Array indices must be positive integers or logical values.

I have a function that keeps giving me this error in line 2. I am trying to plot a third order differential equation. I was previously experiencing a different error, but now I have this. I am creating a function and then calling it when I use ode45. Thank you in advance!

function dYdt = problem03ODEFunction(t,y)
    x1 = y(t);
    X2 = dx1/dt == dy/dt;
    X3 = dx2/dt == d2y/dt2;
    f(t) = heaviside(t-1) - heaviside(t-10);
    dx3dt = d3y/dt3 == f(t) - (19/12)*d2y/dt2 - (19/24)*dydt - (1/8)*y(t);
end

[Dyuman Joshi]

Is y an function of an independent variable t?

What is the purpose of the 2nd line, "x1 = y(t)"?

[Angelina]

The purpose of the second line is to define x1. x1 is used in the following line. Basically the idea is to change the equation from a third order to a second order.

[Dyuman Joshi]

Okay, but what about y and t? What are they supposed to be? y as a function of t? or otherwise? Please specify.

Answer 1

[Askic V]

It seems to me that you actually want this:

[t,y]=ode45(@problem03ODEFunction,[0 20],[2 0 0]);   
plot(t,y(:,1))
hold on; grid on;
plot(t,y(:,2))
leg = legend('y(t)', '$\frac{dy}{dt}$','interpreter','latex');
leg.FontSize = 12;

function dy = problem03ODEFunction(t,y)
    dy = zeros(3,1);
    dy(1) = y(2);
    dy(2) = y(3);
    f = heaviside(t-1) - heaviside(t-10);
    dy(3) = f - (19/12)*dy(2) - (19/24)*dy(1) - (1/8)*y(1);
end