Subscripted assignment dimension mismatch.

I am working on modeling a physical pendulum using the 4th order Runge-Kutta method and I don't understand why I am getting this error. As far as I can tell my dimensions match and am at a bit of a loss how to proceed. This is the line that is causing the error

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

w0 = 1;
alpha = 0.2;
f = 0.52;
w = 0.666;
h = 1;
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....this will plot as it should be
F_t_theta_v = @(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

Answer 1

[Image Analyst]

You say:

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

as if theta is a 2D matrix. It's not. It's a 1 by 100 row vector. Get rid of the colon and index dtheta:

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

Now look at your equation for v and fix that similarly.

[John]

Thank You!! brilliant!