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<

Answer 1

You could also use numpy's triu and scipy.linalg's circulant. Make a circulant version of your matrix. Then, select the upper triangular part starting at the first diagonal, (the default option in triu). The row index will correspond to the number of padded zeros you want.

If you don't have scipy you can generate a nXn circulant matrix by making an (n-1) X (n-1) identity matrix and stacking a row [0 0 ... 1] on top of it and the column [1 0 ... 0] to the right of it.

Answer 2

Elaborating on the answer by Hooked (since it took me a few minutes to understand it)

The code below first pads a certain amount of zeros in the up, down, left and right margins and then selects the original matrix inside the padded one. A perfectly useless code, but good for understanding np.pad.

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

print np.all(x==y)

now to make an upwards shift of 2 combined with a rightwards shift of 1 position one should do

print np.pad(x,((0,2),(1,0)), mode='constant')[2:0,0:-1]

Answer 3

I just wrote the following. It could be more optimized by avoiding zeros_like and just computing the shape for zeros directly.

import numpy as np
def roll_zeropad(a, shift, axis=None):
    """
    Roll array elements along a given axis.

    Elements off the end of the array are treated as zeros.

    Parameters
    ----------
    a : array_like
        Input array.
    shift : int
        The number of places by which elements are shifted.
    axis : int, optional
        The axis along which elements are shifted.  By default, the array
        is flattened before shifting, after which the original
        shape is restored.

    Returns
    -------
    res : ndarray
        Output array, with the same shape as `a`.

    See Also
    --------
    roll     : Elements that roll off one end come back on the other.
    rollaxis : Roll the specified axis backwards, until it lies in a
               given position.

    Examples
    --------
    >>> x = np.arange(10)
    >>> roll_zeropad(x, 2)
    array([0, 0, 0, 1, 2, 3, 4, 5, 6, 7])
    >>> roll_zeropad(x, -2)
    array([2, 3, 4, 5, 6, 7, 8, 9, 0, 0])

    >>> x2 = np.reshape(x, (2,5))
    >>> x2
    array([[0, 1, 2, 3, 4],
           [5, 6, 7, 8, 9]])
    >>> roll_zeropad(x2, 1)
    array([[0, 0, 1, 2, 3],
           [4, 5, 6, 7, 8]])
    >>> roll_zeropad(x2, -2)
    array([[2, 3, 4, 5, 6],
           [7, 8, 9, 0, 0]])
    >>> roll_zeropad(x2, 1, axis=0)
    array([[0, 0, 0, 0, 0],
           [0, 1, 2, 3, 4]])
    >>> roll_zeropad(x2, -1, axis=0)
    array([[5, 6, 7, 8, 9],
           [0, 0, 0, 0, 0]])
    >>> roll_zeropad(x2, 1, axis=1)
    array([[0, 0, 1, 2, 3],
           [0, 5, 6, 7, 8]])
    >>> roll_zeropad(x2, -2, axis=1)
    array([[2, 3, 4, 0, 0],
           [7, 8, 9, 0, 0]])

    >>> roll_zeropad(x2, 50)
    array([[0, 0, 0, 0, 0],
           [0, 0, 0, 0, 0]])
    >>> roll_zeropad(x2, -50)
    array([[0, 0, 0, 0, 0],
           [0, 0, 0, 0, 0]])
    >>> roll_zeropad(x2, 0)
    array([[0, 1, 2, 3, 4],
           [5, 6, 7, 8, 9]])

    """
    a = np.asanyarray(a)
    if shift == 0: return a
    if axis is None:
        n = a.size
        reshape = True
    else:
        n = a.shape[axis]
        reshape = False
    if np.abs(shift) > n:
        res = np.zeros_like(a)
    elif shift < 0:
        shift += n
        zeros = np.zeros_like(a.take(np.arange(n-shift), axis))
        res = np.concatenate((a.take(np.arange(n-shift,n), axis), zeros), axis)
    else:
        zeros = np.zeros_like(a.take(np.arange(n-shift,n), axis))
        res = np.concatenate((zeros, a.take(np.arange(n-shift), axis)), axis)
    if reshape:
        return res.reshape(a.shape)
    else:
        return res

Answer 4

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 5

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.

Answer 6

A bit late, but feels like a quick way to do what you want in one line. Perhaps would work best if wrapped inside a smart function (example below provided just for horizontal axis):

import numpy

a = numpy.arange(1,10).reshape(3,3)  # an example 2D array

print a

[[1 2 3]
 [4 5 6]
 [7 8 9]]

shift = 1
a = numpy.hstack((numpy.zeros((a.shape[0], shift)), a[:,:-shift]))

print a

[[0 1 2]
 [0 4 5]
 [0 7 8]]