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Topic: fminunc + transformation
Replies: 5   Last Post: Apr 12, 2013 1:37 PM

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Saad

Posts: 23
Registered: 11/26/12
Re: fminunc + transformation
Posted: Apr 11, 2013 2:27 PM
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Alan_Weiss <aweiss@mathworks.com> wrote in message <kjjqng$i08$1@newscl01ah.mathworks.com>...
> On 4/4/2013 3:47 AM, Saad wrote:
> >
> >
> > Alan_Weiss <aweiss@mathworks.com> wrote in message
> > <kjhqkj$ioi$1@newscl01ah.mathworks.com>...

> >> On 3/30/2013 5:26 PM, Saad wrote:
> >> > Dear all,
> >> >
> >> > I am replicating a paper who recommends using fminunc to do a >

> >> "constrained" optimization through a transformation. I have tried to
> >> > use directly fmincon (with different algorithms) but the function >
> >> doesnt optimize, thats the reason I would like to follow the paper >
> >> advice and use a transformation as follows:

> >> >
> >> > C_bar=lamda*(exp(C)/1+ exp(C)) where lamda is a constant, C is the
> >> > unconstrained variable and C_bar is the constrained variable. I

> >> would > really appreciate if you could show me how I could use the >
> >> transformation in matlab. Do I have to create a seperate function?
> >> How > can I link it to the optimizer please?

> >> >
> >> > Here is my code
> >> >
> >> > C=[1; 1; 1 ; 1; 1; 1; 1; 1; 1];
> >> >

> >> options=optimset('Diagnostics','on','Display','iter','TolX',0.001,'TolFun',0.001,'LargeScale','off','HessUpdate','bfgs');
> >> >
> >> > [beta,fval,exitflag,output,grad,hessian] =fminunc(@mll,C,options)
> >> >
> >> > Thanks a lot for your help
> >> >
> >> > Best Regards
> >> >
> >> > S

> >>
> >> It looks to me as if C_bar is a multidimensional variable constrained
> >> to be between 0 and 1. Is that right?
> >>
> >> You can certainly include this transformation in a MATLAB statement:
> >>
> >> C_bar = lamda.*(exp(C)./(1+exp(C));
> >>
> >> The ./ and .* statements mean componentwise division and multiplication.
> >>
> >> However, I think you could avoid this complicated and (it seems to
> >> me) fragile transformation by simply setting bounds on fmincon and
> >> using the interior-point or sqp algorithms, which respect bounds.
> >> http://www.mathworks.com/help/optim/ug/writing-constraints.html#br9p_ry
> >>
> >> Good luck,
> >>
> >> Alan Weiss
> >> MATLAB mathematical toolbox documentation

> >
> > Hi Alan
> >
> > Thank you for your reply. I did try fmincon with different
> > algorithms (interior-point and sqp algorithms) but could not locate
> > the minimum. I also tightened the tolerance but still could not
> > minimize (for info i am working on a kalman filtering problem). I
> > think the issue is that I am not able to find a good initial guess.
> > Any thoughts on how to handle that? Thanks a lot
> >
> > Best
> >
> > S

>
> Well, assuming that you have upper and lower bounds on all components,
> why not try a lot of random initial points?
>
> x0 = lb + rand(size(lb)).*(ub - lb);
>
> There are more suggestions here (written for Global Optimization
> Toolbox, but you can use the suggestions anyway):
> http://www.mathworks.com/help/gads/refining-the-start-points.html#bsfjxr9
> http://www.mathworks.com/help/gads/isolated-global-minimum.html
>
> Alan Weiss
> MATLAB mathematical toolbox documentation


Dear Alan

Hope you are well. I tried your suggestions and used Global Optimization Toolbox and my new problem is defined as follows:
%%%%%%%
opts = optimset('Algorithm','interior-point','Hessian','lbfgs');
problem = createOptimProblem('fmincon','x0',[2; 2; 2 ; 2; 3; 3; 3; 3; 3],'objective',@(beta)mll(beta,y,R,xx),...
'lb',[0;0; 0; 0; 0; 0; 0; 0; 0],'ub',[10; 10; 10; 10; 10; 10; 10; 10; 10], 'options',opts);
gs = GlobalSearch;
rng(14,'twister') % for reproducibility
gs.NumTrialPoints = 1000;%1e5;
gs.MaxTime = 300;
[xg fvalg] = run(gs,problem)

%%%%%

I was just wondering how can I retrieve the hessian matrix once the optimization is finished? Thanks a lot
Regards

S



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