Went ahead and added an enhancement request: http://projects.scipy.org/scipy/numpy/ticket/293 This is something I've wanted in the past too. --bb On 9/25/06, Charles R Harris wrote:

On 9/24/06, Bill Baxter wrote:

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Howdy Angus, Yeh, that does seem like a hole in the API. Travis added a rollaxis() but there's still no simple way to roll the elements themselves.

I took a look at numpy.fft.fftshift, which is a function that has to do a similar thing. It does it by concatenating aranges and then doing a take(). Presumably Pearu knew what he was doing when he wrote that, so we can assume this is probably close to the best possible. :-) From that idea here's a function that implements roll().

def roll(y,shift,axis): """Roll the elements in the array by 'shift' positions along the given axis.""" from numpy import asanyarray,concatenate,arange y = asanyarray(y) n = y.shape[axis] shift %= n # does the right thing for negative shifts, too return y.take(concatenate((arange(shift,n),arange(shift))), axis)

It is possible to do a shift inplace using two reflections implemented with swaps. This works because two reflections is the same as a rotating twice the distance between the centers of the reflections. I don't know if it is worth implementing this, however.

Chuck

Hmm. Yes maybe a shift w/fill would be useful too. I can't recall needing such a thing, but it could probably also be implemented easily in a way similar to roll() above. A multi-axis roll might also be nice. Could just allow roll's shiftby and axis args to be tuples. Looking closer at the existing 'rollaxis', numpy.rollaxis(a, axis, start=0) if a.shape is (3,4,5,6) rollaxis(a, 3, 1).shape is (3,6,4,5) rollaxis(a, 2, 0).shape is (5,3,4,6) rollaxis(a, 1, 3).shape is (3,5,4,6) rollaxis(a, 1, 4).shape is (3,5,6,4) it occurs to me that what it is actually doing is not what we've been calling 'rolling'. It's just a move, really (remove value from one place and re-insert in another). So perhaps the name should be 'moveaxis' instead? --bb On 9/25/06, Alan G Isaac wrote:

On Mon, 25 Sep 2006, Bill Baxter apparently wrote:

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Went ahead and added an enhancement request: http://projects.scipy.org/scipy/numpy/ticket/293 This is something I've wanted in the past too.

GAUSS draws a useful distinction between "shifting" and "rotating":

Works roughly like this for the 2D case:

#rotater: rotate row elements # Format: y = rotater(x,r) # rotater(x,r) # Input: x RxK array # rotateby size R integer array, or integer (rotation amounts) # Output: y RxK array: # rows rotated by rotateby

#shiftr: shift row elements and fill with fv # Format: y = shiftr(x,shiftby,fv) # Input: x RxC array # shiftby Rx1 array or scalar (shift amounts) # fv Rx1 array or scalar (fill values) # Output: y RxC array: # rows shifted by shiftby # rows filled with fill

fwiw, Alan Isaac