fitcont.C

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/// \file
/// \ingroup tutorial_fit
/// \notebook
/// Example illustrating how to draw the n-sigma contour of a Minuit fit.
/// To get the n-sigma contour the ERRDEF parameter in Minuit has to set
/// to n^2. The fcn function has to be set before the routine is called.
///
/// WARNING!!! This test works only with TMinuit
///
/// The TGraph object is created via the interpreter. The user must cast it
/// to a TGraph*
///
/// \macro_image
/// \macro_output
/// \macro_code
///
/// \author Rene Brun

#include "TMinuit.h"

void fitcont()
{
   //be sure default is Minuit since we will use gMinuit
   TVirtualFitter::SetDefaultFitter("Minuit");

   TCanvas *c1 = new TCanvas("c1");
   TH1F *h = new TH1F("h","My histogram",100,-3,3);
   h->FillRandom("gaus",6000);
   h->Fit("gaus");
   c1->Update();

   TCanvas *c2 = new TCanvas("c2","contours",10,10,600,800);
   c2->Divide(1,2);
   c2->cd(1);
   /*get first contour for parameter 1 versus parameter 2*/
   TGraph *gr12 = (TGraph*)gMinuit->Contour(40,1,2);
   gr12->Draw("alp");
   c2->cd(2);
   /*Get contour for parameter 0 versus parameter 2  for ERRDEF=2*/
   gMinuit->SetErrorDef(4); //note 4 and not 2!
   TGraph *gr2 = (TGraph*)gMinuit->Contour(80,0,2);
   gr2->SetFillColor(42);
   gr2->Draw("alf");
   /*Get contour for parameter 0 versus parameter 2 for ERRDEF=1*/
   gMinuit->SetErrorDef(1);
   TGraph *gr1 = (TGraph*)gMinuit->Contour(80,0,2);
   gr1->SetFillColor(38);
   gr1->Draw("lf");
}