## main231

Back to index.

    // main231.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2026 Torbjorn Sjostrand.
    // PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
    // Please respect the MCnet Guidelines, see GUIDELINES for details.

    // Keywords:
    //            Basic usage
    //            Command line option
    //            Command file
    //            Analysis

    // This is a simple test program.
    // It illustrates (a) how to collect the analysis code in a separate class
    // and (b) how to provide the .cmnd filename on the command line

    // Once you have linked the main program you can run it with a command line
    //     ./main231 -c main231.cmnd > main231.log

    #include "Pythia8/Pythia.h"
    #include "Pythia8Plugins/InputParser.h"

    using namespace Pythia8;

    //==========================================================================

    // Put all your own analysis code in the myAnalysis class.

    class MyAnalysis {

    public:

      // Constructor can be empty.
      MyAnalysis() {}

      // Initialization actions.
      void init();

      // Analysis of each new event.
      void analyze(Event& event);

      // Show final results.
      void finish();

    private:

      // Declare variables and objects that span init - analyze - finish.
      int  nEvt;
      Hist brH, yH, etaChg, mult;

    };

    //--------------------------------------------------------------------------

    // The initialization code.

    void MyAnalysis::init() {

      // Initialize counter for number of events.
      nEvt = 0;

      // Book histograms.
      brH.book("Higgs branching ratios by flavour", 30, -0.5, 29.5);
      yH.book("Higgs rapidity", 100, -10., 10.);
      etaChg.book("charged pseudorapidity", 100, -10., 10.);
      mult.book( "charged multiplicity", 100, -0.5, 799.5);

    }

    //--------------------------------------------------------------------------

    // The event analysis code.

    void MyAnalysis::analyze(Event& event) {

      // Increase counter.
      ++nEvt;

      // Find latest copy of Higgs and plot its rapidity.
      int iH = 0;
      for (int i = 0; i < event.size(); ++i)
        if (event[i].id() == 25) iH = i;
      yH.fill( event[iH].y() );

      // Plot flavour of decay channel.
      int idDau1 = event[ event[iH].daughter1() ].idAbs();
      int idDau2 = event[ event[iH].daughter2() ].idAbs();
      int iChan  = 29;
      if (idDau2 == idDau1 && idDau1 < 25) iChan = idDau1;
      if (min( idDau1, idDau2) == 22 && max( idDau1, idDau2) == 23) iChan = 26;
      brH.fill( iChan);

      // Plot pseudorapidity distribution. Sum up charged multiplicity.
      int nChg = 0;
      for (int i = 0; i < event.size(); ++i)
      if (event[i].isFinal() && event[i].isCharged()) {
        etaChg.fill( event[i].eta() );
        ++nChg;
      }
      mult.fill( nChg );

    }

    //--------------------------------------------------------------------------

    // The finishing code.

    void MyAnalysis::finish() {

      // Normalize histograms.
      double binFactor = 5. / nEvt;
      yH     *= binFactor;
      etaChg *= binFactor;

      // Print histograms.
      cout << brH << yH << etaChg << mult;

    }

    //==========================================================================

    // You should not need to touch the main program: its actions are
    // determined by the .cmnd file and the rest belongs in MyAnalysis.

    int main(int argc, char* argv[]) {

      // Set up command line options.
      InputParser ip("Illustrates how collect analysis code and read commands.",
        {"./main231 -c main231.cmnd"});
      ip.require("c", "Use this user-written command file.", {"-cmnd"});

      // Initialize the parser and exit if necessary.
      InputParser::Status status = ip.init(argc, argv);
      if (status != InputParser::Valid) return status;

      // Confirm that external file will be used for settings.
      cout << " PYTHIA settings will be read from file "
           << ip.get<string>("c") << endl;

      // Declare generator. Read in commands from external file.
      Pythia pythia;
      pythia.readFile(ip.get<string>("c"));

      // If Pythia fails to initialize, exit with error.
      if (!pythia.init()) return 1;

      // Declare user analysis class. Do initialization part of it.
      MyAnalysis myAnalysis;
      myAnalysis.init();

      // Read in number of event and maximal number of aborts.
      int nEvent = pythia.mode("Main:numberOfEvents");
      int nAbort = pythia.mode("Main:timesAllowErrors");
      bool hasPL = pythia.flag("PartonLevel:all");

      // Begin event loop.
      int iAbort = 0;
      for (int iEvent = 0; iEvent < nEvent; ++iEvent) {

        // Generate events. Quit if too many failures.
        if (!pythia.next()) {
          if (++iAbort < nAbort) continue;
          cout << " Event generation aborted prematurely, owing to error!\n";
          break;
        }

        // User Analysis of current event.
        myAnalysis.analyze( (hasPL ? pythia.event : pythia.process) );

      // End of event loop.
      }

      // Final statistics.
      pythia.stat();

      // User finishing.
      myAnalysis.finish();

      // Done.
      return 0;
    }