Additional output with fsolve

I'm asking what seems to be an identical question to this one. Specifically, I simply want to output additional variables after fsolve completes its task. Suppose that I am using fsolve with this function:

function [F G] = myfunc(x)
F = x^2;
G = 2;

I will be using fsolve with

xsol = fsolve(@(x)myfunc(x), guess);

This will give me the solution. However, if I want the value of G, I need to call myfunc again

[F, G] = myfunc(xsol)

I'd like to NOT evaluate myfunc again. How do I get fsolve to also output additional variables? I'd like to do so without global variables or outputting to files...

Answer 1

[Andrew Newell]

Is the G you're really calculating costly to evaluate? If so, I'd suggest a different approach. Define your function as

function [F G] = myfunc(x)
F = x^2;

if nargout > 1
  G = 2;
end

so it doesn't evaluate G during the solving (which would involve, generally, several calls to myfunc ). Note that you can use a more compact notation:

xsol = fsolve(@myfunc,guess);

Then go ahead and calculate G once:

[F G] = myfunc(x)

You'll probably save a lot more computation with this approach.

[Theo]

I'm sorry I was not more specific (I tried to simplify the problem as much as I could).

> Is the G you're really calculating costly to evaluate?

Sort of. But it is also needed to determined F (and hence for fsolve). Think of G is a function and F as its norm (or some other property). Thus, fsolve will solve for G as it solves F = 0.

[Andrew Newell]

I do appreciate your obvious effort to distill the essence of this problem.

Answer 2

[Andrew Newell]

Based on your comments, here is another approach. Define a function

function G = myGfunc(x)
persistent G
if nargin > 0
  % calculate G
end

This saves the value of G you calculate. Then define

function [F G] = myfunc(x)
G = myGfunc(x);
F = GtoF(G); % Your method of calculating F using G

After your run, you can enter

G = myGfunc;

and it will simply return the saved value of G.

Caveat: it would be wise to enter

clear functions

before each run to make sure that G isn't saved from a previous run.