example_EventSelection.cc

// Matt.Dobbs@Cern.CH, Feb 2000
// Example of applying an event selection to the events written to file
// using example_MyPythia.cxx
// Events containing a photon of pT > 25 GeV pass the selection and are
// written to "example_EventSelection.dat"
// To Compile: go to the HepMC directory and type:
// gmake examples/example_EventSelection.exe
//

#include "HepMC/IO_GenEvent.h"
#include "HepMC/GenEvent.h"


class IsEventGood {
public:
    bool operator()( const HepMC::GenEvent* evt ) { 
        for ( HepMC::GenEvent::particle_const_iterator p 
                  = evt->particles_begin(); p != evt->particles_end(); ++p ){
            if ( (*p)->pdg_id() == 22 && (*p)->momentum().perp() > 25. ) {
                //std::cout << "Event " << evt->event_number()
                //     << " is a good event." << std::endl;
                //(*p)->print();
                return 1;
            }
        }
        return 0;
    }
};

int main() { 
    // declare an input strategy to read the data produced with the 
    // example_MyPythia
    { // begin scope of ascii_in and ascii_out
        HepMC::IO_GenEvent ascii_in("example_MyPythia.dat",std::ios::in);
        // declare another IO_GenEvent for writing out the good events
        HepMC::IO_GenEvent ascii_out("example_EventSelection.dat",std::ios::out);
        // declare an instance of the event selection predicate
        IsEventGood is_good_event;
        //........................................EVENT LOOP
        int icount=0;
        int num_good_events=0;
        HepMC::GenEvent* evt = ascii_in.read_next_event();
        while ( evt ) {
            icount++;
            if ( icount%50==1 ) std::cout << "Processing Event Number " << icount
                                          << " its # " << evt->event_number() 
                                          << std::endl;
            if ( is_good_event(evt) ) {
                ascii_out << evt;
                ++num_good_events;
            }
            delete evt;
            ascii_in >> evt;
        }
        //........................................PRINT RESULT
        std::cout << num_good_events << " out of " << icount 
                  << " processed events passed the cuts. Finished." << std::endl;
    } // end scope of ascii_in and ascii_out
    return 0;
}