Re: Accesssing fit parameters and errors

Arthur E. Snyder wrote:
> One thing you want to be careful of is if you wish to keep the covariance
> matrix. If you've done another fit keeping your TF1 won't keep
> convariance. TF1 caches the errors and values but not the full convariance
> matrix. This could be a problem if, e.g, you wanted to do a systematic
> study by varying fit parameters according to correlated errors. It would
> not be hard to had covariance cache to TF1 and perhaps it will be done
> someday. In the meanwhile if you want to keep covariance you have to hang
> on to it for yourself.
>

Art, Roger,

See the small script cov.C below showing how to save the covariance matrix with the fitted histogram.

Rene Brun

//macro cov.C
void cov() {

   // shows how to save the Fit covariance matrix as an object    // in the histogram list of functions   

   covwrite();
   covread();
}

void covwrite() {

   // shows how to save the Fit covariance matrix as an object    // in the histogram list of functions  

   TH1F *h = new TH1F("h","sho cov matrix",100,-3,3);    h->FillRandom("gaus",5000);
   h->Fit("gaus");
   TVirtualFitter *fitter = TVirtualFitter::GetFitter();    TMatrixD *matrix = new TMatrixD(3,3,fitter->GetCovarianceMatrix());    h->GetListOfFunctions()->Add(matrix);    h->GetListOfFunctions()->ls();
   TFile f("h.root","recreate");
   h->Write();
}

void covread() {

   TFile *f = TFile::Open("h.root");
   TH1F *h = (TH1F*)f->Get("h");
   h->Draw();
   TMatrixD *m =
(TMatrixD*)h->GetListOfFunctions()->FindObject("TMatrixT<double>");

   m->Print();
}

> -Art
>
>
> [a] I'm usually just a consumer of root help, but since this one is
> something I knew about I thought I try to help out. A real root expert
> might have more or better answers.
>
> A.E. Snyder, Group EC \!c*p?/
> SLAC Mail Stop #95 ((. .))
> Box 4349 |
> Stanford, Ca, USA, 94309 '\|/`
> e-mail:snyder_at_slac.stanford.edu o
> phone:650-926-2701 _
> http://www.slac.stanford.edu/~snyder BaBar
> FAX:650-926-2657 Collaboration
>
>
>
> On Tue, 6 Feb 2007, Roger Mason wrote:
>
>
>> Hello Art,
>>
>> "Arthur E. Snyder" <snyder_at_slac.stanford.edu> writes:
>>
>>
>>> Well in c/c++ an array is just a pointer with a certain amount of space
>>> reseved beyond it. So GetParErrors give you a pointer (i.e. the array held
>>> by your TF1 object (fParErrors)).
>>>
>> I'm still at the stage with c++ where the pointer syntax, combined
>> with object syntax, eludes me. Such was the case here.
>>
>>
>>> If you want a copy that will not go away when your TF1 goes away (to be
>>> safe) then I think you just have to copy it element by element, or use it
>>> to construct a TMatrix or TVector or something.
>>>
>> Well, as long as I have the TF1 in the root file I am OK because I can
>> always extract the parameters and errors and dump the into a text
>> file. Why do I need this? Because I always tabulate results in
>> drafts of my publications just so the referees can tell me to toss
>> 'em and keep the pictures.
>>
>>
>>> If your a fortran old-foggie like me this can be very confusing!
>>>
>>>
>> Fortran was my first programming language too. I'm just glad that
>> root is'nt written in Java.
>>
>> Thanks again,
>> Roger
>>
>>
>>
Received on Tue Feb 06 2007 - 20:59:23 CET