嵌套 for 循环的输出 - Python

Question

I am trying to get the output of a nested for loop into an array or matrix. For instance, in the code example below, I want to have a (3 by 2)-matrix of the form:

[[5 6],
 [6 7],
 [7 8]]

But my code is giving out of bound error.

import numpy as np

num = [1,2,3]
sep = [4, 5]
M = np.zeros((3,2))
for i in num:
    for j in sep:
        M[i, j] = i + j
M

However, I realized that changing the initialization to np.zeros((4,6)) seems to work but with some irrelevant cells. Can someone explain how this works or possibly how I can achieve this (3 by 2)-matrix.

Answer 1

You are using the values in your num and sep lists as indexes. You need to use indexes instead:

import numpy as np

num = [1,2,3]
sep = [4, 5]
M = np.zeros((3,2))
for i_i,i in enumerate(num):
    for i_j,j in enumerate(sep):
        M[i_i, i_j] = i + j

print(M)

Output as required.