ode45 system of equations

I used the lotkademo file as a template for solving a set of ODEs

When I change the initial conditions or time span the program still returns a solution vector from 0<t<15 (which was the default for the demo file) I've edited the function containing the equations as well and can't seem to see why these don't change. code below:

t0 = 0;

tfinal = 20;

y0 = [500 10]';

tfinal = tfinal*(1+eps);

[t,y] = ode45('MonodMod',[t0 tfinal],y0,[T Y]);

[T Y]

Given a function called MonodMod:

function yp = MonodMod(t,y)

yp = diag([- ((1*y(1))/(.5+y(1))) + 0 + 0,((1*y(1)*.8)/(.5+y(1))) - (0.7*.8) - (0.3*.8/1.42)])*y;

So when I update values for y0 or tfinal and evaluate [T Y] the ouput is still 0<t<15, and the initial values are still [20 20]..

Answer 1

[ME]

I have just been over this and found that

t0=0;
tfinal=20;
y0=[500 10]';
tfinal-tfinal*(1+eps);
[t,y]=ode45('MonodMod',[t0 tfinal],y0);
[t,y]

with the function

function yp = MonodMod(t,y)
yp = diag([- ((1*y(1))/(.5+y(1))) + 0 + 0,((1*y(1)*.8)/(.5+y(1))) - (0.7*.8) - (0.3*.8/1.42)])*y;
end

seems to work for me for a range of tfinal choices.

Answer 2

[Jan]

This is a very strange example. Using strings to provide the function to be integrated is outdated for 15 years now. Appending [T, Y] as parameter is deprecated for the same time also - see http://www.mathworks.com/matlabcentral/answers/1971.

[T,Y] is not defined or used anywhere. So what it is purpose? All you need and use is [t,y].

Perhaps this helps:

t0     = 0;
tfinal = 20;
y0     = [500 10]';
tfinal = tfinal*(1+eps);  % Are you sure that this is useful?
[t,y]  = ode45(@MonodMod, [t0 tfinal], y0);

function yp = MonodMod(t, y)
k  = -0.72901408450704225;  % -0.56 -(0.024 / 1.42)
yp = [-(y(1)      / (0.5 + y(1))      * y(1); ...
       (y(1)*0.8  / (0.5 + y(1)) + k) * y(2)];