Question regarding ode45 utilization

Hey, I'm working on a homework assignment that requires the use of the ode45 function, and I'm having some trouble wrapping my head around it. Here's my code so far:

t = 0:0.1:50; %timescale

[t,y] = ode45('MyStates', t, [2 -0.5 1.047 -2.094]);

and here's the MyStates function:

function dy = MyStates(t,y)

dy = zeros(4,1);

dy(1) = x(2);

dy(2) = (-14/50)*x(1) + ((14*10)/50)*theta(1);

dy(3) = theta(2);

dy(4) = ((3*14)/(50*10))*x(1) - ((6*14)/50)*theta(1);

end

The function keeps returning the message "Error: Function definitions are not permitted in this context." I really don't know what that means and what I'm doing wrong, let alone how to fix it, so I've been spinning my wheels for the past 30 minutes.

Thanks in advance!

Answer 1

[Star Strider]

You have to save your ‘MyStates’ function to its own separate function file, ‘MyStates.m’.

You need to add ‘theta’ as an argument because it will not pick it up from the workspace.

function dy = MyStates(t,y,theta)
dy = zeros(4,1);
dy(1) = x(2);
dy(2) = (-14/50)*x(1) + ((14*10)/50)*theta(1);
dy(3) = theta(2);
dy(4) = ((3*14)/(50*10))*x(1) - ((6*14)/50)*theta(1);
end

Your ode45 call needs to change as well:

[t,y] = ode45(@(t,y) MyStates(t,y,theta), t, [2 -0.5 1.047 -2.094]);

The ODE function argument to ode45 is now an anonymous function that will pick up ‘theta’ from your workspace and pass it to ‘MyStates’, as well as make ode45 happy with the version of ‘MyStates’ it sees.

I didn’t run your code but this should work.

Answer 2

[Torsten]

Replace x(1),x(2),theta(1) and theta(2) in MyStates by y(1),y(2),y(3) and y(4).

Best wishes

Torsten.