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// main42.cc is a part of the PYTHIA event generator.
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// Copyright (C) 2012 Mikhail Kirsanov, Torbjorn Sjostrand.
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// PYTHIA is licenced under the GNU GPL version 2, see COPYING for details.
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// Please respect the MCnet Guidelines, see GUIDELINES for details.
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// This is a simple test program.
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// It illustrates how a file with HepMC events can be generated by Pythia8.
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// Input and output files are specified on the command line, e.g. like
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// ./main42.exe main42.cmnd hepmcout41.dat > out
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// The main program contains no analysis; this is intended to happen later.
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// It therefore "never" has to be recompiled to handle different tasks.
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// WARNING: typically one needs 25 MB/100 events at the LHC.
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// Therefore large event samples may be impractical.
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#include "HepMCInterface.h"
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#include "HepMC/GenEvent.h"
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#include "HepMC/IO_GenEvent.h"
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// Following line is a deprecated alternative, removed in recent versions.
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//#include "HepMC/IO_Ascii.h"
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//#include "HepMC/IO_AsciiParticles.h"
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// Following line to be used with HepMC 2.04 onwards.
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#ifdef HEPMC_HAS_UNITS
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#include "HepMC/Units.h"
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using namespace Pythia8;
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int main(int argc, char* argv[]) {
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// Check that correct number of command-line arguments
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cerr << " Unexpected number of command-line arguments. \n You are"
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<< " expected to provide one input and one output file name. \n"
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<< " Program stopped! " << endl;
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// Check that the provided input name corresponds to an existing file.
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cerr << " Command-line file " << argv[1] << " was not found. \n"
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<< " Program stopped! " << endl;
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// Confirm that external files will be used for input and output.
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cout << "\n >>> PYTHIA settings will be read from file " << argv[1]
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<< " <<< \n >>> HepMC events will be written to file "
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<< argv[2] << " <<< \n" << endl;
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// Interface for conversion from Pythia8::Event to HepMC one.
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HepMC::I_Pythia8 ToHepMC;
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// ToHepMC.set_crash_on_problem();
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// Specify file where HepMC events will be stored.
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HepMC::IO_GenEvent ascii_io(argv[2], std::ios::out);
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// Following line is a deprecated alternative, removed in recent versions
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// HepMC::IO_Ascii ascii_io("hepmcout32.dat", std::ios::out);
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// Line below is an eye-readable one-way output, uncomment the include above
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// HepMC::IO_AsciiParticles ascii_io("hepmcout32.dat", std::ios::out);
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// Read in commands from external file.
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pythia.readFile(argv[1]);
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// Extract settings to be used in the main program.
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int nEvent = pythia.mode("Main:numberOfEvents");
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int nAbort = pythia.mode("Main:timesAllowErrors");
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for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
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// If failure because reached end of file then exit event loop.
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if (pythia.info.atEndOfFile()) {
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cout << " Aborted since reached end of Les Houches Event File\n";
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// First few failures write off as "acceptable" errors, then quit.
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if (++iAbort < nAbort) continue;
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cout << " Event generation aborted prematurely, owing to error!\n";
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// Construct new empty HepMC event.
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#ifdef HEPMC_HAS_UNITS
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// This form with arguments is only meaningful for HepMC 2.04 onwards,
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// and even then unnecessary if HepMC was built with GeV and mm as units.
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HepMC::GenEvent* hepmcevt = new HepMC::GenEvent(
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HepMC::Units::GEV, HepMC::Units::MM);
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// This form is needed for backwards compatibility.
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// In HepMCInterface.cc a conversion from GeV to MeV will be done.
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HepMC::GenEvent* hepmcevt = new HepMC::GenEvent();
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// Fill HepMC event, including PDF info.
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ToHepMC.fill_next_event( pythia, hepmcevt );
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// This alternative older method fills event, without PDF info.
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// ToHepMC.fill_next_event( pythia.event, hepmcevt );
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// Write the HepMC event to file. Done with it.
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ascii_io << hepmcevt;
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// End of event loop. Statistics.
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examples/main42.cc
