ODE45 for a system of diff eq with a vector eq

Hi,

I am trying to solve a 2 equations system of ODE where the first function is a 2x1 vector and the second a scalar. I tried a simple example but I get an error such as: "in an assignment A(I)=B, the number of elements in B and I must be the same"

also before I got an error claiming that the outcome of @rigid is a 2x1 and my input as initial conditions is a 3x1 and thus is wrong. So this means that matlab is not recognizing that my outcome y should be a 3x1. I dont know how to fix this.

My code is:

function dy = rigid(t,y);

rho1=zeros(2,1)) rho1(1,1)=0.5; rho1(2,1)=2; rho0=1; mu1=zeros(2,2); mu1(1,1)=2.2; mu1(1,2)=1.2; mu1(2,1)=0.4; mu1(2,2)=2; mu0=zeros(2,1); mu0(1,1)=1.2; mu0(2,1)=3.2;

dy=zeros(3,1);

dy(1)=rho1-mu1*y(1:2); dy(2)=rho0-mu0'*y(1:2);

[t,y] = ode45(@rigid,[0 12],[0 0 0]);

Any help with this?

thanks in advance.

Answer 1

[Mischa Kim]

Barbara, check out

 function odetest()
 [t,y] = ode45(@rigid,[0 12],[0 0 0]);
 plot(t,y)
 end

 function dy = rigid(t,y)
 rho1 = zeros(2,1);
 rho1(1,1) = 0.5;  rho1(2,1) = 2;  rho0 = 1;
 mu1 = zeros(2,2); mu1(1,1) = 2.2; mu1(1,2) = 1.2;
 mu1(2,1) = 0.4; mu1(2,2) = 2; mu0 = zeros(2,1); mu0(1,1) = 1.2;
 mu0(2,1) = 3.2;

 dy      = zeros(3,1);
 dy(1:2) = rho1 - mu1*y(1:2);   % Note 1
 dy(3)   = rho0 - mu0'*y(1:2);  % Note 2

1. The right-hand side is a 2-by-1 vector, which you are assigning to a scalar. Not sure what you need to do, but in my code the vector get's assigned to a 2-by-1, dy(1:2).
2. The second statement is syntactically correct (scalar to scalar), but since I assigned the 2-by-1 to the first two elements of dy, I assigned the scalar right-hand side to dy(3). Again, only you know what exactly needs to happen.

[Barbara]

Many many thanks Mischa Kim. For taking the time and for the good answer.It is fixed now.