[Numpy-discussion] ndarray of object dtype

Ioan Ferencik ioan.ferencik@tkk...
Wed Oct 6 01:27:29 CDT 2010


Thanks Robert,

that worked just great.


I was wandering whether anyone has experience with calling Fortran 90  
functions that return derived types and cast them to C/Python objects.

I have found excellent reference in a couple of websites, however all  
examples deal with returning at best 2D arrays of fundamental/basic  
types. When it comes to derived types/structures I have following  
dilemma:

should I use Fortran subroutines, allocate memory in C and pass the  
structures as arguments or should I use functions, allocate memory in  
Fortran and return the data to C/Python.

I should mention I have little experience in C/Fortran.


All thoughts are welcome.





Quoting Robert Kern <robert.kern@gmail.com>:

> On Mon, Oct 4, 2010 at 00:41, Ioan Ferencik <ioan.ferencik@tkk.fi> wrote:
>> Hello list,
>>
>> I am trying to pass elements of a custom defined with C API.
>> I have successfully wrapped a type around a C struct.
>> I intent to create a list with these objects and process it using   
>> numpy C API.
>> So i  create an array:
>>
>> array = (PyArrayObject *)PyArray_ContiguousFromObject(input,
>> PyArray_OBJECT, 0, 0);
>>
>> to my understanding each el. in this array is a pointer to my type so
>> a cast to this type
>> should work.
>>
>>
>> this is my custom type
>>
>> typedef struct val{
>>         PyObject_HEAD
>>         float q;
>>         float wl;
>>         int cssid;
>>         int br;
>> }hm1dval;
>>
>> I am passing only one element for testing purposes.
>>
>> in python
>> a = [hm1d1.hm1dval() for i in range(0,1)]
>> for c in a:
>>         c.set_values(q=3.0, wl=2.5, cssid=6, br=7)
>>
>>
>> So following code should be valid in C:
>>
>> hm1dval s* = PyArray_DATA(array);
>>
>> but the members are 0 after casting in spite they were set previously.
>
> The elements of a dtype=object array are PyObject* pointers. In C, an
> array of pointers would look like this:
>
> hm1dval **s = PyArray_DATA(array);
>
> --
> Robert Kern
>
> "I have come to believe that the whole world is an enigma, a harmless
> enigma that is made terrible by our own mad attempt to interpret it as
> though it had an underlying truth."
>   -- Umberto Eco
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Ioan Ferencik
PhD student
Aalto University
School of Science and Technology
Faculty Of Civil and Env. Engineering
Lahti Center
Tel: +358505122707




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