Question

Say I have a matrix ‘R’ filled with random integers

import numpy as np
matR = np.random.randint(-10,10,size=(4,6))
>>> matR = [[-4 -4  1 -8 -2  5]
            [ 9  2 -4 -1  4  2]
            [ 7  8 -2 -9  3  8]
            [ 9 -3  3  6  4  3]]

Now I know I can sample it like this:

>>> matR[::2,::2] = [[-4  1 -2]
                     [ 7 -2  3]]

What I really want, however, is a clean way of doing this:

>>> matR.?? = [[-4  0  1  0 -2  0]
               [ 0  0  0  0  0  0]
               [ 7  0 -2  0  3  0]
               [ 0  0  0  0  0  0]]

I’d like to avoid python loops, it would be easy that way using enumerate.

Answer 1

Do you want something like this?

>>> import numpy as np
>>> m = np.random.randint(-10,10,size=(4,6))
>>> m
array([[  7,   4,   7,   7,   5,   9],
       [  5,  -7,  -2,   4,   2,  -4],
       [ -9,   4,   6,   8,   5, -10],
       [ -6,  -8,   8,  -5,   2,  -3]])
>>> m2 = np.zeros_like(m) # or m2 = m*0
>>> m2[::2, ::2] = m[::2, ::2]
>>> m2
array([[ 7,  0,  7,  0,  5,  0],
       [ 0,  0,  0,  0,  0,  0],
       [-9,  0,  6,  0,  5,  0],
       [ 0,  0,  0,  0,  0,  0]])