python numpy roll with padding

Question

I'd like to roll a 2D numpy in python, except that I'd like pad the ends with zeros rather than roll the data as if its periodic.

Specifically, the following code

import numpy as np  x = np.array([[1, 2, 3], [4, 5, 6]])  np.roll(x, 1, axis=1) 

returns

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

but what I would prefer is

array([[0, 1, 2], [0, 4, 5]]) 

I could do this with a few awkward touchups, but I'm hoping that there's a way to do it with fast built-in commands.

Thanks

Answer 1

There is a new numpy function in version 1.7.0 numpy.pad that can do this in one-line. Pad seems to be quite powerful and can do much more than a simple "roll". The tuple ((0,0),(1,0)) used in this answer indicates the "side" of the matrix which to pad.

import numpy as np x = np.array([[1, 2, 3],[4, 5, 6]])  print np.pad(x,((0,0),(1,0)), mode='constant')[:, :-1] 

Giving

[[0 1 2]  [0 4 5]] 

Answer 2

I don't think that you are going to find an easier way to do this that is built-in. The touch-up seems quite simple to me:

y = np.roll(x,1,axis=1) y[:,0] = 0 

If you want this to be more direct then maybe you could copy the roll function to a new function and change it to do what you want. The roll() function is in the site-packages\core\numeric.py file.