How to replace values in a numpy array based on another column?

Question

Let say i have the following:

import numpy as np

data = np.array([
     [1,2,3],
     [1,2,3],
     [1,2,3],
     [4,5,6],         
     ])

How would I go about changing values in column 3 based on values in column 2? For instance, If column 3 == 3, column 2 = 9.

[[1,9,3],
 [1,9,3],
 [1,9,3],
 [4,5,6]]

I've looked at np.any(), but I can't figure out how to alter the array in place.

Answer 1

You can use Numpy's slicing and indexing to achieve this. Take all the rows where the third column is 3, and change the second column of each of those rows to 9:

>>> data[data[:, 2] == 3, 1] = 9
>>> data
array([[1, 9, 3],
       [1, 9, 3],
       [1, 9, 3],
       [4, 5, 6]])