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:

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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

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

I tried using

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

but I got an error that says:

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

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

Answer 1

You can use numpy.column_stack:

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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:

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