Getting indices of a specific value in numpy array

Question

I have the foll. numpy array:

arr = [0,0,0,1,0,0,0,0,0,0,0,1,0,1,0,1,0,0,0,0,0,0,0,0,0,0,1]

This is how I am getting the indices of all 0's in the array:

inds = []
for index,item in enumerate(arr):     
    if item == 0:
        inds.append(index)

Is there a numpy function to do the same?

arr = np.array([0,0,0,1,0,0,0,0,0,0,0,1,0,1,0,1,0,0,0,0,0,0,0,0,0,0,1]) convert to array for clarity and then ... >>> np.where(arr==0) (array([ 0, 1, 2, 4, 5, ..., 21, 22, 23, 24, 25]),) is one method. slice the where with [0] just to get the indices — user1121588
– user1121588, Commented Dec 7, 2015 at 4:12
is numpy.argwhere what you're after? Something like numpy.argwhere(arr == 0) — chappers
– chappers, Commented Dec 7, 2015 at 4:14

Anton Protopopov · Accepted Answer · 2017-10-25 08:20:14Z

5

You could use numpy.argwhere as @chappers pointed out in the comment:

arr = np.array([0,0,0,1,0,0,0,0,0,0,0,1,0,1,0,1,0,0,0,0,0,0,0,0,0,0,1])

In [34]: np.argwhere(arr == 0).flatten()
Out[34]:
array([ 0,  1,  2,  4,  5,  6,  7,  8,  9, 10, 12, 14, 16, 17, 18, 19, 20,
       21, 22, 23, 24, 25], dtype=int32)

Or with inverse of astype(bool):

In [63]: (~arr.astype(bool)).nonzero()[0]
Out[63]:
array([ 0,  1,  2,  4,  5,  6,  7,  8,  9, 10, 12, 14, 16, 17, 18, 19, 20,
       21, 22, 23, 24, 25], dtype=int32)

edited Oct 25, 2017 at 8:20

answered Dec 7, 2015 at 5:49

Anton Protopopov

31.9k13 gold badges93 silver badges96 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

John La Rooy · Accepted Answer · 2015-12-07 04:22:47Z

3

>>> arr = np.array([0,0,0,1,0,0,0,0,0,0,0,1,0,1,0,1,0,0,0,0,0,0,0,0,0,0,1])
>>> (arr==0).nonzero()[0]
array([ 0,  1,  2,  4,  5,  6,  7,  8,  9, 10, 12, 14, 16, 17, 18, 19, 20,
       21, 22, 23, 24, 25])

answered Dec 7, 2015 at 4:22

John La Rooy

306k54 gold badges378 silver badges513 bronze badges

2 Comments

user308827 Over a year ago

thanks @John, what does the [0] at the end of nonzero do?

John La Rooy Over a year ago

@user308827, nonzero returns a tuple of arrays, one for each dimension of arr. You only have one dimension, but you still need [0] there to pull it out of the tuple

Collectives™ on Stack Overflow

Getting indices of a specific value in numpy array

2 Answers 2

Comments

2 Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Related