numpy - 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]]