Output of a Nested for Loop - 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.

for j in sep: with sep = [4, 5] results in that j becomes 4 in first iteration and 5 in second. So, this results in M[i, 4] or M[i, 5] which is an out of bound access for a 3x2 matrix, isn't it. Somehow, you confused the iteration "index" with the value picked from given list in iteration. — Scheff's Cat
– Scheff's Cat, Commented Oct 24, 2020 at 12:06

quamrana · Accepted Answer · 2020-10-24 12:06:43Z

2

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.

answered Oct 24, 2020 at 12:06

quamrana

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