Undefined function or variable in ode45

I have this differantial equation set.

http://postimg.org/image/ctudcrzqh/

k1, k2, k3, k4 are constants.

I need to find value of x and y at t=500.

I want to solve this set simultaneously. I wrote that code:

 function da=fun(t,a)

    k1=0.02;    % day^-1 %constant for growth of rabbits
    k2=0.00004; % (day*foxes)^-1 %constant for death of rabbits
    k3=0.0004;  % (day*rabbits)^-1 %constant for growth of 
                % foxes after eating rabbits
    k4=0.04;    % day^-1 %constant for death of foxes

    x=a(1,:);
    y=a(2,:);   
    da(1,:)=k1*x-k2*x*y;
    da(2,:)=k3*x*y-k4*y;

 clear all
 timerange=[0 500];
 initial=[500 200];
 [t,a]=ODE45(@fun,timerange,initial);

When i run, i get "Undefined function or variable 't'." error.

Can anyone help?

[Cedric]

Just to be sure (because I formatted your question this way), the lines

 function da=fun(t,a)

to

 da(2,:)=k3*x*y-k4*y;

(note that in your question, it was initially written a(2,:)=.. and not da(2,:)=..) are in a separate file named fun.m, and from

 clear all

to the end, this is a separate file/script (?)

[Ugur Bozuyuk]

I deleted "clear all". It still doesn't work. Can you be more spesific?

Answer 1

[Cedric]

You cannot have all these lines of code in the same file. You must have two M-files, one named fun.m which contains

 function da=fun(t,a)

    k1=0.02;    % day^-1 %constant for growth of rabbits
    k2=0.00004; % (day*foxes)^-1 %constant for death of rabbits
    k3=0.0004;  % (day*rabbits)^-1 %constant for growth of 
                % foxes after eating rabbits
    k4=0.04;    % day^-1 %constant for death of foxes

    x=a(1,:);
    y=a(2,:);   
    da(1,:)=k1*x-k2*x*y;
    da(2,:)=k3*x*y-k4*y;

And the other named as you prefer, which contains:

 clear all
 timerange=[0 500];
 initial=[500 200];
 [t,a]=ode45(@fun,timerange,initial);

Note that MATLAB is case sensitive, so you have to call ode45 and not ODE45 (beware the forum for that, as we often capitalize function names outside of code blocks, to differentiate them from the rest of the text).

I tested and I am attaching a zip file with the two files that you should have. You can rename main.n into whatever you want, but fun.m cannot be changed, as the file name must match the function name (or you can change both the file name and the function name).

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