Index must be a positive integer or logical

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)

Answer 1

[Star Strider]

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.

[John]

Okay that fixed one problem now it's saying "In an assignment A(I) = B, the number of elements in B and I must be the same.

Error in PhyFinal (line 58) theta(i + h) = theta(i) + dtheta;"

I thought have everything from 1 to length(t) should be right unless I'm missing something

[Star Strider]

Good!

The new problem is with ‘dtheta’. You defined ‘v’ as a vector, and that propagates through to ‘dtheta’. You defined ‘theta(i+h)’ as a scalar, and you cannot assign a vector to a scalar.

If you want to define it as a vector,

theta(i + h, :) = theta(i) + dtheta;

should work.

If you don’t want to define it as a vector, you need to select one element of ‘dtheta’ instead of the entire vector.

Answer 2

[Azzi Abdelmalek]

Check your indices, if a is an array you can't write

a(1.8)

the index must be a positive integer or logical