Re: Problems in ODE solver

"Qiming " <qzfyc@mst.edu> wrote in message <kbhsn8$sa5$1@newscl01ah.mathworks.com>...
> Hi:
>
> I have a question using ODE solvers (say, ode45).
>
> I want to save some parameter values inside the ODE function. For instance, I would like to save parameter "u" which is inside the ODE function for future use. I define the global "u", and also a counter "COUNT".
>
> As I know, ode45 is essentially a "loop", it integrates the ODE function as an adaptive stepsize or a pre-defined fixed step size. The functions are given as below:
>
> Main function:
>
> global u COUNT;
> COUNT = 0;
>
> t0 = 0; tf = 5;
> tSpan = [ t0, tf ];
> y0 = [ 1; 2; 3 ];
> [t y] = ode45(@myode, tSpan, y0);
>
>
>
> ODE function:
>
> function ydot = myode(~,y)
>
> global u COUNT;
> COUNT = COUNT + 1;
>
> a = 0.01;
> b = 0.1;
> ydot (1) = -a*y(1)*y(2);
> ydot (2) = b*y(1)*y(2)-b*y(2);
> ydot (3) = b*y(2);
>
> u(COUNT) = y(1)+y(2)+y(3);
>
> ydot = [ydot (1) ydot (2) ydot (3)]';
>
> The code can be ran without errors or warnings. But the things is, when I checked the workspace, "COUNT = 61", but "length(y) = 41". I also tried using a fixed stepsize, but they're still different. How come they're different?I think these two values should be the same.
>
> Thanks for any help.
>

COUNT is always greater than "length(y)", it seems that ode45 ignores some points where the function is evaluated, it only returns a less number of points. BUT I really want to get the values at each specific points of length(y), not for all the COUNT points. How can I do that?
Thanks a million.