How to replace the diagonal elements of a numpy 2d array with another array?

Question

I have a numpy 2d array:

import numpy as np
A=np.array([[1, 2, 3, 4],
            [5, 6, 7, 8],
            [9, 10, 11, 12],
            [13, 14, 15, 16]])
print (A)

I would like to replace the diagonal elements with a = np.array([0,2,15,20]). That is the desired output should be:

A=[[0, 2, 3, 4],
    [5, 2, 7, 8],
    [9, 10, 15, 12],
    [13, 14, 15, 20]]

I tried with the following code:

import numpy as np
A=np.array([[1, 2, 3, 4],
            [5, 6, 7, 8],
            [9, 10, 11, 12],
            [13, 14, 15, 16]])
a = np.array([0,2,15,20])
print(np.fill_diagonal(A, a))

But it resulted in None

The np.fill_diagonal function modifies the input array in-place, it does not return a value. Try print(A) afterwards to see result. — Andrej Kesely
– Andrej Kesely, Commented Mar 31, 2022 at 22:50

Ali_Sh · Accepted Answer · 2022-04-01 01:30:06Z

2

As an alternative method if a be the main array and b the modified values array:

a_mod = a.ravel()
a_mod[::a.shape[0]+1] = b
result = a_mod.reshape(a.shape)

It can handle where the other diagonals of a matrix (instead the main diagonal) is of interest, by some modification. np.fill_diagonal works on the main diagonal.

edited Apr 1, 2022 at 1:30

answered Mar 31, 2022 at 22:52

Ali_Sh

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Kyriakos · Accepted Answer · 2022-04-01 07:55:37Z

1

try this

A[np.arange(4), np.arange(4)] = a

array([[ 0,  2,  3,  4],
       [ 5,  2,  7,  8],
       [ 9, 10, 15, 12],
       [13, 14, 15, 20]])

answered Apr 1, 2022 at 7:55

Kyriakos

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