Undefined function 'rk' for input arguments of type 'function_handle'.

Hi guys, good day,

I have the following code to obtain the solution for an equation system, using Runge Kutta second order. But when I try to call the function, the "Undefined function 'rk' for input arguments of type 'function_handle'." message appears. What can I do? Please, help. Thanks

a=input('parámetro a: ');
b=input('parámetro b: ');
s0=input('valor inicial de s: ');
c0=input('valor inicial de c: ');
r0=input('valor inicial de r: ');
tf=input('tiempo final, tf: ');
n=input('número de pasos, n: ');
f=@(t,s,c,r) -a*s*c; 
g=@(t,s,c,r) a*s*c-b*c;
j=@(t,s,c,r) b*c;
%condiciones iniciales
t0=0; 
[t,s,c,r] =rk(f,g,j,t0,tf,s0,c0,r0,n);
hold on
plot(t,s,'b')
plot(t,c,'r')
plot(t,r,'g')
xlabel('t')
ylabel('s,c,r');
legend('s(t)','c(t)','r(t)')
title('ds/dt=-a*s*c, dc/dt=a*s*c-b*c, dr/dt=b*c')
hold off
function [t,s,c,r] =rk(f,g,j,t0,tf,s0,c0,r0,n)
      h=(tf-t0)/n;
      t=t0:h:tf;
      s=zeros(n+1,1); %reserva memoria para n+1 element(i)os del vect(i)or x(i)
      c=zeros(n+1,1);
      r=zeros(n+1,1);
      s(1)=s0; c(1)=c0; r(1)=r0;

        for i=1:n
          k1=h*f(t(i),s(i),c(i),r(i));
          l1=h*g(t(i),s(i),c(i),r(i));
          m1=h*j(t(i),s(i),c(i),r(i));
          k2=h*f(t(i)+h/2,s(i)+k1/2,c(i)+l1/2,r(i)+m1/2);
          l2=h*g(t(i)+h/2,s(i)+k1/2,c(i)+l1/2,r(i)+m1/2);
          m2=h*j(t(i)+h/2,s(i)+k1/2,c(i)+l1/2,r(i)+m1/2);

          s(i+1)=s(i)+k2;
          c(i+1)=c(i)+l2;
          r(i+1)=r(i)+m2;
          %s(i+1)=s(i)+(k1+2*k2+2*k3+k4)/6;
          %c(i+1)=c(i)+(l1+2*l2+2*l3+l4)/6;
          %r(i+1)=r(i)+(m1+2*m2+2*m3+m4)/6;
        end
  end

Answer 1

[Walter Roberson]

At the top of your code, you need to insert the line

function run_my_rk

except that you need to replace "run_my_rk" with the name of the file you have all of this code stored in (and that file name has to be a valid MATLAB identifier.)

[Brian Eduardo Arreola Tuda]

Yep, that was right.... thank you so much. I must improve my skills. Thank you again.