[Numpy-discussion] Anyone have a well-tested SWIG-based C++ STL valarray <=> numpy.array typemap to share?

[David Goldsmith]

Thanks!

DG

Lisandro Dalcin wrote:
> David, I'll try to show you what I do for a custom C++ class, of
> course this does not solve the issue resizing (my class does not
> actually support resizing, so this is fine for me):
>
> My custom class is a templatized one called DTable (is like a 2d
> contiguous array), but currently I only instantiate it for 'int' and
> 'double'. This class have two relevant methods: getShape(), returning
> a std::pair, and getArray(), returning a reference to an underliing
> std::vector. So I am able to automatically support array interface by
> using this:
>
> First, I define a templatized utility function 'array_interface'
>
> %header %{
> namespace numpy {
>
>   template<typename T> static char typechar() { return '\0'; }
>   template<> static char typechar<int>()      { return  'i'; }
>   template<> static char typechar<double>()   { return  'f'; }
>
>   template<typename T>
>   static PyObject*
>   array_interface(const DTable<T>* self)
>   {
>     const std::pair<int,int>& shape = self->getShape();
>     const std::vector<T>&     data  = self->getArray();
>     void* array = const_cast<T*>(&data[0]);
>     char endian = PyArray_NATIVE;
>     char kind   = typechar<T>();
>     int  elsize = sizeof(T);
>     return Py_BuildValue("{sNsNsNsN}",
> 			 "shape",   Py_BuildValue("ii", shape.first, shape.second),
> 			 "typestr", PyString_FromFormat("%c%c%d", endian, kind, elsize),
> 			 "data",    Py_BuildValue("NO", PyLong_FromVoidPtr(array), Py_False),
> 			 "version", PyInt_FromLong(3));
>   }
> }
> %}
>
> Now define a SWIG macro to apply it to instantiations of my class
>
> %define %array_interface(Class)
> %extend Class { PyObject* __array_interface__; }
> %{
> #define %mangle(Class) ##_## __array_interface__ ## _get(_t) \
>         numpy::array_interface(_t)
> #define %mangle(Class) ##_## __array_interface__ ## _set(_t, _val) \
>         SWIG_exception_fail(SWIG_AttributeError, "read-only attribute")
> %}
> %enddef
>
> and finally instantiate my class with different names for 'int' and
> 'double' in SWIG and finally apply previous macro
>
>
> %template(DTableI) DTable<int>;
> %template(DTableS) DTable<double>;
>
> %array_interface( DTable<int> );
> %array_interface( DTable<double> );
>
> I think you can implement someting similar for std::vector,
> std::valarray, or whatever... For use other data types, the only you
> need is to specialize the 'typecode' function.
>
> Hope this help you.
>
>
> On 9/4/07, David Goldsmith <David.L.Goldsmith at noaa.gov> wrote:
>   
>> Anyone have a well-tested SWIG-based C++ STL valarray <=> numpy.array
>> typemap to share?  Thanks!
>>
>> DG
>> --
>> ERD/ORR/NOS/NOAA <http://response.restoration.noaa.gov/emergencyresponse/>
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>>     
>
>
>