[Numpy-discussion] Question about flags of fancy indexed array

[Travis Oliphant]

Charles R Harris wrote:
>
>
> On 5/23/07, *Albert Strasheim* <fullung at gmail.com 
> <mailto:fullung at gmail.com>> wrote:
>
>     Hello all
>
>     On Wed, 23 May 2007, Anne Archibald wrote:
>
>     > On 23/05/07, Albert Strasheim <fullung at gmail.com
>     <mailto:fullung at gmail.com>> wrote:
>     >
>     > > Consider the following example:
>     >
>     > First a comment: almost nobody needs to care how the data is stored
>     > internally. Try to avoid looking at the flags unless you're
>     > interfacing with a C library. The nice feature of numpy is that it
>     > hides all that junk - strides, contiguous storage, iteration, what
>     > have you - so that you don't have to deal with it.
>
>     As luck would have it, I am interfacing with a C library.
>
>     > > Is it correct that the F_CONTIGUOUS flag is set in the case of
>     the fancy
>     > > indexed x? I'm running NumPy 1.0.3.dev3792 here.
>     >
>     > Numpy arrays are always stored in contiguous blocks of memory with
>     > uniform strides. The "CONTIGUOUS" flag actually means something
>     > totally different, which is unfortunate, but in any case, "fancy
>     > indexing" can't be done as a simple reindexing operation. It
>     must make
>     > a copy of the array. So what you're seeing is the flags of a
>     fresh new
>     > array, created from scratch (and numpy always creates arrays in C
>     > order internally, though that is an implementation detail you should
>     > not rely on).
>
>     If you are correct that this is in fact a fresh new array, I really
>     don't understand where the values of these flags. To recap:
>
>     In [19]: x = N.zeros((3,2))
>
>     In [20]: x.flags
>     Out[20]:
>       C_CONTIGUOUS : True
>       F_CONTIGUOUS : False
>       OWNDATA : True
>       WRITEABLE : True
>       ALIGNED : True
>       UPDATEIFCOPY : False
>
>     In [21]: x[:,[1,0]].flags
>     Out[21]:
>       C_CONTIGUOUS : False
>       F_CONTIGUOUS : True
>       OWNDATA : False
>       WRITEABLE : True
>       ALIGNED : True
>       UPDATEIFCOPY : False
>
>     So since x and x[:,[1,0]] are both new arrays, shouldn't their
>     flags be
>     identical? I'd expect at least C_CONTIGUOUS and OWNDATA to be True.
>
>
> The contiguous refers to how stuff is layed out in memory. In this 
> case it appears that fancy indexing creates the new array by first 
> copying column 1, then column 2, so that the new array is indeed 
> F_CONTIGUOUS. Assuming I correctly understand the behaviour of the 
> tostring argument, which is debatable, that is indeed what happens.

Yes, you are using the 'A' argument correctly.  For contiguous arrays, 
you can always get a look at the actual buffer, using

x.data[:]
or
y.data[:]

You can't get a buffer for discontiguous arrays.

-Travis