Help with ode45 error.

I have a higher order system of ode's that I am trying to solve using ode45. I am receiving an error:

Subscript indices must either be real positive integers or logicals.

My two files are below, followed by a complete list of errors.

function xdot = fdynamics(t,y)

F = 0;
T = 0;
m1 = 2;
m2 = 3;
g = 9.81;
l1 = 1;
l2 = 1;

xdot(1) = y(2);

xdot(2) = (F-m2*g*sin(y(3)))/m2 + (l1+y(1))*y(4)^2;

xdot(3) = y(4);

xdot(4) = (T-(2*m2*(l1+y(1)))*y(4)*y(2) - g*cos(y(3))*(m1*(l1/2)+m2*(l1+y(1))) /((1/3)*m1*(l1)^2 + (1/12)*m2*l2^2 + m2(l1+y(1))^2));

xdot = xdot';

end

//AND//

clc
clear

options = odeset('Refine',1','MaxStep',1/60);

[t,y]=ode45(@fdynamics,[0 5],[0; 0; 0; 0],options);

plot(t,y(:,1),'-x')

title('D vs t');

    xlabel('Time t');

    ylabel('D');

    legend('D')

figure

    plot(t,y(:,2),'-x')
    title('D-Dot vs t');
    xlabel('Time t');
    ylabel('D-Dot');
    legend('D-Dot')

    figure

    plot(t,y(:,3),'-x')
    title('Theta vs t');
    xlabel('Time t');
    ylabel('Theta');
    legend('Theta')

    figure

    plot(t,y(:,4),'-o')
    title('Theta-Dot vs t');
    xlabel('Time t');
    ylabel('Theta-Dot');
    legend('Theta-Dot')

//ERRORS//

Subscript indices must either be real positive integers or
logicals.

Error in fdynamics (line 16)
xdot(4) = (T-(2*m2*(l1+y(1)))*y(4)*y(2) -
g*cos(y(3))*(m1*(l1/2)+m2*(l1+y(1))) /((1/3)*m1*(l1)^2 +
(1/12)*m2*l2^2 + m2(l1+y(1))^2));

Error in ode45 (line 262)
    f(:,3) = feval(odeFcn,t+hA(2),y+f*hB(:,2),odeArgs{:});

Error in attempt2 (line 9)
[t,y]=ode45(@fdynamics,[0 5],[0; 0; 0; 0],options);

Answer 1

[Star Strider]

You’re missing a multiplication operator:

xdot(4) = (T-(2*m2*(l1+y(1)))*y(4)*y(2) - g*cos(y(3))*(m1*(l1/2)+m2*(l1+y(1))) /((1/3)*m1*(l1)^2 + (1/12)*m2*l2^2 + m2*(l1+y(1))^2));
                                                                                                                      ↑ — INSERT MULTIPLICATION OPERATOR HERE

Without the operator (I assume you intend multiplication), MATLAB assumes ‘m2’ is an array, and throws that error.

[Christopher Secrest]

Thank you, I've been stuck on this system for weeks.

[Star Strider]

My pleasure.

You have my sympathies. If it makes you feel any better, I’ve had the same sort of problem, and I’m confident many if not most here have.