how to call functions properly: Index in position 1 is invalid. Array indices must be positive integers or logical values.

I'm trying to work with multiple functions but I keep getting this error, its for the runge kutta method solving rossler equations.

% The code Im trying to run

 clear
 u0=[0.116365;0.376326;0.946238];
 [ t, y ] = runge ( 'ross', [ 0.0, 200.0 ], u0, 15000 );
 plot3(y(1,:),y(2,:),y(3,:))

Which calls my runge function

 function [t,y]=runge(f,timespan,u0,N);


N=round(timespan(2)*10);
y=u0;
h=(timespan(2)-timespan(1))/N;
t=linspace(timespan(1),timespan(2),N+1);   
                       
for i=1:N
  k1=f(t(i),y(:,i));   %here is where the error occurs
  k2=f(t(i)+0.5*h,y(:,i)+0.5*h*k1);
  k3=f(t(i)+0.5*h,y(:,i)+0.5*h*k2);
  k4=f(t(i)+h,y(:,i)+h*k3);
  y(:,i+1)=y(:,i)+h*((k1+k4)/6+(k2+k3)/3);
end

I also have ross as

function y0 = ross ( t, y )

y0 = [ (-1*(y(2))-y(3)); y(1)+(0.1*y(2)); 0.1+y(3)*(y(1)-10)];

Command window prints

Index in position 1 is invalid. Array indices must be positive integers or logical values.

Error in runge(line 10)
  k1=f(t(i),y(:,i));

Error in untitled11 (line 3)
 [ t, y ] = runge ( 'ross', [ 0.0, 200.0 ], u0, 15000 );
 

Answer 1

[Praveen Iyyappan Valsala]

In your runge function, f is a string not a function.

   k1=eval(strcat(f,'(t(i),y(:,i));'));%f(t(i),y(:,i));   %here is where the error occurs
    k2=eval(strcat(f,'(t(i)+0.5*h,y(:,i)+0.5*h*k1);'));%f(t(i)+0.5*h,y(:,i)+0.5*h*k1);
    k3=eval(strcat(f,'(t(i)+0.5*h,y(:,i)+0.5*h*k2);'));%f(t(i)+0.5*h,y(:,i)+0.5*h*k2);
    k4=eval(strcat(f,'(t(i)+h,y(:,i)+h*k3);'));%f(t(i)+h,y(:,i)+h*k3);
    

Alternatively, you can pass function handles

  ross =@( t, y ) [ (-1*(y(2))-y(3)); y(1)+(0.1*y(2)); 0.1+y(3)*(y(1)-10)]; % your ross function
   [ t, y ] = runge ( ross, [ 0.0, 200.0 ], u0, 15000 );

[jacob Mitch]

wow thank you so much I've deleted

function y0 = ross ( t, y )
y0 = [ (-1*(y(2))-y(3)); y(1)+(0.1*y(2)); 0.1+y(3)*(y(1)-10)];

and am running it as so as you've suggested and it seems to be working well!

 clear
 u0=[0.116365;0.376326;0.946238];
  ross =@( t, y ) [ (-1*(y(2))-y(3)); y(1)+(0.1*y(2)); 0.1+y(3)*(y(1)-10)]; % your ross function
   [ t, y ] = runge ( ross, [ 0.0, 200.0 ], u0, 15000 );
 plot3(y(1,:),y(2,:),y(3,:))

Just double checking but thank you!