[Numpy-discussion] [NumPy/Swig] Return NumPy array with same size as input array (no additional length argument)

[Chris Barker]

On Fri, Oct 30, 2015 at 11:15 PM, laurentes <laurentelshafey at gmail.com>
wrote:

> Using Swig, I don't manage to (properly) create the Python Binding for the
> following C-like function:
>
> void add_array(double* input_array1, double* input_array2, double*
> output_array, int length);
>
> where the three arrays have all the same length.
>

do you have to use SWIG? this would be really easy in Cython....

cdef cdef extern from "your_header.h":
    void add_array(double* input_array1,
                   double* input_array2,
                   double* output_array,
                   int length)


def py_add_array( np.ndarray[double, ndim=1] arr1,
                  np.ndarray[double, ndim=1] arr2):

    cdef int length

    if arr1.shape != arr2.shape:
        raise ValueError("Arrays must be the same size")

    length = arr1.shape[0]

    cdef np.ndarray[double, ndim=1] out_arr = np.empty((length),
dtype=np.float64)

    add_array(&arr1[0],
              &arr2[0],
              &out_arr[0],
              length)

    return out_arr


Untested and from memory -- but you get the idea.

-CHB


> >
> > This is similar to  this thread
> > <
> http://numpy-discussion.10968.n7.nabble.com/Numpy-SWIG-td25709.html#a25710
> >
> > , which has never been fully addressed online.
> >
> > From Python, I would like to be able to call:
> >
> add_array(input_array1, input_array2)
>
> which would return me a newly allocated NumPy array (output_array) with the
> result.
>
> In my Swig file, I've first used the wrapper function trick described  here
> <
> http://web.mit.edu/6.863/spring2011/packages/numpy_src/doc/swig/doc/numpy_swig.html#a-common-example
> >
> , that is:
>
> %apply (double* IN_ARRAY1, int DIM1) {(double* input_array1, int length1),
> (double* input_array2, int length2)};
> %apply (double* ARGOUT_ARRAY1, int DIM1) {(double* output_array, int
> length3)};
>
> %rename (add_array) my_add_array;
> %exception my_add_array {
>     $action
>     if (PyErr_Occurred()) SWIG_fail;
> }
> %inline %{
> void my_add_array(double* input_array1, int length1, double* input_array2,
> int length2, double* output_array, int length3) {
>   if (length1 != length2 || length1 != length3) {
>       PyErr_Format(PyExc_ValueError,
>                    "Arrays of lengths (%d,%d,%d) given",
>                    length1, length2, length3);
>   }
>   else {
>     add_array(input_array1, input_array2, output_array, length1);
>   }
> }
> %}
>
> This allows me to call the function from Python using
> add_array(input_array1, input_array2, length). But the third argument of
> this function is useless and this function does not look 'Pythonic'.
>
> Could someone help me to modify my Swig file, such that only the first two
> arguments are required for the Python API?
>
> Thanks a lot,
> Laurent
>
>
>
> --
> View this message in context:
> http://numpy-discussion.10968.n7.nabble.com/NumPy-Swig-Return-NumPy-array-with-same-size-as-input-array-no-additional-length-argument-tp41601.html
> Sent from the Numpy-discussion mailing list archive at Nabble.com.
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