# How to add a number to a matrix diagonal elements in python ?

Published: August 06, 2019

Example of how to add a number to a matrix diagonal elements in python ?

### Replace the diagonal element by a same number

To replace the diagonal element by a same number, a solution is to use the numpy function numpy.fill_diagonal

````>>> import numpy as np`
`>>> A = np.arange(9).reshape(3,3)`
`>>> A`
`array([[0, 1, 2],`
`       [3, 4, 5],`
`       [6, 7, 8]])`
`>>> np.fill_diagonal(A, 100)`
`>>> A`
`array([[100,   1,   2],`
`       [  3, 100,   5],`
`       [  6,   7, 100]])`
```

### Add a number to the diagonal elements of a matrix

It is also possible to add a number to the diagonal elements of a matrix using the numpy function numpy.diagonal pour ajouter un nombre aux éléments de la diagonale

````>>> A = np.arange(9).reshape(3,3)`
`>>> A`
`array([[0, 1, 2],`
`       [3, 4, 5],`
`       [6, 7, 8]])`
`>>> np.fill_diagonal(A, A.diagonal() + 100)`
`>>> A`
`array([[100,   1,   2],`
`       [  3, 104,   5],`
`       [  6,   7, 108]])`
```