HepMC Reference Documentation HepMC

example_UsingIterators.cc

// Matt.Dobbs@Cern.CH, Feb 2000
// This example shows low to use the particle and vertex iterators
// To Compile: go to the HepMC directory and type:
// gmake examples/example_UsingIterators.exe
//

#include "HepMC/IO_Ascii.h"
#include "HepMC/GenEvent.h"
#include <math.h>
#include <algorithm>
#include <list>


class IsPhoton {
public:
    bool operator()( const HepMC::GenParticle* p ) { 
        if ( p->pdg_id() == 22 
             && p->momentum().perp() > 10. ) return 1;
        return 0;
    }
};


class IsW_Boson {
public:
    bool operator()( const HepMC::GenParticle* p ) { 
        if ( abs(p->pdg_id()) == 24 ) return 1;
        return 0;
    }
};


class IsFinalState {
public:
    bool operator()( const HepMC::GenParticle* p ) { 
        if ( !p->end_vertex() && p->status()==1 ) return 1;
        return 0;
    }
};

int main() {
    { // begin scope of ascii_in
        // an event has been prepared in advance for this example, read it
        // into memory using the IO_Ascii input strategy
        HepMC::IO_Ascii ascii_in("example_UsingIterators.txt",std::ios::in);
        if ( ascii_in.rdstate() == std::ios::failbit ) {
            std::cerr << "ERROR input file example_UsingIterators.txt is needed "
                      << "and does not exist. "
                      << "\n Look for it in HepMC/examples, Exit." << std::endl;
            return 1;
        }

        HepMC::GenEvent* evt = ascii_in.read_next_event();

        // if you wish to have a look at the event, then use evt->print();

        // use GenEvent::vertex_iterator to fill a list of all 
        // vertices in the event
        std::list<HepMC::GenVertex*> allvertices;
        for ( HepMC::GenEvent::vertex_iterator v = evt->vertices_begin();
              v != evt->vertices_end(); ++v ) {
            allvertices.push_back(*v);
        }

        // we could do the same thing with the STL algorithm copy
        std::list<HepMC::GenVertex*> allvertices2;
        copy( evt->vertices_begin(), evt->vertices_end(), 
              back_inserter(allvertices2) );

        // fill a list of all final state particles in the event, by requiring
        // that each particle satisfyies the IsFinalState predicate
        IsFinalState isfinal;
        std::list<HepMC::GenParticle*> finalstateparticles;
        for ( HepMC::GenEvent::particle_iterator p = evt->particles_begin();
              p != evt->particles_end(); ++p ) {
            if ( isfinal(*p) ) finalstateparticles.push_back(*p);
        }

        // an STL-like algorithm called HepMC::copy_if is provided in the
        // GenEvent.h header to do this sort of operation more easily,
        // you could get the identical results as above by using:
        std::list<HepMC::GenParticle*> finalstateparticles2;
        HepMC::copy_if( evt->particles_begin(), evt->particles_end(), 
                        back_inserter(finalstateparticles2), IsFinalState() );

        // lets print all photons in the event that satisfy the IsPhoton criteria
        IsPhoton isphoton;
        for ( HepMC::GenEvent::particle_iterator p = evt->particles_begin();
              p != evt->particles_end(); ++p ) {
            if ( isphoton(*p) ) (*p)->print();
        }

        // the GenVertex::particle_iterator and GenVertex::vertex_iterator
        // are slightly different from the GenEvent:: versions, in that
        // the iterator starts at the given vertex, and walks through the attached 
        // vertex returning particles/vertices.
        // Thus only particles/vertices which are in the same graph as the given
        // vertex will be returned. A range is specified with these iterators,
        // the choices are:
        //    parents, children, family, ancestors, descendants, relatives 
        // here are some examples.

        // use GenEvent::particle_iterator to find all W's in the event,
        // then 
        // (1) for each W user the GenVertex::particle_iterator with a range of
        //     parents to return and print the immediate mothers of these W's.
        // (2) for each W user the GenVertex::particle_iterator with a range of
        //     descendants to return and print all descendants of these W's.
        IsW_Boson isw;
        for ( HepMC::GenEvent::particle_iterator p = evt->particles_begin();
              p != evt->particles_end(); ++p ) {
            if ( isw(*p) ) {
                std::cout << "A W boson has been found: " << std::endl;
                (*p)->print();
                // return all parents
                // we do this by pointing to the production vertex of the W 
                // particle and asking for all particle parents of that vertex
                std::cout << "\t Its parents are: " << std::endl;
                if ( (*p)->production_vertex() ) {
                    for ( HepMC::GenVertex::particle_iterator mother 
                              = (*p)->production_vertex()->
                              particles_begin(HepMC::parents);
                          mother != (*p)->production_vertex()->
                              particles_end(HepMC::parents); 
                          ++mother ) {
                        std::cout << "\t";
                        (*mother)->print();
                    }
                }
                // return all descendants
                // we do this by pointing to the end vertex of the W 
                // particle and asking for all particle descendants of that vertex
                std::cout << "\t\t Its descendants are: " << std::endl;
                if ( (*p)->end_vertex() ) {
                    for ( HepMC::GenVertex::particle_iterator des 
                              =(*p)->end_vertex()->
                              particles_begin(HepMC::descendants);
                          des != (*p)->end_vertex()->
                              particles_end(HepMC::descendants);
                          ++des ) {
                        std::cout << "\t\t";
                        (*des)->print();
                    }
                }
            }
        }
        // cleanup
        delete evt;
        // in analogy to the above, similar use can be made of the
        // HepMC::GenVertex::vertex_iterator, which also accepts a range.
    } // end scope of ascii_in

    return 0;
}

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