How to append a last column (1d array) to a 2d numpy array in python?

Question

I have two arrays:

import numpy as np
a = np.array([[1,2,3], [4,5,6]])
b = np.array([5, 6])

when I try to append b to a as a's last column using the below code

 np.hstack([a, b])



it thows an error:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<__array_function__ internals>", line 6, in hstack
  File "C:\Users\utkal\anaconda3\envs\AIML\lib\site-packages\numpy\core\shape_base.py", line 345, in hstack
    return _nx.concatenate(arrs, 1)
  File "<__array_function__ internals>", line 6, in concatenate
ValueError: all the input arrays must have same number of dimensions, but the array at index 0 has 2 dimension(s) and the array at index 1 has 1 dimension(s)

I want the final array as:

1 2 3 5
4 5 6 6

b isn't a column.

hpaulj
– hpaulj

2020-10-17 11:57:05 +00:00
Commented Oct 17, 2020 at 11:57 — hpaulj
– hpaulj, Commented Oct 17, 2020 at 11:57

flabons · Accepted Answer · 2020-10-17 07:51:21Z

4

You can do it like this:

np.hstack((a,b.reshape(2,1)))
array([[1, 2, 3, 5],
       [4, 5, 6, 6]])

answered Oct 17, 2020 at 7:51

flabons

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Comments

Andy L. · Accepted Answer · 2020-10-17 08:06:44Z

2

np.hstack is a shortcut for np.concatenate on axis=1. So, it requires both arrays have the same shapes with all dimensions matching except on concatenate axis. In your case, you need both array in 2-d and matching on dimension 0

So you need

np.hstack([a, b[:,None]])

Out[602]:
array([[1, 2, 3, 5],
       [4, 5, 6, 6]])

Or use np.concatenate

np.concatenate([a, b[:,None]], axis=1)

Out[603]:
array([[1, 2, 3, 5],
       [4, 5, 6, 6]])

edited Oct 17, 2020 at 8:06

answered Oct 17, 2020 at 7:59

Andy L.

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Comments

ali abedzadeh · Accepted Answer · 2025-04-20 10:18:59Z

0

You can do it like this:

import numpy as np
a = np.array([[1,2,3], [4,5,6]])
b = np.array([5, 6])
c=np.concatenate([a,b[:,None]],axis=1)

answered Apr 20 at 10:18

ali abedzadeh

1

2 Comments

André Apr 22 at 8:33

This is exactly what has already been suggested in 2020. Please don't just duplicate existing answers, but only post if you can provide additional insights.

André Apr 22 at 8:34

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

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