Handles: How do They Work?

Hey all,

I'm trying to create an ODE function which measures the abundance of L and G. My problem is getting other non-variable parameters into the darn thing. I think that I can accomplish this with the use of handles, but I can't for the life of me figure out how to set up and call them.

This is the function I run...

function [g]=IFFL_TC(t0,t1,t2,A0,A1,I,Smax,Smin,EffG,dl,dg,L,G)
x = [L,G];
P = [t1,t2,A0,A1,I,Smax,Smin,EffG,dl,dg];
Param = @P;   % This is where it all gets a little hazy...
[t,x] = ode45('IFFL_TC_eqns',[0 t0],x);
%%Plus more stuff that works fine

IFFL_TC_eqns is a system of 2 ODEs that spits out L and G. What I can't figure out how to pack all the necessary parameters into P and then unpack them from within IFFL_TC_eqns. I'm pretty sure it's simple, but the syntax is killing me.

Thanks in advance,

-DM

Answer 1

[Richard Brown]

The easiest way to do this is to define a constructor function that passes out a handle to be used in the ode solver by using nested functions. For example, you've got a code that needs parameters a and b to evaluate the derivative. This is basically what you've got, but yours has heaps more parameters, and no explicit t dependence.

function dy = myfunc(t, y, a, b)
  dy = a*y + b*y.^2;
end

One way to do it is to create an anonymous function

a = 1;
b = 2;
f = @(t,y) myfunc(t, y, a, b);

But probably nicer if you've got a bigger problem is to do it like this:

function hf = create_ode_fun(a, b)
  % a and b are available in this scope - nested funs can see them
  hf = @myfun;

    function dy = myfun(t, y)
      dy = a*y + b*y.^2
    end
  end

You'd then call it like this

hf = create_ode_fun(1, 2);
ode45(hf, blah, blah, blah)

Performance-wise both methods seem to be reasonably similar, the nested one may be slightly faster. Last time I tested this I didn't see a huge difference.

[Menn]

Thank a lot. I think I'm going to have to figure out how nested functions work, but this gives me some pretty good starting direction.

-DM