Trying to modify np array diagonal

Question

I am trying to modify the diagonal values of a 6 x 5 2D numpy array (It's an exercise in this scipy tutorial: http://scipy-lectures.org/intro/numpy/array_object.html#basic-visualization). I'm supposed to change the values of a diagonal from zeroes to 2,3,4,5,6. Since it's a 6 x 5 matrix, there's not really a "main" diagonal, and so I need to change the diagonal starting from the second row ([1][0]) to [5][4]. They suggest reading the docstring for diag. I did, and I still can't figure out how to do this. Any suggestions?

arr[[1,2,3,4,5], [0,1,2,3,4]] indexes that diagonal. Make np.arange(30).reshape(6,5) and test it. — hpaulj
– hpaulj, Commented Jul 1, 2019 at 2:48

Akavall · Accepted Answer · 2019-07-01 02:29:26Z

2

You can just slice an array, and fill_diagonal of that:

In [13]: import numpy as np                                                     

In [14]: a = np.zeros((6,5), int)                                               

In [15]: np.fill_diagonal(a[1:], [2,3,4,5,6])                                   

In [16]: a                                                                      
Out[16]: 
array([[0, 0, 0, 0, 0],
       [2, 0, 0, 0, 0],
       [0, 3, 0, 0, 0],
       [0, 0, 4, 0, 0],
       [0, 0, 0, 5, 0],
       [0, 0, 0, 0, 6]])

answered Jul 1, 2019 at 2:29

Akavall

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