Duplicate array dimension with numpy (without np.repeat)

Question

I'd like to duplicate a numpy array dimension, but in a way that the sum of the original and the duplicated dimension array are still the same. For instance consider a n x m shape array (a) which I'd like to convert to a n x n x m (b) array, so that a[i,j] == b[i,i,j]. Unfortunately np.repeat and np.resize are not suitable for this job. Is there another numpy function I could use or is this possible with some creative indexing?

>>> import numpy as np
>>> a = np.asarray([1, 2, 3])
>>> a
array([1, 2, 3])
>>> a.shape
(3,)
# This is not what I want...
>>> np.resize(a, (3, 3))
array([[1, 2, 3],
       [1, 2, 3],
       [1, 2, 3]])

In the above example, I would like to get this result:

array([[1, 0, 0],
       [0, 2, 0],
       [0, 0, 3]])

akuiper · Accepted Answer · 2017-07-28 01:17:42Z

2

From 1d to 2d array, you can use the np.diagflat method, which Create a two-dimensional array with the flattened input as a diagonal:

import numpy as np
a = np.asarray([1, 2, 3])

np.diagflat(a)
#array([[1, 0, 0],
#       [0, 2, 0],
#       [0, 0, 3]])

More generally, you can create a zeros array and assign values in place with advanced indexing:

a = np.asarray([[1, 2, 3], [4, 5, 6]])

result = np.zeros((a.shape[0],) + a.shape)
idx = np.arange(a.shape[0])
result[idx, idx, :] = a

result
#array([[[ 1.,  2.,  3.],
#        [ 0.,  0.,  0.]],

#       [[ 0.,  0.,  0.],
#        [ 4.,  5.,  6.]]])

edited Jul 28, 2017 at 1:17

answered Jul 28, 2017 at 0:43

akuiper

216k33 gold badges362 silver badges379 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

orange Over a year ago

np.diagflat(a) doesn't work correctly. a.shape = (2, 3) turns into np.diagflat(a).shape = (6, 6), but I need a shape of (2, 2, 3).

akuiper Over a year ago

np.diagflat only works if you have 1d array and want to convert it to 2d array with 1d as diagonal, for a 2d array, you'll have to use the second more general solution.

Collectives™ on Stack Overflow

Duplicate array dimension with numpy (without np.repeat)

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related