How to append a column to a 2d numpy array in Python 2.7?

Question

I have two numpy arrays. One is a 2d matrix that has 3 columns and 4 rows. The second numpy array is a 1d array with 4 values. Is there a way to append the second numpy array as a column to the first numpy array in Python 2.7?

For example if these were my two numpy arrays:

arr2d = np.matrix(
[[1, 2, 3],
[4, 5, 6], 
[7, 8, 9], 
[10, 11, 12]])

column_to_add = np.array([10, 40, 70, 100])

I'd like the output to look like this

    [[1, 2, 3, 10],
    [4, 5, 6, 40], 
    [7, 8, 9, 70], 
    [10, 11, 12, 100]]

I tried using

output = np.hstack((arr2d, column_to_add))

but I got an error that says:

ValueError: all the input arrays must have the same number of dimensions.

Any and all help is appreciated. Thank you so much!

Correct the number of dimensions: np.concatenate((arr2d, column_to_add[:,None]), axis=1). The [:,None] makes the 1d array into a 2d column array. — hpaulj
– hpaulj, Commented Jun 20, 2017 at 16:09

FJSevilla · Accepted Answer · 2017-06-20 10:39:47Z

10

You can use numpy.column_stack:

import numpy as np

arr2d = np.matrix(
[[1, 2, 3],
[4, 5, 6], 
[7, 8, 9], 
[10, 11, 12]])

column_to_add = np.array([10, 40, 70, 100])

output = np.column_stack((arr2d, column_to_add))

Output:

matrix([[  1,   2,   3,  10],
        [  4,   5,   6,  40],
        [  7,   8,   9,  70],
        [ 10,  11,  12, 100]])

answered Jun 20, 2017 at 10:39

FJSevilla

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