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Additional output with fsolve

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Theo
Theo on 14 May 2011
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...

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Accepted Answer

Andrew Newell
Andrew Newell on 14 May 2011
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.

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Theo
Theo on 14 May 2011
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
Andrew Newell on 14 May 2011
I do appreciate your obvious effort to distill the essence of this problem.

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More Answers (1)

Andrew Newell
Andrew Newell on 14 May 2011
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.

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