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Re: Questions on bsimp
- From: Slaven Peles <peles at cns dot physics dot gatech dot edu>
- To: <gsl-discuss at sources dot redhat dot com>
- Date: Thu, 1 Aug 2002 13:06:54 -0400
- Subject: Re: Questions on bsimp
On Thursday 01 August 2002 12:19 pm, you wrote:
> When I use bsimp as the stepper function, it seems it just jumps too each
> step. I need bsimp because the speed is an important issue in my project.
When using adaptive step size different integrators will produce different
number of intermediate values during integration from t to t1. bsimp produces
fewer intermmediate values so that is why it seems to you like your results
are messy. Have look at the second example in the documentation at
http://sources.redhat.com/gsl/ref/gsl-ref_25.html#SEC381
It's explained there how to "fix" this problem ;-).
Slaven
> --------------------------------------------------------------
>
> #include <stdio.h>
> #include <math.h>
> #include <gsl/gsl_errno.h>
> #include <gsl/gsl_matrix.h>
> #include <gsl/gsl_odeiv.h>
>
> int func (double t, const double y[], double f[], void *params)
> {
> f[0] = -y[1];
> f[1] = y[0];
> return GSL_SUCCESS;
> }
>
>
> int jac (double t, const double y[], double *dfdy, double dfdt[],
> void *params)
> {
> gsl_matrix_view dfdy_mat
> = gsl_matrix_view_array (dfdy, 2, 2);
> gsl_matrix * m = &dfdy_mat.matrix;
> gsl_matrix_set (m, 0, 0, 0.0);
> gsl_matrix_set (m, 0, 1, -1.0);
> gsl_matrix_set (m, 1, 0, 1.0);
> gsl_matrix_set (m, 1, 1, 0.0);
> dfdt[0] = 0.0;
> dfdt[1] = 0.0;
> return GSL_SUCCESS;
> }
>
> int main (void)
> {
> const gsl_odeiv_step_type * T
> = gsl_odeiv_step_rkf45;
>
> gsl_odeiv_step * s
> = gsl_odeiv_step_alloc (T, 2);
> gsl_odeiv_control * c
> = gsl_odeiv_control_standard_new(1e-30, 1e-10,
> 1.0, 1.0);
> gsl_odeiv_evolve * e
> = gsl_odeiv_evolve_alloc (2);
>
> gsl_odeiv_system sys = {func, jac, 2, NULL};
>
> double t = 0.0, t1 = 10.0;
> double h = 1e-6;
> double y[2] = {1.0, 0.0};
>
> while (t < t1)
> {
> int status = gsl_odeiv_evolve_apply(e, c, s,
> &sys,
> &t, t1,
> &h, y);
> if (status != GSL_SUCCESS)
> break;
> printf ("%.5e %.5e %.5e\n", t, y[0], y[1]);
> }
>
> gsl_odeiv_evolve_free (e);
> gsl_odeiv_control_free (c);
> gsl_odeiv_step_free(s);
> return 0;
> }
-------------------------------------------------------