Bishwajit Purkaystha - 1 year ago 142
Python Question

# Adding a column in front of a numpy array

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.

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]])
``````
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