[C++-sig] Retrieving lvalues using custom converters (numpy 1-D converters attached)

[Ravi]

On Thursday 02 October 2008 07:41:04 Neal Becker wrote:
> This is very interesting!  But how do I use it?
>
> What I tried:
> your numpy.h (but comment out)
>  #ifndef IN_FILE_IMPORTING_ARRAY
>  #define NO_IMPORT_ARRAY
>  #endif
>  #define PY_ARRAY_UNIQUE_SYMBOL Wicket_Numpy

Unless you removed the above for simplicity in you trivial module, you should 
generally have something like the above if you plan to include numpy.h from 
multiple source files for a single extension module; please see the numpy book 
for details.

> My code:
> ,----[ /home/nbecker/numpy.new/num.cc ]
>
> | #include "numpy.h"
> | #include <boost/python/module.hpp>
> | #include <boost/python/def.hpp>
> | #include <numeric>
> |
> | namespace ublas=boost::numeric::ublas;
> | using namespace boost::python;
> |
> | double do_sum (ublas::vector<double> const& u) {
> |   return std::accumulate (u.begin(), u.end(), 0.0);
> | }

Replace the above with:
double do_sum( numpy::array_from_py<double>::type u ) { ... }

The reason is that arrays coming *from* numpy are converted to this type which 
is actually ublas::vector< double, numpy_storage_array<double> >. The 
allocator (numpy_storage_array<double>) is what converts storage in a 
PyArrayObject to something that ublas can use. I expect that the argument type 
above will be used only for functions dealing with values from python. I have 
attached an example of using numpy.h. Note that, in dump_vec(), a "regular" 
ublas vector is subtracted from the numpy vector with the results visible in 
python.

Also, FYI, since you pass by const reference, you cannot modify the contents 
of 'u', but if you pass by value, you can. Passing by value costs you the two 
pointers instead of one but nothing more since the underlying storage is 
reference counted.

> |
> | BOOST_PYTHON_MODULE (num) {
> |   import_array();
> |   numpy::register_ublas_from_python_converters();
> |   numpy::register_default_ublas_to_python();
> |   def ("do_sum", &do_sum);
> | }
>
> `----
> ,----[ /home/nbecker/numpy.new/test_num.py ]
>
> | from num import *
> | from numpy import *
> |
> | u = array (xrange(10), dtype=float64)
> | v = do_sum (u)
>
> `----

Regards,
Ravi

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