Where in my code using runge- kutta four step method did I make a mistake?

% compute our trajectory
dt = .01;
tspan = 0:dt:4;

Y(:,1) = y0;
yk = y0;
for i=1:length(tspan)
    time= i*dt;
    ykplus1=rk4singlestep(@(t,y)lorenz(t,y,sigma,beta,rho),dt,time,yk);
    Y =[Y ykplus1];
    yk = ykplus1;
end

plot3(Y(1,:),Y(2,:),Y(3,:),'b')

[darova]

Can you show rk4singlestep function?

[Charles Saulse]

The highlighted rk4singlestep says that ive used the viriable, but never set it. This is what ive done in the first editor followed by the seccond editor window.

1. function dy = lorenz(t,y,sigma,beta,rho)

% y is a three dimentional state-vector

dy = [

sigma*(y(2)-y(1));

y(1)*(rho-y(3))-y(2);

y(1)*y(2)-beta*y(3);

];

2. function yout = rk4singlestep(fun,dt,tk,yk)

f1 = fun(tk,yk);

f2 = fun(tk+dt/2,yk+(dt/2)*f1);

f3 = fun(tk+dt/2,yk+(dt/2)*f2);

f4 = fun(tk+dt,yk+dt*f3);

yout = yk + (dt/6)*(f1+2*f2+2*f3+f4);

[James Tursa]

Where are sigma, beta, and rho set? You need to do that before you use them in the function handle.

Can you post the complete error message, including the offending line?

[Charles Saulse]

??? Undefined variable "rk4singlestep" or class "rk4singlestep".

[James Tursa]

Is your rk4singlestep function in a file called rk4singlestep.m?

[Charles Saulse]

?? I don't know how to put it in a file or make one.

[Charles Saulse]

Thanks, but I found out that I mixed up my code and not saving the code to matlab. But I did get my answer. Thanks

Answer 1

[James Tursa]

Type the following at the command line:

edit rk4singlestep.m

Then copy your code into that file and save it.

[Charles Saulse]

Okay, thank you James.

[Charles Saulse]

Good day. When I do that it takes me straight to the code which I wrote already in the editing window.

[Charles Saulse]

And when i run the code it gives the same error answer.