[Numpy-discussion] when did column_stack become C-contiguous?

[josef.pktd at gmail.com]

On Mon, Oct 19, 2015 at 5:16 AM, Sebastian Berg <sebastian at sipsolutions.net>
wrote:

> On Mo, 2015-10-19 at 01:34 -0400, josef.pktd at gmail.com wrote:
> >
>
> <snip>
>
>
> >
> > It looks like in 1.9 it depends on the order of the 2-d arrays, which
> > it didn't do in 1.6
> >
>
> Yes, it uses concatenate, and concatenate probably changed in 1.7 to use
> "K" (since "K" did not really exists before 1.7 IIRC).
> Not sure what we can do about it, the order is not something that is
> easily fixed unless explicitly given. It might be optimized (as in this
> case I would guess).
> Whether or not doing the fastest route for these kind of functions is
> faster for the user is of course impossible to know, we can only hope
> that in most cases it is better.
> If someone has an idea how to decide I am all ears, but I think all we
> can do is put in asserts/tests in the downstream code if it relies
> heavily on the order (or just copy, if the order is wrong) :(, another
> example is change of the output order in advanced indexing in some
> cases, it makes it faster sometimes, and probably slower in others, what
> is right seems very much non-trivial.
>

To understand the reason:

Is this to have more efficient memory access during copying?

AFAIU, column_stack needs to create a new array which has to be either F or
C contiguous, so we always have to pick one of the two. With a large number
of 1d arrays it seemed more "intuitive" to me to copy them by columns.

Josef



>
> - Sebastian
>
>
> >
> > >>> np.column_stack((np.ones(10), np.ones((10, 2), order='F'))).flags
> >   C_CONTIGUOUS : False
> >   F_CONTIGUOUS : True
> >   OWNDATA : True
> >   WRITEABLE : True
> >   ALIGNED : True
> >   UPDATEIFCOPY : False
> >
> >
> >
> >
> > which means the default order looks more like "K" now, not "C", IIUC
> >
> >
> > Josef
> >
> >
> >
> >
> >
> >         Josef
> >
> >
> >
> >
> >
> >                 Stephan
> >
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> >
> >
> >
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