Re: [Help-gsl] problem

[caoj]

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."
> }
> 
> 
>