??? Undefined function or variable 'x'

I am trying to use ode45 to solve a system of ODEs.

What I did was:

function Testfunction
tspan = [...]
%specify initial conditions
x0 = [...,...,...,...]
[t,x] = ode45(@somefn, tspan, x0)
%defining the output variables
y=x(:,1);
etc...
%plot stuff
plot(...)
return
%now define somefn
function g = somefn(t,x)
g = zeros(size(x));
g(1) = y;
etc...
return

But when I tried to run this code MATLAB keeps saying ??? Undefined function or variable 'x' in somefn

please could someone help me? Thanks.

Answer 1

[Wayne King]

Hi Richard, save your function

   function g = somefn(t,x)
   g = zeros(size(x));
   g(1) = y;
   end

in some folder on the MATLAB path and then call

   [t,x] = ode45(@somefn,tspan,x0);

at the command line.

You are going to have a problem though, because somefn.m has to know what y is. It looks like you are using ode45 to return x and then using x to compute y, but you never pass y to the function somefn.m.

You have to give somefn.m everything it needs. Perhaps you can define that y internally inside the function?? Although you appear to have some recursive situation going on where you need the output of ode45() to feedback into ode45()...

[Richard]

Thank you, Wayne. I am just trying to follow the template on pages 5-6 in http://www.me.berkeley.edu/~adila/ODE45.pdf. I still have no idea what has gone wrong?

How do I define y internally?

[Wayne King]

If you look at the template he gives you for the integrating function, everything that is computed in somefn.m (your name) is something that is either based a variable passed to the function, or something declared inside the function. Once ode45() returns x, then you can do things like

y = x(:,1);

you have inside of somefn.m

g(1) = y;

but somefn.m has no idea what y is. You do not pass y to somefn.m as an input.

[Richard]

Thanks, again, @Wayne!