoshAdel Over a year ago

I would just say that having played around with the timings, @eumiro's solution is surprisingly efficient for a pretty wide range of array sizes. I would recommend benchmarking for your particular case to see which is better, also noting that np.setdiff1d has an option if both arrays contain only unique values, which can speed things up.

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