/*
Name of the programmer: Diksha Garg
Date: 16 May, 2017
Comment: This program loads a root file and then fits 2 gaus functions(pre-defined) to the trkmm parameter of the root file. 
*/


{
  TFile func("test_10GeV_reco.root"); /*loading root file */

  TH1F *h= new TH1F("h","trkmm gaussian fit",50,4,16); /*creating empty 
                                                          histogram */
  
  /* projecting the values of trkmm present in root file on the empty histogram,h */
  T1-> Project ("h","abs(trkmm[0])","abs(trkmm[0])> 4 && abs(trkmm[0])<16"); 
 
  /* creating gaussian curves */
  g1= new TF1 ("m1","gaus",4,9);
  g2= new TF1 ("m2","gaus",8,14);
  //  g3= new TF1 ("m3","gaus",12,17);

  TF1 *f1= new TF1("double_gaus","gaus(0)+gaus(3)",4,16); /* define combined function*/

  /* Fitting each gaus fn and adding them to the existing list of functions */
  /* the + sign adds the newly fitted fn to the existing fn. */
  h-> Fit (g1,"R");
  h-> Fit (g2,"R+");
  //  h-> Fit (g3,"R+");

  Double_t par[9]; /* parameter array */

   /* get parameters from the fit, it first fits & takes the parameter from there */
  g1-> GetParameters (&par[0]);
  g2-> GetParameters (&par[3]);
  //  g3-> GetParameters (&par[6]);

  /* use the parameters on the combined function */
  double_gaus-> SetParameters (par);

  h->Fit("double_gaus","+","",4,16); /*fitting gaussian curve on the histogram */ 
  h->Draw("e1");
  gStyle->SetOptFit(1); // To print the parameters on the canvas

}
/* Refer: https://root.cern.ch/root/html534/guides/users-guide/FittingHistograms.html */