求和 numpy 二维数组中的列

Question

I have a 2D NumPy array V :

import numpy as np
np.random.seed(10)

V = np.random.randint(-10, 10, size=(6,8))

This gives V as:

[[ -1  -6   5 -10   7   6   7  -2]
 [ -1 -10   0  -2  -6   9   6  -6]
 [  5   1   1  -9  -2  -6   4   7]
 [  9   3  -5   3   9   3   2  -9]
 [ -6   8   3   1   0  -1   5   8]
 [  6  -3   1   7   4  -3   1  -9]]

Now, I have 2 lists, r1 and r2 , containing column indices as follows:

r1 = [1, 2, 5]
r2 = [3, 4, 7]

What I want is to add the columns of V based on the indices pair (r1, r2) and store it in column indices r1 . That is, for this case, add columns (1, 3) , (2, 4) and (5, 7) and store them respectively in columns 1 , 2 and 5 of V .

It can be done easily like this:

V[:, 1] = V[:, [1,3]].sum(axis=1)
V[:, 2] = V[:, [2,4]].sum(axis=1)
V[:, 5] = V[:, [5,7]].sum(axis=1)

which gives V as:

[[ -1 -16  12 -10   7   4   7  -2]
 [ -1 -12  -6  -2  -6   3   6  -6]
 [  5  -8  -1  -9  -2   1   4   7]
 [  9   6   4   3   9  -6   2  -9]
 [ -6   9   3   1   0   7   5   8]
 [  6   4   5   7   4 -12   1  -9]]

My concern is that is there a way we can do it without loops? Thanks in advance:)

Answer 1

Just add V[:, r2] at V[:, r2] , like below:

V[:, r1] += V[:, r2]
print(V)

Output

[[ -1 -16  12 -10   7   4   7  -2]
 [ -1 -12  -6  -2  -6   3   6  -6]
 [  5  -8  -1  -9  -2   1   4   7]
 [  9   6   4   3   9  -6   2  -9]
 [ -6   9   3   1   0   7   5   8]
 [  6   4   5   7   4 -12   1  -9]]