Basic how-to for C - Swig - Python/Numpy interfacing

cfunction.c

void demo(size_t nr, size_t nc, double *arr)
{
  for(int r(0); r < nr; ++r)
    for(int c(0); c < nc; ++c)
      arr[r*nc+c] = 1.; // fills arr[r][c]
}

python.py

#import demo

a = np.zeros((10,10))

demo.demo(a)

print a

swig.i

/* Source.i */

/* module name needs be the name of the VS project in current setup */
%module demo
%{
/* only once in project/i files */
#define SWIG_FILE_WITH_INIT
/* headers as #include header.h or explicit declarations, only of functions that need interface */
extern void demo(size_t nr, size_t nc, double *arr);
%}

/* only once in project / i file */
%include "numpy.i"
%init %{
import_array();
%}
 
/* declare what interfaces to replace how - parameter names must be unique, replaced as given here for all */
%apply (int DIM1, int DIM2, double* INPLACE_ARRAY2) {(size_t nr, size_t nc, double *arr)};

/* now all that need interfaced, and replaced by above swig signature */
extern void demo(size_t nr, size_t nc, double *arr);