I'll also point out that np.copy has an "order" argument, so you can get
back a Fortran-ordered array by doing
>>> X_train = X_train.copy(order='F') # lets materialize the view
On Tue, Nov 26, 2013 at 11:53 PM, Peter Prettenhofer <
peter.prettenho...@gmail.com> wrote:
>
>
>
> 2013/11/26 Olivier Grisel <olivier.gri...@ensta.org>
>
>> 2013/11/26 Peter Prettenhofer <peter.prettenho...@gmail.com>:
>> > Hi all,
>> >
>> > I'm currently modifying our tree code so that it runs on both fortran
>> and c
>> > continuous arrays. After some benchmarking I got aware of the following
>> > numpy behavior that was contrary to what I was expecting::
>> >
>> > >>> X = # some feature matrix
>> > >>> X = np.asfortranarray(X)
>> > >>> X.flags.f_contiguous
>> > True
>> > >>> # so far so good
>> > >>> X_train = X[:1000]
>> > >>> X_train.flags.f_contiguous
>> > False
>> > >>> X_train.flags.c_contiguous
>> > False
>> > >>> # damn - seems like a view is neither c nor fortran continuous
>>
>> Only if you slice the rows of a fortran aligned 2D array, this is
>> expected. If you slices the rows of a C-contiguous 2D array or the
>> columns of a F-contiguous 2D array it stays contiguous.
>>
>
> Actually, now that I think about it, it totally makes sense -- next time I
> think before I write ;-)
>
> thanks guys!
>
>
>
>>
>> >>> import numpy as np
>> >>> a_c = np.arange(12).reshape(3, 4)
>> >>> a_f = np.asfortranarray(a_c)
>> >>> a_c.flags
>> C_CONTIGUOUS : True
>> F_CONTIGUOUS : False
>> OWNDATA : False
>> WRITEABLE : True
>> ALIGNED : True
>> UPDATEIFCOPY : False
>> >>> a_f.flags
>> C_CONTIGUOUS : False
>> F_CONTIGUOUS : True
>> OWNDATA : True
>> WRITEABLE : True
>> ALIGNED : True
>> UPDATEIFCOPY : False
>> >>> a_c[1:].flags
>> C_CONTIGUOUS : True
>> F_CONTIGUOUS : False
>> OWNDATA : False
>> WRITEABLE : True
>> ALIGNED : True
>> UPDATEIFCOPY : False
>> >>> a_f[:, 1:].flags
>> C_CONTIGUOUS : False
>> F_CONTIGUOUS : True
>> OWNDATA : False
>> WRITEABLE : True
>> ALIGNED : True
>> UPDATEIFCOPY : False
>>
>>
>>
>> --
>> Olivier
>> http://twitter.com/ogrisel - http://github.com/ogrisel
>>
>>
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>
>
>
> --
> Peter Prettenhofer
>
>
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