## main262

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    // main262.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:
    //            Analysis
    //            Tuning
    //            Uncertainty bands
    //            Gnuplot

    // Illustrate how to set up automatic uncertainty band calculations.

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

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

    int main() {

      // Initialize Pythia.
      Pythia pythia;
      pythia.readFile("main262.cmnd");

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

      // Define multiple histograms, one for each variation.
      int nWeights = pythia.info.nWeights();
      vector<double> sumOfWeights;
      vector<Hist> pTtop, nCh;
      vector<string> names;
      vector<string> weightStrings = pythia.settings.wvec("UncertaintyBands:List");

      // Loop through weights to initialize the histograms.
      for (int iWeight=0; iWeight < nWeights; ++iWeight) {
        names.push_back( (iWeight==0)
          ? "baseline" : pythia.info.getGroupName(iWeight));
        pTtop.push_back ( Hist("top transverse momentum",       100,  0., 200.));
        nCh.push_back   ( Hist("charged particle multiplicity", 100, -1., 399.));
        sumOfWeights.push_back(0.);
      }

      // Event generation loop.
      int nEvent = pythia.mode("Main:numberOfEvents");
      for (int iEvent=0; iEvent < nEvent; ++iEvent) {

        // Generate next event. Break out of event loop if at end of an LHE file.
        if ( !pythia.next() ) {
          if ( pythia.info.atEndOfFile() ) break;
          else continue;
        }

        // Find last top in event record and count number of charged particles.
        int iTop = 0;
        int nChg = 0;
        for (int i=0; i < pythia.event.size(); ++i) {
          if (pythia.event[i].id() == 6) iTop = i;
          if (pythia.event[i].isFinal() && pythia.event[i].isCharged()) ++nChg;
        }
        // Get top pT.
        double pT = pythia.event[iTop].pT();

        // Fill histograms with variation weights.
        for (int iWeight = 0; iWeight < nWeights; ++iWeight) {
          // Get weight
          double w = pythia.info.getGroupWeight(iWeight);
          // Add the weight of the current event to the wsum of weights.
          sumOfWeights[iWeight]  += w;
          // Fill histograms.
          pTtop[iWeight].fill(pT, w);
          nCh[iWeight].fill(nChg, w);
        }

      }

      // Print cross section information.
      pythia.stat();

      // Normalize histograms and print data tables. Also, construct a simple
      // gnuplot command to plot the histograms.
      stringstream gnuplotCommand;
      gnuplotCommand << "gnuplot -e \"plot";
      for (int iWeight=0; iWeight < nWeights; ++iWeight) {
        if (sumOfWeights[iWeight] != 0) {
          cout << "Normalize histogram for weight " << names[iWeight] << " to "
               << scientific << setprecision(8) << sumOfWeights[iWeight] << endl;
          pTtop[iWeight] *= pythia.info.sigmaGen() / sumOfWeights[iWeight]
            / 2.; // ... and divide by bin width.
          nCh[iWeight]   *= pythia.info.sigmaGen() / sumOfWeights[iWeight]
            / 4.; // ... and divide by bin width.
        }
        // Print data tables.
        ofstream write;
        write.open( ("pTtop-" + names[iWeight] + ".dat").c_str());
        pTtop[iWeight].table(write);
        write.close();
        write.open( ("nCh-" + names[iWeight] + ".dat").c_str());
        nCh[iWeight].table(write);
        write.close();
        // Construct gnuplot command.
        gnuplotCommand << " \'pTtop-" << names[iWeight]
        << ".dat\' using 1:2 w steps title \'" << names[iWeight] << "\',";
      }
      gnuplotCommand.seekp(-1, std::ios_base::end);
      gnuplotCommand << "; pause -1\"";

      // Print suggested gnuplot command.
      cout << "You can plot the variation by e.g. using the command:" << endl;
      cout << gnuplotCommand.str() << endl << endl;

      // Done.
      return 0;

    }