[Numpy-discussion] Inplace shift
Sat Jun 7 20:48:17 CDT 2008
2008/6/7 Keith Goodman <email@example.com>:
> On Fri, Jun 6, 2008 at 10:46 PM, Anne Archibald
> <firstname.lastname@example.org> wrote:
>> 2008/6/6 Keith Goodman <email@example.com>:
>>> I'd like to shift the columns of a 2d array one column to the right.
>>> Is there a way to do that without making a copy?
>>> This doesn't work:
>>>>> import numpy as np
>>>>> x = np.random.rand(2,3)
>>>>> x[:,1:] = x[:,:-1]
>>> array([[ 0.44789223, 0.44789223, 0.44789223],
>>> [ 0.80600897, 0.80600897, 0.80600897]])
>> As a workaround you can use backwards slices:
>> In : x = np.random.rand(2,3)
>> In : x[:,:0:-1] = x[:,-2::-1]
>> In : x
>> array([[ 0.20183084, 0.20183084, 0.08156887],
>> [ 0.30611585, 0.30611585, 0.79001577]])
> Neat. It makes sense to go backwards. Thank you.
>> Less painful for numpy developers but more painful for users is to
>> warn them about the status quo: operations on overlapping slices can
>> happen in arbitrary order.
> Now I'm confused. Could some corner case of memory layout cause numpy
> to work from right to left, breaking the workaround? Or can I depend
> on the workaround working with numpy 1.0.4?
I'm afraid so. And it's not such a corner case as that: if the array
is laid out in "C contiguous" order, you have to go backwards, while
if the array is laid out in "FORTRAN contiguous" order you have to go
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