Adding a column in front of a numpy array

Question

I have a numpy array:

array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])

Now I want to add a column of all ones in front of it to get:

array([[1, 1, 2, 3],
       [1, 4, 5, 6],
       [1, 7, 8, 9]])

I saw many posts pertaining to my question, but none of them could solve my problem. I tried np.concatenate np.append np.hstack but unfortunately none of them worked.

Answer 1

Simply use np.concatenate:

>>> import numpy as np
>>> arr = np.array([[1, 2, 3],
                    [4, 5, 6],
                    [7, 8, 9]])

>>> np.concatenate((np.array([0,0,0])[:, np.newaxis], arr), axis=1)
array([[0, 1, 2, 3],
       [0, 4, 5, 6],
       [0, 7, 8, 9]])

But hstack works too:

>>> np.hstack((np.array([0,0,0])[:, np.newaxis], arr))
array([[0, 1, 2, 3],
       [0, 4, 5, 6],
       [0, 7, 8, 9]])

The only "tricky" part is that both arrays must have the same number of dimensions, that's why I added the [:, np.newaxis] - which adds a new dimension.

How to change the zeros to ones is left as an (easy) exercise :-)

If you want to prepend a 2D array you have to drop the [:, np.newaxis] part:

np.concatenate((np.zeros((3,3), dtype=int), arr), axis=1)
array([[0, 0, 0, 1, 2, 3],
       [0, 0, 0, 4, 5, 6],
       [0, 0, 0, 7, 8, 9]])