multifit.C: Fitting multiple functions to different ranges of a 1-D histogram

#include "TH1.h"
#include "TF1.h"

void multifit() {
//  Fitting multiple functions to different ranges of a 1-D histogram
// To see the output of this macro, click begin_html <a href="gif/multifit.gif" >here</a> end_html
//      Example showing how to fit in a sub-range of an histogram
//  An histogram is created and filled with the bin contents and errors
//  defined in the table below.
//  3 gaussians are fitted in sub-ranges of this histogram.
//  A new function (a sum of 3 gaussians) is fitted on another subrange
//  Note that when fitting simple functions, such as gaussians, the initial
//  values of parameters are automatically computed by ROOT.
//  In the more complicated case of the sum of 3 gaussians, the initial values
//  of parameters must be given. In this particular case, the initial values
//  are taken from the result of the individual fits.
//Author: Rene Brun

   const Int_t np = 49;
   Float_t x[np] = {1.913521, 1.953769, 2.347435, 2.883654, 3.493567,
                    4.047560, 4.337210, 4.364347, 4.563004, 5.054247,
                    5.194183, 5.380521, 5.303213, 5.384578, 5.563983,
                    5.728500, 5.685752, 5.080029, 4.251809, 3.372246,
                    2.207432, 1.227541, 0.8597788,0.8220503,0.8046592,
                    0.7684097,0.7469761,0.8019787,0.8362375,0.8744895,
                    0.9143721,0.9462768,0.9285364,0.8954604,0.8410891,
                    0.7853871,0.7100883,0.6938808,0.7363682,0.7032954,
                    0.6029015,0.5600163,0.7477068,1.188785, 1.938228,
                    2.602717, 3.472962, 4.465014, 5.177035};

   TH1F *h = new TH1F("h","Example of several fits in subranges",np,85,134);
   h->SetMaximum(7);

   for (int i=0;i<np;i++) {
      h->SetBinContent(i+1,x[i]);
   }

   Double_t par[9];
   TF1 *g1    = new TF1("g1","gaus",85,95);
   TF1 *g2    = new TF1("g2","gaus",98,108);
   TF1 *g3    = new TF1("g3","gaus",110,121);
   TF1 *total = new TF1("total","gaus(0)+gaus(3)+gaus(6)",85,125);
   total->SetLineColor(2);
   h->Fit(g1,"R");
   h->Fit(g2,"R+");
   h->Fit(g3,"R+");
   g1->GetParameters(&par[0]);
   g2->GetParameters(&par[3]);
   g3->GetParameters(&par[6]);
   total->SetParameters(par);
   h->Fit(total,"R+");
}