Solving a system of ODEs in MATLAB? - matlab

Question

this is my first time posting, so if you need anymore info or I havn't done something correct please let me know!
I want to plot a system of ODES, which would be no problem as I would do the following;
F=#(t,x) [
.the RHS of my first order ODES.
];
[t x]=ode45(F,[Range], [Initial conditions]);
Where a vector would be created containing x'(1), x'(2),... where x(1), x(2) would be the time dependant variable.
However my system is a little more complicated. I have 6 differential equations which are made up of equations that include differential equations and the time dependant variables. For example, the RHS of my ODES are something like
2*x(2)*x'(1)*f(a)
Where f(a) could be another function based on constants, a.
I have set up my code in the following order;
Constants, then equations of the form f(a), then my differential equations followed by the ode45 solver and the plot commands. However I am getting several errors "Undefined function or variable" as early equations depend on variables/equations that are not defined until later.
Thank you ever so much for you help :)
As suggested, here an example of the type of my code;
`%Constants
a=34
b=31
c=20
%Equations
A=b*cos(2*pi)
B=a*EQ1
C=c*x(2)
%DifferentialEquations
EQ1=x(1)*A
EQ2=(EQ3-EQ1)*(B-C)
EQ3=x(2)*x(3)
F=#(t,x) [EQ1;EQ2;EQ3;];[t x]=ode45(F,[0 10], [0 0 0 ]);
Provides the error.
Produces the undefined function or variable 'DEQ1A'.

Answer 1

Based on the comment you are trying to use a variable before it is defined. In MATLAB you need to define your variables and functions before you use them. Because the equations are functions of each other you can use anonymous functions to define F.
Example:
a=34;
b=31;
c=20;
A = b*cos(2*pi);
EQ1 = #(x) x(1)*A;
B = #(x) a*EQ1(x);
C = #(x) c*x(2);
EQ3 = #(x) x(2)*x(3);
EQ2 = #(x) (EQ3(x) - EQ1(x))*(B(x)-C(x));
F = #(t,x) [EQ1(x);EQ2(x);EQ3(x)];
[t,x] = ode45(F,[0 10], [0 0 0]);