## main121

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    // main121.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2025 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
    //            LHE file

    // This is a simple test program.
    // It illustrates how Les Houches Event File input can be used in Pythia8.
    // It uses the ttsample.lhe(.gz) input file, the latter only with 100 events.

    #include "Pythia8/Pythia.h"
    using namespace Pythia8;

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

    int main() {

      // You can always read an plain LHE file,
      // but if you ran "./configure --with-gzip" before "make"
      // then you can also read a gzipped LHE file.
    #ifdef GZIP
      bool useGzip = true;
    #else
      bool useGzip = false;
    #endif
      cout << " useGzip = " << useGzip << endl;

      // Generator. We here stick with default values, but changes
      // could be inserted with readString or readFile.
      Pythia pythia;

      // Initialize Les Houches Event File run. List initialization information.
      pythia.readString("Beams:frameType = 4");
      if (useGzip) pythia.readString("Beams:LHEF = ttbar.lhe.gz");
      else         pythia.readString("Beams:LHEF = ttbar.lhe");
      // If Pythia fails to initialize, exit with error.
      if (!pythia.init()) return 1;

      // Book histogram.
      Hist nCharged("charged particle multiplicity",100,-0.5,399.5);

      // Allow for possibility of a few faulty events.
      int nAbort = 10;
      int iAbort = 0;

      // Begin event loop; generate until none left in input file.
      while (iAbort < nAbort) {

        // Generate events, and check whether generation failed.
        if (!pythia.next()) {

          // If failure because reached end of file then exit event loop.
          if (pythia.info.atEndOfFile()) break;
          ++iAbort;
          continue;
        }

        // Sum up final charged multiplicity and fill in histogram.
        int nChg = 0;
        for (int i = 0; i < pythia.event.size(); ++i)
        if (pythia.event[i].isFinal() && pythia.event[i].isCharged())
          ++nChg;
        nCharged.fill(nChg);

      // End of event loop.
      }

      // Give statistics. Print histogram.
      pythia.stat();
      cout << nCharged;

      // Done.
      return 0;
    }