How to convert a column or row matrix to a diagonal matrix in Python?

Question

I have a row vector A, A = [a1 a2 a3 ..... an] and I would like to create a diagonal matrix, B = diag(a1, a2, a3, ....., an) with the elements of this row vector. How can this be done in Python?

UPDATE

This is the code to illustrate the problem:

import numpy as np a = np.matrix([1,2,3,4]) d = np.diag(a) print (d) 

the output of this code is [1], but my desired output is:

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

Answer 1

You can use diag method:

import numpy as np  a = np.array([1,2,3,4]) d = np.diag(a) # or simpler: d = np.diag([1,2,3,4])  print(d) 

Results in:

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

If you have a row vector, you can do this:

a = np.array([[1, 2, 3, 4]]) d = np.diag(a[0]) 

Results in:

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

For the given matrix in the question:

import numpy as np a = np.matrix([1,2,3,4]) d = np.diag(a.A1) print (d) 

Result is again:

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