Re: [Numpy-discussion] Please help - pointer to slice

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

A Divendres 07 Mar=E7 2003 01:42, Sebastian Haase va escriure:
> if I try to debug this by doing:
> for z in range(nz):
>     print repr(arr[z]._data)
>
> that also tells be that python / numarray thinks all slices are located=
 at
> the same address, that is:
> every slice looks just like the full array.

You can access the slices by taking into account the byte offset
attribute. Look at the next example:

In [20]: a=3Dnumarray.arange(100, shape=3D(10,10))

In [21]: a._data
Out[21]: <memory at 083e58e8 with size:400 held by object at 083e58d0>

In [22]: a[1]._data
Out[22]: <memory at 083e58e8 with size:400 held by object at 083e58d0>

as you already know, both memory buffers point to the same address. I gue=
ss
this is implemented in that way so as to not copy data unnecessarily.

now, look at:

In [23]: a._byteoffset
Out[23]: 0

In [24]: a[1]._byteoffset
Out[24]: 40

So, you can know where the data actually starts by looking at the
_byteoffset property. In fact, you should always look at it in order to g=
et
proper results!.

In C, you can access to this information by looking at the byteoffset fie=
ld
in the PyArrayObject struct (look at chapter 10 in the User's Manual).

Hope that helps,

--=20
Francesc Alted