Peter Smit Peter Smit - 1 month ago 17
Python Question

How to add an extra column to an numpy array

Lets say I have an numpy array a:

a = np.array([[1,2,3],[2,3,4]])


And I would like to add a column of zeros to get array b:

b = np.array([[1,2,3,0],[2,3,4,0]])


How can I do this easily in numpy?

Answer

I think a more straightforward solution and faster to boot is to do the following:

import numpy as np
N = 10
a = np.random.rand(N,N)
b = np.zeros((N,N+1))
b[:,:-1] = a

And timings:

In [23]: N = 10

In [24]: a = np.random.rand(N,N)

In [25]: %timeit b = np.hstack((a,np.zeros((a.shape[0],1))))
10000 loops, best of 3: 19.6 us per loop

In [27]: %timeit b = np.zeros((a.shape[0],a.shape[1]+1)); b[:,:-1] = a
100000 loops, best of 3: 5.62 us per loop