Iterate over the output of `np.where`

Question

I have a 3D array and use np.where to find elements that meet a certain condition. The output of np.where is a tuple of three 1D arrays, each givin

Answer 1

Use np.transpose over zip, it's faster for large arrays

import numpy as np
myarray = np.random.randint(0, 7, size=1000000)
%timeit indices = zip(*np.where(myarray == 0))
%timeit indices = np.transpose(np.where(myarray == 0))

10 loops, best of 3: 31.8 ms per loop
100 loops, best of 3: 15.9 ms per loop

Answer 2

Use zip

indices = zip(*np.where(myarray == 0))

Then you can do

for i, j, k in indices:
    print ...

For example,

In [1]: x = np.random_integers(0, 1, (3, 3, 3))
In [2]: np.where(x) # you want np.where(x==0)
Out[2]: (array([0, 0, 0, 0, 0, 1, 1, 1, 1, 2]),
         array([0, 1, 1, 2, 2, 0, 0, 1, 1, 2]),
         array([1, 0, 1, 0, 1, 1, 2, 0, 2, 2]))
In [3]: zip(*np.where(x))
Out[3]: [(0, 0, 1),
         (0, 1, 0),
         (0, 1, 1),
         (0, 2, 0),
         (0, 2, 1),
         (1, 0, 1),
         (1, 0, 2),
         (1, 1, 0),
         (1, 1, 2),
         (2, 2, 2)]