Getting an error using theta

running this code:

function [tvals, wvals] = RK4SYS(rhsf,init_t, init_y, stop_t, Nsteps)
h=(stop_t-init_t)/Nsteps;
tvals(1)=init_t;
wvals(:,1)=init_y;
for index=1:Nsteps
    tvals(index+1)=tvals(index)+h;
    k1=h*rhsf(tvals(index),wvals(:,index));
    k2=h*rhsf(tvals(index)+h/2,wvals(:,index)+k1/2);
    k3=h*rhsf(tvals(index)+h/2,wvals(:,index)+k2/2);
    k4=h*rhsf(tvals(index)+h,wvals(:,index)+k3);
    wvals(:,index+1)=wvals(:,index)+(k1/6+k2/3+k3/3+k4/6);
end


end

trying to do:

Rhsprob=@ (t,U)[U(2),-32.17/2*sin(theta )]

initU1=[pi/6 ; 0]

tv, wv] = RK4SYS(Rhsprob,0, initU1, 2, 5);

but i get an error when doing this line says theta requires something and error in line 15 of my code

[Walter Roberson]

Where do you define a value for theta?

Answer 1

[Steven Lord]

Rhsprob=@ (t,U)[U(2),-32.17/2*sin(theta )]

Rhsprob = function_handle with value:
    @(t,U)[U(2),-32.17/2*sin(theta)]

Nowhere in the code you posted before this line do you define a variable named theta. So if I were to try to call this anonymous function:

Rhsprob(0, [1; 2])

Unrecognized function or variable 'theta'.

Error in solution>@(t,U)[U(2),-32.17/2*sin(theta)] (line 1)
Rhsprob=@ (t,U)[U(2),-32.17/2*sin(theta )]

MATLAB doesn't know what theta means in this context and so correctly throws an error alerting you to that fact. I don't know what theta means in this context either so I can't tell you how to correct the code.

[allison]

it is a math probelm tring to do the sin of theta sin(theta)

[Walter Roberson]

@allison

Yes, but what is theta ?

If you are trying to create an ODE in terms of θ and then the θ to be used in the function handle would be one of the state variables, either U(1) or U(2)

[Image Analyst]

Looks like @allison posted a separate question with more details.