[ROOT] what's with the fitters? can't get params or error matrix

From: Dr. John Krane (jkrane@netzero.com)
Date: Wed Aug 06 2003 - 21:51:27 MEST


Hi,

I'm trying to get the final fit parameter values and the covariance 
matrix out of TVirtualFitter or TFitter.  Is there no way to do this?

I see

Int_t GetParameter(Int_t ipar,char *parname,Double_t &value,Double_t 
&verr,Double_t &vlow, Double_t &vhigh)

...but this looks like a cut'n'paste of SetParameter.  Why would I have 
to provide the initial values just to get the current one?!  Should this 
not be

Double_t GetParameter(Int_t ipar)

...with a simple inline implementation?  There also appears to be *no 
way* to get the covariance matrix.  Perhaps I should have bypassed 
TVirtualFitter entirely and just used TMinuit directly.  This course 
seemed unwise to me at first, or at least non-aesthetic, but 
TVirtualFitter seems to lack even the basic functionality I need.

If I am wrong, could somebody please tell me how to (for instance) 
modify the code of

http://root.cern.ch/cgi-bin/print_hit_bold.pl/root/html/examples/minexam.cxx.html

to get the parameters and covariance matrix with high-precision?  I am 
looking for something similar to mnemat for the latter.

	- John



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