Date: Dec 27, 2012 11:22 AM
Author: Qiming
Subject: ODE solver


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 (The following codes are only for the purpose of testing):

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;

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 think these two values should be the same. I also tried using a fixed stepsize, but they're still different. How come they're different?

Thanks for any help.