Array indices must be positive integers or logical values.

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Angelina
Angelina 2023-2-21
编辑： Askic V 2023-2-21
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
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Angelina
Angelina 2023-2-21
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
Dyuman Joshi 2023-2-21
Okay, but what about y and t? What are they supposed to be? y as a function of t? or otherwise? Please specify.

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回答（1 个）

Askic V
Askic V 2023-2-21
编辑：Askic V 2023-2-21
Ran in:
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

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