Python How to replace values in specific columns (defined by an array) with zero

Question

I'm trying to replace values in specific columns with zero with python, and the column numbers are specified in another array.

Given the following 2 numpy arrays

a = np.array([[  1,   2,   3,   4],
              [  1,   2,   1,   2],
              [  0,   3,   2,   2]])

and

b = np.array([1,3])

b indicates column numbers in array "a" where values need to be replaced with zero. So the expected output is

            ([[  1,   0,   3,   0],
              [  1,   0,   1,   0],
              [  0,   0,   2,   0]])

Any ideas on how I can accomplish this? Thanks.

Does this answer your question? [How to replace one column by a value in a numpy array?](https://stackoverflow.com/questions/28952946/how-to-replace-one-column-by-a-value-in-a-numpy-array) — Marco Bonelli, Jul 11 '22 at 21:26

constantstranger · Accepted Answer · 2022-07-09T21:45:56.473

2

Your question is:

I'm trying to replace values in specific columns with zero with python, and the column numbers are specified in another array.

This can be done like this:

a[:,b] = 0

Output:

[[1 0 3 0]
 [1 0 1 0]
 [0 0 2 0]]

The Integer array indexing section of Indexing on ndarrays in the numpy docs has some similar examples.

edited Jul 09 '22 at 21:45

answered Jul 09 '22 at 21:30

constantstranger

Do you need additional help with your question? – constantstranger Jul 11 '22 at 23:05

score 1 · Answer 2 · answered Jul 09 '22 at 21:18

1

A simple for loop will accomplish this.

for column in b:
    for row in range(len(a)):
        a[row][column] = 0
        
print(a)

[[1 0 3 0]
 [1 0 1 0]
 [0 0 2 0]]

answered Jul 09 '22 at 21:18

ImajinDevon

2

iterating over a numpy array sort of defeats the point of using one: https://stackoverflow.com/questions/10112745/python-is-the-iteration-of-the-multidimensional-array-super-slow – Hugh Mungus Jul 11 '22 at 10:05

2 Answers2