[SciPy-user] question about ODE output and time steps

[Anne Archibald]

On 26/04/07, Trevis Crane <t_crane at mrl.uiuc.edu> wrote:

> [Trevis Crane]
>
> I assumed it was fixed since you supply the dt value (or in the case of
> odeint, an array of t values).  By you statement above, though, I assume
> these are simply to tell the solver at what times you want y information
> (much like MATLAB's solvers).
>
> What I really want to do, however, requires that I stop the solver at
> some iteration step that, a priori, is unknown.  I'm simulating a
> dynamic system and my condition for stopping the solver is when the
> energy of the system stops changing beyond a specified threshold.  So
> each iteration of the solver I calculated the energy, and if after so
> many iterations it stops changing (much), I want the solver to stop.

I had essentially the same need. I tried various hacks - it turns out
odeint actually allows you to backtrack within one integrator step -
but they failed and were a pain to use. I came up with a wrapper for
ODEPACK's LSODAR, which has a stop-on-contstraint function that does
exactly the right thing but is otherwise identical to LSODA that
odeint uses. But PyDSTool did it right, apparently (I haven't used
it).

Anne