[SciPy-dev] C API

Nariman Habili n_habili at hotmail.com
Fri Nov 25 00:47:36 CST 2005


Travis and Robert, thanks very much for your help. I now have a better 
understanding of how to access ndarray elements from C++.

The application is mainly for image processing. I've been using boost.python 
to pass numeric arrays from Python to C++ by reference, eg

void FeatureDetector(numeric::array &input_image, numeric::array 
&feature_image);

Therefore I need to access every element of the input numeric array (in most 
cases representing images), calculate the image features (eg, corner points) 
in C++ and then output the modified image, again via reference. With the 
information you provided, I can now access the ndarray elements.

Travis, I have purchased a copy of your "Guide to SciPy: Core System", some 
sections seem to be missing, for example section 13.2, "Using the Array 
iterator in C". I would appreciate it if you could explain to me how I can 
use the array iterator.

I'm currently using boost.python to expose C++ classes and methods to 
Python. I've also used swig in the past. I would like to know how I can 
expose C++ classes and methods to Python using just the C API. Any examples 
or references would be appreciated. Sorry if this question sounds obvious; 
I'm new to the C API.

Nariman

>From: Travis Oliphant <oliphant at ee.byu.edu>
>Reply-To: SciPy Developers List <scipy-dev at scipy.net>
>To: SciPy Developers List <scipy-dev at scipy.net>
>Subject: Re: [SciPy-dev] C API
>Date: Thu, 24 Nov 2005 01:53:11 -0700
>
>Nariman Habili wrote:
>
> >Hi there,
> >
> >I would like to know how to extract a value (eg int) from a PyObject that
> >contains an ndarray. I'm using boost.python to expose classes to Python. 
>I'm
> >passing an ndarray to C++ by reference. However, since boost.python 
>hasn't
> >been updated yet to scipy core, I'm using the C API.
> >I've tried PyArray_GETITEM, however it returns a PyObject, and I'm not 
>sure
> >how to extract an int or double etc from it. I need the C types so I can
> >interface python with other parts of my code.
> >
> >
>
>PyArray_DATA(obj)  gives you a pointer to the first element of the ndarray.
>This assumes that obj is actually an ndarray object, of course.
>
>The data flag bits in PyArray_FLAGS(obj) are critical to dealing with
>the data correctly.
>
>It is possible that the data is not in machine byte order, not aligned
>properly (which can cause
>BUS errors on some platforms if you try certain operations), and not
>writeable.  Of course in C
>you can do whatever you want, but if you don't respect the flags, you
>may get gibberish and
>even segfault your machine.
>
>If PyArray_CHKFLAGS(obj, BEHAVED_FLAGS) is true then you don't have to
>worry about the above paragraph, otherwise you do
>
>How to get elements beyond the first depends on PyArray_STRIDES(obj) and
>PyArray_DIMS(obj)
>
>If PyArray_CHKFLAGS(obj, CARRAY_FLAGS) is true then access to elements
>beyond the first is simple
>as the data region is a simple contiguous chunk of memory with each
>element placed right after the next in C-style contiguous fashion (last
>index varies the fastest).
>
>Otherwise you need to understand the concept of striding in order to
>access elements beyond the first.
>
>There is also the Array iterator which you can construct and use the
>PyArray_ITER_GOTO() macro to jump to a particular element in the array
>(regardless of the flags).  If the array is not BEHAVED, then you need
>to be careful about how you access it, but the dataptr field of the
>array iterator structure will point to the right place.
>
>So, is this all clear as mud?   If you were more clear about which
>element of the array you wanted and how the array was created, I could
>be more directly helpful instead of exposing you to all the possibilities.
>
>-Travis
>
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