Concat two arrays of different dimensions numpy

Question

I am trying to concatenate two numpy arrays to add an extra column: array_1 is (569, 30) and array_2 is is (569, )

combined = np.concatenate((array_1, array_2), axis=1)

I thought this would work if I set axis=2 so it will concatenate vertically. The end should should be a 569 x 31 array.

The error I get is ValueError: all the input arrays must have same number of dimensions

Can someone help?

Thx!

Close, but you have only two axis (axis 0 = 569 and axis 1 = 30), try `axis=1`. — umutto, Oct 12 '17 at 01:45
hi - that was a typo.. i just updated my question with my array's shapes and the error i get back — Trung Tran, Oct 12 '17 at 01:48
Ahh your `array_2` only has one dimension, needs to have same number of dimensions with your `array_1`. You can either reshape it `array_2.reshape(-1,1)`, or add a new axis `array_2[:,np.newaxis]` to make it 2 dimensional before concatenation. — umutto, Oct 12 '17 at 01:53

Psidom · Accepted Answer · 2017-10-12T01:58:56.720

27

You can use numpy.column_stack:

np.column_stack((array_1, array_2))

Which converts the 1-d array to 2-d implicitly, and thus equivalent to np.concatenate((array_1, array_2[:,None]), axis=1) as commented by @umutto.

a = np.arange(6).reshape(2,3)
b = np.arange(2)

a
#array([[0, 1, 2],
#       [3, 4, 5]])

b
#array([0, 1])

np.column_stack((a, b))
#array([[0, 1, 2, 0],
#       [3, 4, 5, 1]])

edited Oct 12 '17 at 01:58

answered Oct 12 '17 at 01:53

Psidom

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It uses `array(arr, copy=False, subok=True, ndmin=2).T` to convert a 1d array into 2. I like your `[:, None]` better. :) – hpaulj Oct 12 '17 at 02:56
What will be the equivalent if we want to achieve ```axis=-1``` of the same ? – Jean-Élisée Yao Feb 10 '22 at 14:20

score 0 · Answer 2 · edited Feb 01 '22 at 15:05

0

To stack them vertically try

np.vstack((array1,array2))

edited Feb 01 '22 at 15:05

buddemat

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answered Jan 26 '22 at 20:58

Forough Nasihati

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score -1 · Answer 3 · edited Apr 21 '20 at 09:51

-1

You can simply use numpy's hstack function.

e.g.

import numpy as np

combined = np.hstack((array1,array2))

edited Apr 21 '20 at 09:51

Maximouse

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answered Apr 21 '20 at 08:31

Darshil Shah

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Concat two arrays of different dimensions numpy

3 Answers3

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