On 9/24/06, Bill Baxter <[EMAIL PROTECTED]> wrote:
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
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
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