numpy-discussion

[Numpy-discussion] Accessing data buffers in numpy scalars

[Francesc A. <fa...@ca...>]

Hi,

I'm looking for an easy way to access the data area of the numpy scalars no=
=20
matter its type.  I've seen that numpy/arrayscalars.h define a structure fo=
r=20
each scalar type, so I'd guess that it will not be possible to find a gener=
al=20
way for accessing the data buffer for each type. So, I've decided to look f=
or=20
a workaround and I've devised a couple of possibilities:

1.- Promote the scalar type to a ndarray object and use the regular ndarray=
 C=20
structure to access the data buffer.

2.- Fetch the buffer in scalartype.data and use the buffer protocol in orde=
r=20
to access the pointer to data in memory. However, I lack experience in buff=
er=20
protocol, so suggestions for achieving this are welcome.

If there is some other trivial way that I haven't devised (specially if usa=
ble=20
from pyrex), please tell me about.

TIA,

=2D-=20
>0,0<   Francesc Altet =A0 =A0 http://www.carabos.com/
V   V   C=E1rabos Coop. V. =A0=A0Enjoy Data
 "-"

Re: [Numpy-discussion] Accessing data buffers in numpy scalars

[Travis O. <oli...@ee...>]

Francesc Altet wrote:

>Hi,
>
>I'm looking for an easy way to access the data area of the numpy scalars no 
>matter its type.  I've seen that numpy/arrayscalars.h define a structure for 
>each scalar type, so I'd guess that it will not be possible to find a general 
>way for accessing the data buffer for each type. So, I've decided to look for 
>a workaround and I've devised a couple of possibilities:
>  
>
This problem showed up in writing NumPy several times.

One solution might be to use

PyArray_ScalarAsCtype(PyObject *scalar, void *ctypeptr)

which will copy the data into the area pointed to by ctypeptr (unless 
you have a "flexible scalar" in which case only a pointer to the 
data-area will be copied).


>2.- Fetch the buffer in scalartype.data and use the buffer protocol in order 
>to access the pointer to data in memory. However, I lack experience in buffer 
>protocol, so suggestions for achieving this are welcome.
>  
>
This will also work.   A read-only buffer protocol is exported by all 
the scalars.

scalar.data will return a buffer object.

Or you can use the Python C-API

const char *buffer;
Py_ssize_t buflen;

PyObject_AsReadBuffer(scalar, (const void **)&buffer, &buflen)

to retrieve a pointer to the data in buffer and the size of the data in 
buflen.


-Travis

Re: [Numpy-discussion] Accessing data buffers in numpy scalars

[Francesc A. <fa...@ca...>]

A Dimarts 17 Octubre 2006 18:22, Travis Oliphant va escriure:
> >2.- Fetch the buffer in scalartype.data and use the buffer protocol in
> > order to access the pointer to data in memory. However, I lack experien=
ce
> > in buffer protocol, so suggestions for achieving this are welcome.
>
> This will also work.   A read-only buffer protocol is exported by all
> the scalars.
>
> scalar.data will return a buffer object.
>
> Or you can use the Python C-API
>
> const char *buffer;
> Py_ssize_t buflen;
>
> PyObject_AsReadBuffer(scalar, (const void **)&buffer, &buflen)

Oh, this one seems pretty easy, and as a plus, you don't have to book memor=
y=20
for copying the data area, so I'll use it.

Thanks,

=2D-=20
>0,0<   Francesc Altet =A0 =A0 http://www.carabos.com/
V   V   C=E1rabos Coop. V. =A0=A0Enjoy Data
 "-"

Re: [Numpy-discussion] Accessing data buffers in numpy scalars

[Lisandro D. <da...@gm...>]

On 10/17/06, Travis Oliphant <oli...@ee...> wrote:
> Or you can use the Python C-API
>
> const char *buffer;
> Py_ssize_t buflen;
>
> PyObject_AsReadBuffer(scalar, (const void **)&buffer, &buflen)
>
> to retrieve a pointer to the data in buffer and the size of the data in
> buflen.
>

Travis. Have numpy something similar to a mutable scalar, or the only
way is using an array whit only one element?


--=20
Lisandro Dalc=EDn
---------------
Centro Internacional de M=E9todos Computacionales en Ingenier=EDa (CIMEC)
Instituto de Desarrollo Tecnol=F3gico para la Industria Qu=EDmica (INTEC)
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PTLC - G=FCemes 3450, (3000) Santa Fe, Argentina
Tel/Fax: +54-(0)342-451.1594

Re: [Numpy-discussion] Accessing data buffers in numpy scalars

[Travis O. <oli...@ee...>]

Lisandro Dalcin wrote:

>On 10/17/06, Travis Oliphant <oli...@ee...> wrote:
>  
>
>>Or you can use the Python C-API
>>
>>const char *buffer;
>>Py_ssize_t buflen;
>>
>>PyObject_AsReadBuffer(scalar, (const void **)&buffer, &buflen)
>>
>>to retrieve a pointer to the data in buffer and the size of the data in
>>buflen.
>>
>>    
>>
>
>Travis. Have numpy something similar to a mutable scalar, or the only
>way is using an array whit only one element?
>  
>

0-dim arrays are still mutable scalars.

-Travis