df017_vecOpsHEP.C

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/// \file
/// \ingroup tutorial_dataframe
/// \notebook -draw
/// This tutorial shows how VecOps can be used to slim down the programming
/// model typically adopted in HEP for analysis.
/// In this case we have a dataset containing the kinematic properties of
/// particles stored in individual arrays.
/// We want to plot the transverse momentum of these particles if the energy is
/// greater than 100.
///
/// \macro_code
/// \macro_image
///
/// \date March 2018
/// \author Danilo Piparo, Andre Vieira Silva

auto filename = gROOT->GetTutorialDir() + "/dataframe/df017_vecOpsHEP.root";
auto treename = "myDataset";
using doubles = ROOT::VecOps::RVec<double>;
using RDF = ROOT::RDataFrame;

void WithTTreeReader()
{
   TFile f(filename);
   TTreeReader tr(treename, &f);
   TTreeReaderArray<double> px(tr, "px");
   TTreeReaderArray<double> py(tr, "py");
   TTreeReaderArray<double> E(tr, "E");

   TH1F h("pt", "pt", 16, 0, 4);

   while (tr.Next()) {
      for (auto i=0U;i < px.GetSize(); ++i) {
         if (E[i] > 100) h.Fill(sqrt(px[i]*px[i] + py[i]*py[i]));
      }
   }
   h.DrawCopy();
}

void WithRDataFrame()
{
  RDF f(treename, filename.Data());
   auto CalcPt = [](doubles &px, doubles &py, doubles &E) {
      doubles v;
      for (auto i=0U;i < px.size(); ++i) {
         if (E[i] > 100) {
            v.emplace_back(sqrt(px[i]*px[i] + py[i]*py[i]));
         }
      }
      return v;
   };
   f.Define("pt", CalcPt, {"px", "py", "E"})
    .Histo1D<doubles>({"pt", "pt", 16, 0, 4}, "pt")->DrawCopy();
}

void WithRDataFrameVecOps()
{
   RDF f(treename, filename.Data());
   auto CalcPt = [](doubles &px, doubles &py, doubles &E) {
      auto pt = sqrt(px*px + py*py);
      return pt[E>100];
   };
   f.Define("good_pt", CalcPt, {"px", "py", "E"})
    .Histo1D<doubles>({"pt", "pt", 16, 0, 4}, "good_pt")->DrawCopy();
}

void WithRDataFrameVecOpsJit()
{
   RDF f(treename, filename.Data());
   f.Define("good_pt", "sqrt(px*px + py*py)[E>100]")
    .Histo1D({"pt", "pt", 16, 0, 4}, "good_pt")->DrawCopy();
}

void df017_vecOpsHEP()
{
   // We plot four times the same quantity, the key is to look into the implementation
   // of the functions above
   auto c = new TCanvas();
   c->Divide(2,2);
   c->cd(1);
   WithTTreeReader();
   c->cd(2);
   WithRDataFrame();
   c->cd(3);
   WithRDataFrameVecOps();
   c->cd(4);
   WithRDataFrameVecOpsJit();
}