macro/examples/fastsim/ana_example.C

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int ana_example(TString fname,int nevts=0)
{
        TStopwatch timer;
        timer.Start();
        
        gSystem->Load("libEGPythia6");  // needed for TDatabasePDG
        gSystem->Load("libRho");
        
        // the PndEventReader takes care about file/event handling
        PndEventReader evr(fname);
        
        // to attach more files: evr.Add(fname2);evr.Add(fname3); ...
        
        TH1F *metac=new TH1F("metac","m(eta_c)",100,2.6,3.3);
        
        // the candidates lists we need
        TCandList gam, pi0, eta, etac;
        
        // some mass selectors
        TPidMassSelector *etaMassSel=new TPidMassSelector("eta",0.534,0.03);
        TPidMassSelector *pi0MassSel=new TPidMassSelector("pi0",0.130,0.03);
        
        if (nevts==0) nevts=evr.GetEntries();
        int i=0,j;
        
        // *************
        // this is the loop through the events ... as simple as this...
        // ****************
        while (evr.GetEvent() && i++<nevts)
        {
            // ask the event reader to fill the gamma candidates lists
                evr.FillList(gam,"Neutral");
                
                // do the combinatorics and selection
                pi0.Combine(gam,gam);
                pi0.Select(pi0MassSel);
                
                eta.Combine(gam,gam);
                eta.Select(etaMassSel);
                
                // Combine now works with up to 5 lists
                // double counting and overlap when using the same list
                // more the once is handled properly
                etac.Combine(eta, pi0, pi0); 
                
                // fill a histogram
                for (j=0;j<etac.GetLength();++j) metac->Fill(etac[j].M());              
        }
        
        // ... and plot it
        TCanvas *c1=new TCanvas("c1","c1",500,500);
        c1->cd(); metac->Draw();
        
        // now take the time...
    timer.Stop();
    Double_t rtime = timer.RealTime();
    Double_t ctime = timer.CpuTime();
    
    printf("RealTime=%f seconds, CpuTime=%f seconds\n",rtime,ctime);
  return 0;
}