[Numpy-discussion] numpy C++ swig class example

[Bill Spotz]

Jared,

Typically, public data members would be handled in SWIG with, I  
believe, the "out" typemap.  numpy.i does not support this typemap  
because there is no general way that I know of to tell it that, for  
example, _nrow and _ncol are the dimensions of data.

Faced with this problem, and a desire to have a "data" attribute, I  
would probably do the following (ignoring the fact that the  
constructor has an argument named "data" that clashes with the public  
variable):

%ignore Array2D::data;
%apply (int DIM1, int DIM2, double* IN_ARRAY2) {(int nrow, int ncol,  
double *data)};
%include "Array2D.h"
%extend(Array2D)
{
   PyObject * _getData()
   {
     npy_intp dims[2] = {self->_nrow, self->_ncol};
     return PyArray_SimpleNewFromData(2, dims, NPY_DOUBLE, (void*)data);
   }
}
%pythoncode
%{
     data = property(_getData, doc="docstring for data")
%}

You'll want four spaces as indentation within the %pythoncode  
directive to ensure it fits properly under the Array2D python proxy  
class.  This should allow getting the entire array and setting and  
getting elements of the array:

 >>> a = Array2D([[1,2],[3,4]])
 >>> a.data[0,0] = 5
 >>> print a.data
[[5 2]
  [3 4]]
 >>> print a.data[1,1]
4

I don't think I would do this for an a class that advertises itself as  
an array (for this case, I would make sure that an instance of Array2D  
behaves like an array rather than its "data" attribute), but I could  
see other classes that might have an attribute that is an array, where  
you would want to do it.

On Jun 9, 2009, at 10:52 PM, Rubin, Jared wrote:

> I am using the numpy.i interface file and have gotten the cookbook/ 
> swig example to work from scipy.
> Are there any examples of appyling the numpy.i to a C++ header file.  
> I would like to generate a lightweight
> Array2D class that just uses doubles and would have the following  
> header file
>
> Array2D.h
> =========
> class Array2D {
>   public:
>     int _nrow;
>     int _ncol;
>     double* data;
>   Array2DD(int nrow, int ncol, double *data);
> }
>
> // I would expect the have the following Array2D.i interface file
> %module array2DD
>
> %{
> #define SWIG_FILE_WITH_INIT
> #include "Array2D.h"
> %}
>
> %include "numpy.i"
>
> %init %{
> import_array();
> %}
>
> %ignore Array2D();
> %ignore Array2D(long nrow, long ncol);
> %apply (int DIM1, int DIM2, double* IN_ARRAY2) {(int nrow, int ncol,  
> double *data)}
> %include "Array2D.h"
>

** Bill Spotz                                              **
** Sandia National Laboratories  Voice: (505)845-0170      **
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