Fitter trouble

Hi everyone,

I’m having a bit of a fight with TFitter/TMinuit using ROOT 4.04/02b (3 June 2005). I create a TF1 with seven parameters. In the TF1 they are numbered 0-6. I fix five of these parameters by calling TF1::FixParameter(…). Then I create a TFitter with seven parameters. This creates a TMinuit with seven parameters. Unless I’m mistaken, TMinuit now has seven external parameters numbered 1-7 and two internal parameters numbered 1,2.

So far I think everything’s fine. Now I’m going to ask the fitter if a given parameter is fixed or not: TFitter::IsFixed(par). Looking at the code, I think there are two problems.

Bool_t TFitter::IsFixed(Int_t ipar) const
   //return kTRUE if parameter ipar is fixed, kFALSE othersise)

   for (Int_t i=0;i<fMinuit>fNpfix;i++) {
      if (fMinuit->fNexofi[i] == ipar) return kTRUE;
   return kFALSE;
  1. Since this returns true when an external parameter is found for an internal one, which means it’s a free parameter, I think this returns exactly the opposite of what you would expect (and is documented).
  2. From the code I gather that it wants an external parameter number, so one of the 1-7. Looking at the contents of TMinuit::fNexofi and TMinuit::fNiofex etc. (see below) this makes sense but goes wrong for the last parameter, number seven.
gMinuit->fCpnam[0] = a
gMinuit->fCpnam[1] = b
gMinuit->fCpnam[2] = c
gMinuit->fCpnam[3] = d
gMinuit->fCpnam[4] = e
gMinuit->fCpnam[5] = f
gMinuit->fCpnam[6] = g
gMinuit->fNexofi[0] = 4
gMinuit->fNexofi[1] = 5
gMinuit->fNexofi[2] = 7
gMinuit->fNexofi[3] = 7
gMinuit->fNexofi[4] = 7
gMinuit->fNexofi[5] = 7
gMinuit->fNexofi[6] = 7
gMinuit->fNiofex[0] = 0
gMinuit->fNiofex[1] = 0
gMinuit->fNiofex[2] = 0
gMinuit->fNiofex[3] = 1
gMinuit->fNiofex[4] = 2
gMinuit->fNiofex[5] = 0
gMinuit->fNiofex[6] = 0

I also have no idea why the loop in TFitter::IsFixed only runs to fNpfix?

Am I confusing things? What’s going on here? And how to work around it?

Thanks for your help.


looks strange to me too !

I would recomend to you to use the new Minuit, Minuit2, via the class
TFitterMinuit, which is available in the most recent version of ROOT.


We have found that the problem is also in the HEAD version of TFitter.
It is now fixed in CVS.

Attached is the patch, that, if you want, you can apply to your version

Thanks for reporting the problem,

TFitter.patch.txt (587 Bytes)