Re: [Help-gsl] problem

[Pau Cervera Badia]

I think it means that gsl_odeiv_evolve_apply is trying to adjust h to 
obtain the prescribed presicion in the integration of the equations, but 
the step-size value seems too large to obtain that precision, so the 
routine chooses a smaller step (see [1]). It seems like if one or more 
of your variables or derivatives is unboundedly growing, so your 
solutions are diverging, but I'm not sure if it is the only reason 
because I don't know you're equations.

Maybe you can post your *func* function and we can take a look.

[1] http://www.gnu.org/software/gsl/manual/gsl-ref_25.html#SEC384

address@hidden wrote:

> Hi Pau Cervera Badia:
>
> I check h. I found that h is becoming smaller and smaller and finally get to 
> 0.0. What does it mean? Does it mean my program is wrong? Thanks again.
>
> Jun Cao
>
>
> Quoting Pau Cervera Badia <address@hidden>:
>
>  
>
> > Maybe is changing only a little.
> > Can you monitor the h values, something as
> >
> > while (t < t1) {
> >
> >    int status = gsl_odeiv_evolve_apply(e,c,s,&sys,&t,t1,&h,y);
> >
> >    printf("%.5e %.5e %.5e\n", t, t1, h);
> >
> >    if (status != GSL_SUCCESS)
> >       break;
> >
> > }
> >
> > will do. If h is becoming smaller and smaller, something wrong is happening
> > (maybe your ecuations are diverging somewhere).
> >
> >
> > address@hidden wrote:
> >
> >    
> >
> > > Hi Pau Cervera Badia:
> > >
> > > Thank you for your response. I check my program again. It stuck in 
> > >      
> > t=5.790250, 
> >    
> >
> > > t1=6.467000.  After gsl_odeiv_evolve_apply, t always is 5.790250 and does
> > >      
> > not 
> >    
> >
> > > change. So the program stuck in the "while" loop. I try to change h to 1e-2
> > >      
> > and 
> >    
> >
> > > 1e-10. I get the same result. Do you have anoter idea? Thanks again.
> > >
> > > Jun Cao
> > >
> > >
> > > Quoting Pau Cervera Badia <address@hidden>:
> > >
> > >
> > >
> > >      
> > >
> > > > gsl_odeiv_evolve_apply(e,c,s,&sys,&t,t1,&h,y) will advance from time t 
> > > > to the next integration time with an optimum step-size, provided that 
> > > > the next time is less than t1. Otherwise it will integrate the system to 
> > > > t1 exactly. So after some iterations the while condition will be false.
> > > >
> > > > You can check that gsl_odeiv_evolve_apply is running properly with:
> > > >
> > > > while (t < t1) {
> > > >
> > > >   int status = gsl_odeiv_evolve_apply(e,c,s,&sys,&t,t1,&h,y);
> > > >
> > > >   printf("%.5e %.5e\n", t, t1); // this should print t less than t1
> > > >
> > > >   if (status != GSL_SUCCESS)
> > > >      break;
> > > >
> > > > }
> > > > printf("%.5e %.5e\n",t,t1); // this should print t equal to t1
> > > >
> > > > If I'm not missing something, your program should work. Maybe the 
> > > > problem is that t is always less than t1 for some other reason (maybe 
> > > > the routine couldn't reach the prescribed precision and 
> > > > gsl_odeiv_evolve_apply is trying to reduce the step-size more and more). 
> > > > Maybe you can check the h values.
> > > >
> > > > Maybe something will be of any help.
> > > >
> > > > address@hidden wrote:
> > > >
> > > >   
> > > >
> > > >        
> > > >
> > > > > Hi:
> > > > >
> > > > > I use ordinary differential equations from GSL. The part of my program is
> > > > >     
> > > > >
> > > > >          
> > > > like 
> > > >   
> > > >
> > > >        
> > > >
> > > > > example shown in the document.
> > > > >
> > > > > const gsl_odeiv_step_type * T 
> > > > >  = gsl_odeiv_step_rk8pd;
> > > > >
> > > > > gsl_odeiv_step * s 
> > > > >  = gsl_odeiv_step_alloc (T, 9);
> > > > > gsl_odeiv_control * c 
> > > > >  = gsl_odeiv_control_y_new (1e-6, 0.0);
> > > > > gsl_odeiv_evolve * e 
> > > > >  = gsl_odeiv_evolve_alloc (9);
> > > > >
> > > > > double mu = 10;
> > > > > gsl_odeiv_system sys = {func, NULL, 9, &mu};
> > > > >
> > > > > double t = 0.0;  
> > > > > double h = 1e-6;
> > > > >
> > > > > for (i = 1; i <no_of_data_points; i++) {
> > > > >   t1 = mytable[i];
> > > > >   while (t < t1)
> > > > >   {
> > > > >      int status = gsl_odeiv_evolve_apply (e, c, s,
> > > > >                                         &sys, 
> > > > >                                         &t, t1,
> > > > >                                         &h, y);
> > > > >
> > > > >    if (status != GSL_SUCCESS)
> > > > >        break;
> > > > >    }
> > > > >    ...
> > > > > }
> > > > > gsl_odeiv_evolve_free (e);
> > > > > gsl_odeiv_control_free (c);
> > > > > gsl_odeiv_step_free (s);
> > > > >
> > > > > The weird thing is that at some i, the program stuck inside of "while"
> > > > >          
> > loop
> >    
> >
> > > > >     
> > > > >
> > > > >          
> > > > and 
> > > >   
> > > >
> > > >        
> > > >
> > > > > never come out. gsl_odeiv_evolve_apply will give t=t, so t is always <
> > > > >          
> > t1.
> >    
> >
> > > > >     
> > > > >
> > > > >          
> > > > As 
> > > >   
> > > >
> > > >        
> > > >
> > > > > result the program never stop. Dose anyone know how to solve this problem?
> > > > >          
> > > > >
> > > > > Thanks.
> > > > >
> > > > > Jun Cao
> > > > >
> > > > >
> > > > >
> > > > > _______________________________________________
> > > > > Help-gsl mailing list
> > > > > address@hidden
> > > > > http://lists.gnu.org/mailman/listinfo/help-gsl
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >     
> > > > >
> > > > >          
> > > > --
> > > > Pau Cervera i Badia (e-mail address@hidden)
> > > > {
> > > >  Departament de Física Fonamental                 Martí i Franqués, 1
> > > >  Universitat de Barcelona                   Planta 3, despatx 346 bis
> > > >                                                       08028 Barcelona
> > > >  tel: +34 934 921 155                                           Spain
> > > >
> > > >  "To err is human, but to really foul things up requires a computer."
> > > > }
> > > >
> > > >
> > > >
> > > >   
> > > >
> > > >        
> > >
> > >
> > >
> > >
> > >
> > >      
> > --
> > Pau Cervera i Badia (e-mail address@hidden)
> > {
> >   Departament de Física Fonamental                 Martí i Franqués, 1
> >   Universitat de Barcelona                   Planta 3, despatx 346 bis
> >                                                        08028 Barcelona
> >   tel: +34 934 921 155                                           Spain
> >
> >   "To err is human, but to really foul things up requires a computer."
> > }
> >
> >
> >
> >    
>
>
>
>
>
>  

--
Pau Cervera i Badia (e-mail address@hidden)
{
  Departament de Física Fonamental                 Martí i Franqués, 1
  Universitat de Barcelona                   Planta 3, despatx 346 bis
                                                       08028 Barcelona
  tel: +34 934 921 155                                           Spain

  "To err is human, but to really foul things up requires a computer."
}