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System of Differential Equations- How can I avoid an index error when my first equations depend on second derivatives of later equations?

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Leila
Leila 2015-12-19
关闭： MATLAB Answer Bot 2021-8-20
I am solving a basic system of differential equations. I have the following simple function:
function dy = waspstandardq(t,y)
dy = zeros(2,1);
C1 = 5;
f = 1;
C3 = 15;
C2 = 90;
C4 = 35;
Vo = 2;
R3 = 5;
M = 20;
M2 = 20;
L1 = 20;
L3 = 20;
R4 = 10;
L2 = 20;
L4 = 20;
R1 = 5;
R2 = 5;
G = sin(1.5*t)*2.7^(-t);
y(1) = y(2);
% CS
dy(2) = (dy(4)*M +dy(6)*M - (y(1)/C2) - (R3*y(2)) /(2 * L3));
dy(3) = y(4);
% WF
dy(4) = ((dy(2)*M - (y(3)/C1) - (R1* y(4)) +Vo-G)/(L2));
dy(5) = y(6);
% PF
dy(6) =((dy(4)*M+(dy(8))*M2 - (y(5)/C3) - (R2*y(6))) /(2*L1));
dy(7) = y(8);
% B
dy(8) = ((dy(6)*M2) - (y(7)/C4) - (R4*y(8)) )/(L4);
Then I run it with this:
[t,y] = ode45('waspstandardq',[0 1000], [0 0 0 0 0 0 0 0 ]);
It works of course if the equations don't depend upon the second derivative terms. How can I avoid the error when MATLAB reads dy(4) for instance, before it is defined? Obviously I can not define it sooner.
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