Assignment of matrix through nested loops only works for first row

See the following simplified code

import numpy as np
u=np.zeros((3,3))
i,j=0,0
while i<3:
    while j<3:
        u[i,j]=i+j
        j=j+1
    i=i+1

I was expecting a full matrix u (except for [0,0] element). This code seems to assign to only first row of u. I have used something like this in MATLAB in the past with success

>Solution :

The main problem is that you forgot to reset the index j when the inner loop is complete. After the first row, j will be greater than 3 so u[i, j] will not make sense.
The correct code would be:

import numpy as np
u=np.zeros((3,3))
i=0
while i<3:
    j=0
    while j<3:
        u[i,j]=i+j
        j=j+1
    i=i+1

This should fix your issue and initialize u to the following array:

[[0. 1. 2.]
 [1. 2. 3.]
 [2. 3. 4.]]

Note that although the code is correct, it could be rewritten to be a bit cleaner using for loops and ranges:

import numpy as np
u = np.zeros((3, 3))
for i in range(3):
    for j in range(3):
        u[i, j] = i+j