MATLAB: RK4 error for the code

differential equationsnumerical integrationrk4

Hi, I am integrating a differential equation using the code attached in the image.While i am executing my code i am getting the following error message,Can anyone help me with this.

??? Error using ==> feval

Argument must contain a string or function_handle.

Error in ==> rk4 at 21

k1 = h*feval(f,xo,u);

Best Answer

Hi,

the error tells you, that the first argument to call rk4 should be a function that can be called, so either the name as a string (of a .m file), or a function handle.

Titus

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MATLAB: RK4 orbit problem

dvxdt=@(t,x,vx) GM*x./(x.^2+y.^2).^3;

that accepts vx as a parameter but does not use it.

It does not have any parameter named y but uses y. In that case, it is going to use the variable value that the variable y had at the time the anonymous function was created, which comes from

y = zeros(1,1825);

This is a vector, and even though it only contributes 0 at all components, it makes the result of dvxdt be a vector. That becomes a problem because it means that

    k1vx = dvxdt(t(i)     ,x(i)     ,vx(i));

gives a vector result, so

vx(i+1)=vx(i)+h/6*(k1vx+2*k2vx+2*k3vx+k4vx);

has a vector on the right hand side, but designates a scalar output location.

MATLAB: Difference in ode45 and rk4 code

Hi Debashis,

I think there is a misunderstanding: ode45 uses the result from the 5th order RK method for the solution, whereas rk4 is a 4th order method. The "4" in the ode45 means, that the 4th order is used for the variable step selection. Now: for "nice" functions like 2*t, both rk4 and rk5 reproduce the result. Therefore you see no difference. For t^5 you see, that the 5th order is still exact (error of ode45 small), whereas 4th order rk4 cannot give exact result (as you see).

Titus

Best Answer

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