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Re: [Help-gsl] problem

From: Pau Cervera Badia
Subject: Re: [Help-gsl] problem
Date: Wed, 24 Aug 2005 09:29:39 +0200
User-agent: Mozilla Thunderbird 1.0.6-1.1.fc3 (X11/20050720)

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)

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:


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)
 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

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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."

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