Re: Some (probably) ridiculous questions

[Jean-Max Redonnet <redonnet at lgmt-fab1 dot ups-tlse dot fr>]

Le Vendredi  6 Juillet 2001 17:35, Brian Gough a écrit :

<snip>

>
> Hello,
>
> The gsl_function type is mainly designed for interfacing to the
> univariate gsl routines.  It is not really a general facility so it
> does not handle other cases that are not used in the library, such as
> surfaces.
>
> The way to handle this sort of situation is,
>
> -- define your own surface function type in the way that you normally
> would in your C-programs
>
> -- if you need to use a gsl routine, for example to integrate along a
> line, define a function to create the appropriate gsl_function by
> mapping from your surface type.
>
> Philosophically, calls to GSL routines, and the GSL types, can be at a
> "lower-level" than your program.
>
> regards
> Brian Gough

Thanks for your help to a definitively poor programmer (and mathematician)

But I'm not sure I really understand what you mean...

I think I have defined a surface function type as you proposed.
This part seems to work as expected.
but I don't see how to define a function to create the appropriate 
gsl_function...

Here the code :

#include <stdlib.h>
#include <stdio.h>
#include <gsl/gsl_math.h>
#include <gsl/gsl_diff.h>

#define RAD2DEG(X) (X*180/M_PI)
#define DEG2RAD(X) (X*M_PI/180)

struct surf_function_struct {
  double (* fx)(double u, double v);
  double (* fy)(double u, double v);
  double (* fz)(double u, double v);;
};

typedef struct surf_function_struct surf_function ;

double C0_fx(double u)
{
  return 80*u;
}

double C0_fy(double u)
{
  return 80*tan(DEG2RAD(22.5))*(u-0.5);
}

double C0_fz(double u)
{
  return 40;
}

double C1_fx(double u)
{
  return 80*u;
}

double C1_fy(double u)
{
  return -80*tan(DEG2RAD(22.5))*(u-0.5);
}

double C1_fz(double u)
{
  return -40;
}

double S_fx(double u, double v)
{
  return (1-v)*C0_fx(u)+v*C1_fx(u);
}

double S_fy(double u, double v)
{
  return (1-v)*C0_fy(u)+v*C1_fy(u);
}

double S_fz(double u, double v)
{
  return (1-v)*C0_fz(u)+v*C1_fz(u);
}

int main (void)
{
  double u,v;
  surf_function surf;

  u = 0.5;
  v = 0.5;
  surf.fx = &S_fx;
  surf.fy = &S_fy;
  surf.fz = &S_fz;
  
  printf("              | %f \n", S_fx(u,v));
  printf(" S(%.1f,%.1f) = | %f \n", u, v, S_fy(u,v));
  printf("              | %f \n", S_fz(u,v));

  printf("              | %f \n", surf.fx(u,v));
  printf(" S(%.1f,%.1f) = | %f \n", u, v, surf.fy(u,v));
  printf("              | %f \n", surf.fz(u,v));
  
  return 0;
}

You would be my definitive heroe if you can help me on this point.

Thanks.
-- 
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