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## Re: [Help-gsl] Interpolated Values as function return

 From: Foivos Diakogiannis Subject: Re: [Help-gsl] Interpolated Values as function return Date: Tue, 12 Nov 2013 05:26:26 +1100

```(that awkward moment, when the shortcut apperas and sends an incomplete
email... APOLOGIES!)

Dear all,

This is not a sophisticated example (I am a humble astronomer, not a
software engineer) but may be of help to Klaus.

Say you have initiated interpolation at some point in your code, and you
wan't to use this interpolation as a function in ta numerical integration
scheme.

// Define parameters for Function
struct gsl_params{
gsl_spline * splineE; // Pointer to spline GSL definitions.
gsl_interp_accel * accC; //accelerator for spline interpetation
};

// Degine kernel (function) that goes into the integration scheme.
double gsl_kernel(double r, void *p)
{
gsl_params * params = reinterpret_cast<gsl_params * > (p);
return
gsl_spline_eval (params->splineE, r, params->accC);
};

// Actual numerical integration function. Takes as argument the params
structure.
double gsl_integral( gsl_params * myparams){

gsl_integration_workspace * w    = gsl_integration_workspace_alloc
(1000);
double result, error;

gsl_function func { gsl_kernel, myparams}; // Function that is the
wrapper of the interpolation
gsl_integration_qags (&func, 0, 1, 0, 1e-7, 1000, w, &result,
&error);

gsl_integration_workspace_free (w);

return result;
};

All the best, my deepest apologies for previous incomplete email.
Cheers!
Foivos

On Tue, Nov 12, 2013 at 5:23 AM, Foivos Diakogiannis <

>
>
>
> On Tue, Nov 12, 2013 at 5:04 AM, Rhys Ulerich <address@hidden>wrote:
>
>> > But searching the internet told me, that it is not possible to get the
>> > address from a member function in C++!?
>>
>> I believe that's true but have never looked in-depth, however,...
>>
>> > ...you have to give the GSL function the address of the member
>> > function, in that case the foo::operator()().
>>
>> I'm suggesting storing a pointer to an instance in
>> gsl_function.params.  Then, in the function, retrieving that pointer,
>> casting it to the object type, and calling a method on the instance.
>>
>> > Uh, what do you mean with 'it definitly _does not_complie'? So my idea
>> is
>> > plain wrong?
>>
>> Not at all.  Just that my sample code wouldn't build.
>>
>> - Rhys
>>
>>
>

```