Question

My Matlab program has multiple inputs as a struct (in.a, in.b, etc.)
and multiple outputs (out.a, out.b, etc.)

I would like to use the genetic algorithm solver from teh optimization toolbox to find the best input in.a, while all the other inputs are constant. The fitness is one of the outputs, e.g. out.b(2,3).

How do I “tell” the solver this?

Thanks
Daniel

Answer 1

It is not uncommon in programming to have a situation where what is most convenient for your function and what some library call expects of it don’t agree. The normal resolution to such a problem is to write a small layer in between that allows the two to talk; an interface.

From help ga:

X = GA(FITNESSFCN,NVARS) finds a local unconstrained minimum X to the
FITNESSFCN using GA. […] FITNESSFCN accepts a vector X of size
1-by-NVARS, and returns a scalar evaluated at X.

So, ga expects vector input, scalar output, whereas you have a structure going in and out. You would have to write the following (sub)function:

function Y = wrapper_Objfun(X, in)
    in.a = X; %# variable being optimized
    out  = YOUR_REAL_FUNCTION(in); %# call to your actual function
    Y    = out.b(2,3); %# objective value
end

and then the call to ga will look like

X = ga(@(x) wrapper_Objfun(x,in), N);

where N is however large in.a should be.

Also have a read about it in Matlab’s own documentation on the subject.