MATLAB: ??? Undefined function or variable ‘x’

MATLAB

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.

Best Answer

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()...

Related Solutions

MATLAB: Plot just y or y’ in a second order eq.

syms y(t) x(t)
[V] = odeToVectorField(diff(y, 2) == -1*(1-1*cos(2*t))*y);
M = matlabFunction(V,'vars', {'t','Y'});
[ty,y] = ode45(@(t,Y)M(t,Y),tspan,[0 1]);
[tx,x] = ode45(@(t,Y)M(t,Y),tspan,[1 0]);
hold on
plot(tx, x(:,1));
plot(ty, y(:,2));
hold off
legend({"x", "y'"})

MATLAB: How to pass out extra parameters using ODE23 or ODE45 from the MATLAB ODE suite

There is no method for passing out additional parameters from the ODE function using the ODE solvers provided in the MATLAB ODE suite (e.g., ODE45). These additional parameters, however, can be passed out after ODE45 returns a solution.

For example, consider the ODE:

 y' = x + k*y

where

 k = x^2 + y^2

If you have a need to obtain "k" for specified "x" and "y", you can create the following ODE function:

function [dydx k] = myode(x, y)
k = x.^2 + y.^2;
dydx = x + k.*y;

The solution to the ODE can be solved using the commands:

[X Y] = ode45(@myode, [0 5], 1);

Then, "k" can be obtained via the command:

[dydx k] = myode(X, Y);

If you would like to obtain a series of values that are computed during the solution algorithm, you can declare a variable as PERSISTENT, and append the value computed during each call to the ODE function to the end of this persistent variable.

For example, consider the previous ODE. If you would like to obtain a vector of the "x" and "y" values specified for each call to the ODE function, try the following ODE function:

function [dydx xvt yvt] = myode(x, y)
persistent xv yv
xv = [xv; x];
yv = [yv; y];
k = x.^2 + y.^2;
dydx = x + k.*y;
if nargout>1
      xvt = xv; yvt = yv;
end

The solution to the ODE can be solved using the commands:

[X Y] = ode45(@myode, [0 5], 1);

Then, "xv" and "yv" can be obtained via the command:

[dydx xv yv] = myode([], []);

You could also use the ASSIGNIN, EVALIN, or SAVE functions. The ASSIGNIN function allows you to assign variables into workspace, and the EVALIN function evaluates an expression in the workspace. The SAVE function saves workspace variables to disk. The following example demonstrates how to assign the variable 'dy' from the ODEFUN to the workspace. Save the following into a function file named "odefun.m":

function dy =odefun(t,y)
dy=1;
varToPassOut=2;
assignin('base','varInBase',varToPassOut);
evalin('base','varPassedOut(end+1)=varInBase');

After writing this function, execute the following at the MATLAB command prompt:

varPassedOut=0
[T,Y] = ode45(@odefun,[0 12],[0]);
varPassedOut=varPassedOut(2:end);

Note that you need to define the variable "varPassedOut" in the base workspace before calling ODE45, or you will receive an error.

Best Answer

Related Solutions

MATLAB: Plot just y or y’ in a second order eq.

MATLAB: How to pass out extra parameters using ODE23 or ODE45 from the MATLAB ODE suite

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