#include "TROOT.h"
#include "TH1.h"
#include "TMath.h"
#include "TF1.h"
#include "TFile.h"
#include "TLegend.h"
#include "TCanvas.h"
#include <TDirectory.h> 

Double_t fitf(Double_t *v, Double_t *par)
{
   Double_t arg = 0;
   if (par[2] != 0) arg = (v[0] - par[1])/par[2];
 
   Double_t fitval = par[0]*TMath::Exp(-0.5*arg*arg);
   return fitval;
   
}
 
void fitExample()
{
   TFile *f = new TFile("PtMC.root","READ");
   TList *list = dynamic_cast<TList*> (f->Get("cOutput")); 
   if (!list) { 
      printf("Error retrieving cOutput  \n");
      return;   
   }
   TH1F *fHistMassK0 = dynamic_cast<TH1F*>( list->FindObject("fHistMassK0On") ); 
   if (!fHistMassK0) { 
      printf("Error retrieving fHistMassK0On \n");
      return;   
   }
 
   //create a function with 3 parameters in the range [-3,3]
   TF1 *func = new TF1("fit",fitf,-3,3,3);
   func->SetParameters(5000,fHistMassK0->GetMean(),fHistMassK0->GetRMS());
   func->SetParNames("Constant","Mean_value","Sigma");
   func->SetRange(0.485,0.515);
   fHistMassK0->Fit("fit","R");
   fHistMassK0->Draw();
}