Hi,

I am using Matlab for a class, and unfortunately, I don't have much experience in it.

For my problem, I need to solve an ordinary differential equation using Matlab's built-in ODE45 function. Using some tutorials, I came up with the following code to solve and plot my function.

function jtd
[t,x] = ode45(@dfile,[0,20],[0;0]);
plot(t,x(:,1))
title('nonlinear')
xlabel('t'), ylabel('y'), grid
function xprime = dfile(t,x)
F=13.4;
a=0.1;
xprime = zeros(2,1);
xprime(1) = x(2);
xprime(2) = F*cos(t) - a*x(2) - x(1)^3;


It works well, but I need to be able to vary F and a using a loop to get multiple plots as the two variables change. Yet I can't for the life of me figure out how to get a variable from the main function JTD into the function xprime when I call the ode45 function.

If I could figure that out, it would help me immensely in continueing my program.

You can rewrite the code in this way:

F=13.4;
a=0.1;
fun=@(x,t) dfile(x,t,F,a);
[t,x] = ode45(fun,[0,20],[0;0]);
plot(t,x(:,1))
title('nonlinear')
xlabel('t'), ylabel('y'), grid

function xprime = dfile(t,x,F,a)
xprime = zeros(2,1);
xprime(1) = x(2);
xprime(2) = F*cos(t) - a*x(2) - x(1)^3;

I define the function handle of dfile out of the ode45 function. In this way you can pass the parameter to the function.

Now you can make a for cicle to make different plots, for example:
F=[12.3 13.4];
a=[0.2 0.1];
for i=1:length(F)
figure();
fun=@(x,t) dfile(x,t,F(i),a(i));
...
end

I add the function figure() so that plots on different figures.

That worked like a charm, except for the function name being chopped off, in case anybody reading decides to copy and paste what you wrote for me.

From here on out, it should be smooth sailing. Thanks, Giug!

I'm happy that it runs as you wanted!

How would removing the cos(t) change the code? I am doing something similar but in my diff eqns there are no cos(t) or anything similar.