MATLAB: Index must be a positive integer or logical

4th runge-kuttaindex must be logicalphysical pendulum

I'm am working on modeling the physical pendulum using 4th Runge-Kutta Method and and getting the error
"Attempted to access F_t_theta_v(1,-0.0885,0.8); index must
be a positive integer or logical. Error in PhyFinal (line 42)
    k_v1 = h * F_t_theta_v( t(i), theta(i), v(i) );"
Not sure what is going on or why it's not working. Any explaination would be helpful!
%Define Variables
w0 = 1;
alpha = 0.2;
f = 0.52;
w = 0.666;
%define step size
h = 1;
%set t from 1 to 100 seconds
t = 1:h:100;
%Create placement to put values generated 
theta = zeros ( 1 , length(t) );
v= zeros ( 1 , length(t) );
k_v1=zeros ( 1 , length(t) );
k_theta1=zeros ( 1 , length(t) );
%first initial conditions
theta(1)=-0.0885;
v(1)=0.8;
%Function.
F_t_theta_v = -w0^2 * sin(theta) - alpha * v + f * cos(w*t);
%Apply 4th RK
for i=1:(length(t)-1);  
      k_v1 = h * F_t_theta_v( t(i), theta(i), v(i) );
      k_theta1 = h * v;
      k_v2 = h * F_t_theta_v ( (t(i) + 0.5 * h), (theta(i) + 0.5 * k_theta1), (v + 0.5 * k_v1) );
      k_theta2 = h * (v + k_v1);
      k_v3 = h * F_t_theta_v ( (t(i) + 0.5 * h), ( theta (i) + 0.5 * k_theta2), (v + k_v1) );
      k_theta3 = h * ( v + k_v2 );
      k_v4 = h * F_t_theta_v ( ( t(i) + h ), ( theta(i) + k_theta3), ( v + k_v1));
      k_theta4 = h * ( v + k_v3 );
      dtheta = 1/6 * (k_theta1 + 2 * k_theta2 + 2 * k_theta3 + k_theta4);
      dv = 1/6 * (k_v1 + 2 * k_v2 + 2 * k_v3 + k_v4);
      theta(i + h) = theta(i) + dtheta;
      v (i + h) = v(i) + dv;
  end
%plot theta(t), might change to 'x' for consistency', and v(t)
figure(1);
plot ( v,theta)

Best Answer

You need to define your Anonymous Function correctly:
%Function.
F_t_theta_v = @(t,theta,v) -w0^2 * sin(theta) - alpha * v + f * cos(w*t);
Change the variables in the argument list in ‘@(t,theta,v)’ if I got it wrong. (I used the name of your function as a clue!) Your function should work then. It should pick up the other variables it needs from your workspace, but they have to be defined before you define the function.
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