Hi, please keep list in cc in replies. OK I see, you want single steps, sorry my confusion. The way to carry out a single adaptive step via driver is show in test.c, function sys_driver(). After allocation of gsl_odeiv2_driver *d, you can call evolve_apply like so: s = gsl_odeiv2_evolve_apply (d->e, d->c, d->s, sys, &t, t1, &h, y);
BR, Tuomo On Wed, Mar 31, 2021 at 5:52 PM Tuomo Keskitalo <tuomo.keskit...@iki.fi> wrote: > Hello Andrei, > > I think you could use the driver and let it handle step size control. You > can check the usage examples here: > http://www.gnu.org/software/gsl/doc/html/ode-initval.html#examples > If you want to see more examples, there are some functions in the test.c > file: https://git.savannah.gnu.org/cgit/gsl.git/tree/ode-initval2/test.c > > The idea of the driver level is to hide the low-level objects from the > users who don't need to have manual control over solution process. You can > access them via the driver struct if needed. The driver level is documented > here: http://www.gnu.org/software/gsl/doc/html/ode-initval.html#driver > > BR, > Tuomo > > > On Tue, Mar 30, 2021 at 4:59 PM Andrei Kramer <andre...@kth.se> wrote: > >> Hello, >> I have a question regarding ODE steppers. I would like to integrate a >> stiff system, and obtain output which is fine grained where needed and >> has bold time steps where the trajectory is more flat. This is ideal >> for plotting the result I think, and gives me some insight into which >> step sizes the integrator thought reasonable. >> >> So, I wanted to try to use the low-level evolve_apply functions (is >> that the right way to go about this?). But the really good solvers for >> stiff problems require a driver object, which allocates all of the >> arguments for the evolve_apply function internally. I don't see any way >> to access those objects. So, I don't know how to call the evolve_apply >> function at all, other than making those objects a second time >> (manually) and hoping that they are similar enough (i.e. that I don't >> specify two very different control functions by accident). It just >> seems unatural to have the driver internal step/control function and >> another (manually made) step/control function. >> >> Should I... not try to obtain adaptively spaced output and just pick a >> fixed time-step for the trajectory? (this is what I'm doing now) >> >> This would probably mean that I won't know which steps the solver made. >> Currently I pick a safe (small) step size so that the plotted curves >> look smooth enough. >> >> I suspect that I don't fully understand the reasoning behind the odeiv2 >> interface. >> >> Best, >> Andrei Kramer >> >> >> > > -- > tuomo.keskit...@iki.fi > http://iki.fi/tuomo.keskitalo > -- tuomo.keskit...@iki.fi http://iki.fi/tuomo.keskitalo
