[Numpy-discussion] Numeric and swig (or, how to wrap functions).

Roger Hansen rogerha at ifi.uio.no
Fri Jan 5 08:48:15 CST 2001


* Roy Dragseth

[snip Numeric and swig questions about how to give a numpy array to a
swigged c function]

> Any hints is greatly appreciated.

I've done this a few times. What you need to do is help swig
understand that a numpy array is input and how to treat this as a C
array. With swig you can do this with a typemap. Here's an example:

we can create a swig interface file like

%module exPyC

%{
#include <Python.h>
#include <arrayobject.h>   /* Remember this one */
#include <math.h>
%}

%init %{
        import_array()   /* Initial function for NumPy */
%}

%typemap(python,in) double * {
  PyArrayObject *py_arr;

  /* first check if it is a NumPy array */
  if (!PyArray_Check($source)) {
    PyErr_SetString(PyExc_TypeError, "Not a NumPy array");
    return NULL;
  } 
  
  if (PyArray_ObjectType($source,0) != PyArray_DOUBLE) {
    PyErr_SetString(PyExc_ValueError, 
                    "Array must be of type double");  
    return NULL;
  }

  /* check that array is 1D */
  py_arr = PyArray_ContiguousFromObject($source, 
                                        PyArray_DOUBLE, 1, 1);

  /* set a double pointer to the NumPy allocated memory */
  $target = py_arr->data; 
}
/* end of typemap */

%inline %{

void arrayArg(double *arr, int n)
{
  int i;
  
  for (i = 0; i < n; i++) {
    arr[i] = f(i*1.0/n);
  }
}
%}

Now, compile your extension, and test

>>> import exPyC
>>> import Numeric
>>> a = Numeric.zeros(10,'d')
>>> a
array([ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.])
>>> exPyC.arrayArg(a,10)
>>> a
array([ 0.        ,  0.00996671,  0.0394695 ,  0.08733219,  0.15164665,  0.22984885,
             0.31882112,  0.41501643,  0.51459976,  0.61360105])


You can also get rid of the length argument with a
typemap(python,ignore), see the swig docs. For more info about the
safety checks in the typemap check the numpy docs.

Lykke til! :-)


HTH,
Roger




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