numpy array slicing and masking

Question

I've come across numpy syntax I don't entirely understand. Here it is:

array[:, mask] = a

array is a numpy array of indeces, mask is a numpy array of bool values and a is an integer.

Could someone please translate this notation for me?

I couldn't find the answer anywhere online.

Answer 1

Translation:

• in the array array
• for the rows [:] (all in this case)
• do for every column where mask contains True
• the operation: assign a

So it writes a into all matrix cells there masks contains a True value.

here is an example:

import numpy as np

array = np.array([
    [1,5,3],
    [1,5,3],
    [1,5,3],
])
mask = np.array(
    [True,False,True]
)
array[:, mask] = 10
print(array)

results in:

[[10  5 10]
 [10  5 10]
 [10  5 10]]