Copy NumPy matrix into NumPy array

Question

I am using cvxpy to solve an optimization problem. I want to store the output, a matrix into a ndarray.

This is the reduced test case. a representing the returned value from cvxpy.

import numpy as np

z = np.zeros((3,7))
a = np.matrix("[1; 2; 3]")
z[0, 0] = a[0]
z[1, 0] = a[1]
z[2, 0] = a[2]

I would like to replace the last three lines with something better, but everything I try results in an error. E.g.,

>>> z[:, 0] = a
Traceback (most recent call last):
  File "<input>", line 1, in <module>
ValueError: could not broadcast input array from shape (3,1) into shape (3)
>>> np.copyto(z[:, 0], a)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
ValueError: could not broadcast input array from shape (3,1) into shape (3)

I would appreciate a little help. Thank you.

Divakar · Accepted Answer · 2017-05-18 09:31:18Z

1

One simple way would be to use a list of the column index for indexing into z and then simply assign the NumPy matrix a there -

z[:,[0]] = a

Another way would be to use the transpose of the matrix a, which would be a row vector to assign into the sliced version of the first column in z -

z[:,0] = a.T

answered May 18, 2017 at 9:31

Divakar

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3 Comments

hpaulj Over a year ago

a.A1 might work here. I.e. convert the 2d matrix into a 1d array.

Divakar Over a year ago

@hpaulj You love that .A1 thing, don't you? :)

Imanol Luengo Over a year ago

@hpaulj I didn't know about a.A1, such a nice syntactic sugar :P

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