Fail to concatenate two numpy array

Question

Given two numpy array a1 and a2:

>>>np.shape(a1)
(4465, 5000)
>>>np.shape(a2)
(4465, )

However,

>>>np.concatenate((a1, a2), axis=1)
ValueError: all the input arrays must have the same number of dimensions

And I also tried:

np.concatenate((a1, a2), axis=1),
np.concatenate((a1, a2.T), axis=0),
np.concatenate((a1, a2.T), axis=1)

But also got the same error.

Could you please tell me what's wrong with my code? Thanks!

What output are you expecting? An array of length 4465 by 5001? — Ed Smith
– Ed Smith, Commented Jun 6, 2016 at 7:29
@Alex Totally different. I didn't lost the parentheses. Just remove the duplicate. — u3728666
– u3728666, Commented Jun 6, 2016 at 7:31

P. Camilleri · Accepted Answer · 2016-06-06 07:30:40Z

2

As the error message states, a1 and a2 do not have the same number of dimensions (accessible via attribute ndims). Make a2 2-dimensional with a2 = a2[:, None]. You can also use the more explicit syntax a2 = a2[:, np.newaxis], but it is strictly equivalent.

answered Jun 6, 2016 at 7:30

P. Camilleri

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3 Comments

u3728666 Over a year ago

a2 = a2[:, np.newaxis] works but a2 = a2[:, None] doesn't. Why??

Ed Smith Over a year ago

Strange, which version of python/numpy as you using? You could also use a2 = np.expand_dims(a2, 1) or a2 = np.atleast_2d(a2).T

P. Camilleri Over a year ago

@ZICHAOLI try to avoid "doesn't work" on SO : do you get an error message or do you get unexpected results ? In both case, include them

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