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 2015-12-19

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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.