Index exceeds matrix dimensions

Hello I am working on a double inverted pendulum. I come up with this error 'index exceeds Matrix Dimensions' Error in Test_1>pend (line 8) x3=x(3);

    function pend1
    pendsss
    end

 function dx= pend(t,x)
 x1=x(1);
 x2=x(2);
 x3=x(3);
 x4=x(4);

 % parameters
 m1=1;
 m2=1;
 a1=0.5;
 L1=1;
 a2=0.5;
 L2=1;
 g=9.8;
 I1=0.0126;
 I2=0.0185;

 d1=I1+a1+a1+m1+L1*L1*m2;
 d2=a2*L1*m2;
 d3=(a1*m1+L1*m2)*g;
 d4=a2*L1*m2;
 d5=a1*m1+L1*m2;
 d6=I2+a2*a2*m2;
 d7=a2*m2*g;
 d8=a2*m2;
 e1=0.015;
 e2=0.015;

 D=[d1 d2*cos(x1-x3);d4*cos(x1-x3) d6];
 C=[d4*sin(x1-x3)*x4;-d4*sin(x1-x3)*x2];
 G=[d3*sin(x1);d7*sin(x3)];
 F=[-d5*cos(x1);-d8*cos(x3)];

 I= inv(D);
 dx=I*(-C-G+F);
 end

 function pendsss
 tspan=[0 1]; %time span
 ic=[0;pi];%initial conditions

 [t,x]=ode45(@pend,tspan,ic);

 plot (t,x), 
 grid on
 xlabel('Time(s)');
 legend('theta1', 'theta2')
 end

Answer 1

[Jan]

If the initial conditions have 2 components "ic=[0;pi]", why do you think, that the state variable has 4? The size of x in pend is the same as the size of the initial conditions.

By the way: This is a bad idea:

 I  = inv(D);
 dx = I*(-C-G+F);

As explained in doc inv, calculating the inverse explicitly is slower and less accurate than using the slash operator:

dx = D \ (-C -G + F)

[Aravind Krishnamoorthi]

So if i give the initial conditions vector as ic=[0;pi;-pi;0] would that be good enough.

And thank you for the idea.