MATLAB: Additional output with fsolve

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…

Best Answer

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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MATLAB: Problem using ‘deal’ for objective and gradient in comb. with ‘DerivativeCheck’

An internal routine (in this case finDiffEvalAndChkErr) is calling your routine with only one output. For a regular function that returns two outputs, this is fine, as you are allowed to do this (the second output gets ignored).

But for your function with deal, you are requiring it to always be called with two outputs, rather than optionally.

I suggest you just resign yourself to writing a wrapper function for your two routines (you can also save yourself evaluating the gradient too if it's not required).

function [f, g] = fun_all(x)
  f = fun_obj(x);
  if nargout == 2
    g = fun_grad(x);
  end
end

MATLAB: Best practice for saving only one output from a vector-outputting function

Simply

 WantThis = myfun(x, y, ... )

if you just need/want the first output. More generally

 [a, b, c, d] = myfun(x, y, ... )

even if there could be more output args ( e, f, g, ..).

If don't want to name args that you don't need, which are preceding args that you do need, you can use ~ as a placeholder:

 [~, ~, c, ~, e] = myfun(x, y, ... )

Note that ~ is supported since 2009 if I remember well (edit: ref).

Best Answer

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MATLAB: Problem using ‘deal’ for objective and gradient in comb. with ‘DerivativeCheck’

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