[Numpy-discussion] Shift or rotate command?
wbaxter at gmail.com
Sun Sep 24 20:12:43 CDT 2006
Went ahead and added an enhancement request:
This is something I've wanted in the past too.
On 9/25/06, Charles R Harris <charlesr.harris at gmail.com> wrote:
> On 9/24/06, Bill Baxter <wbaxter at gmail.com> wrote:
> > 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.
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