df002_dataModel.py

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## \file
## \ingroup tutorial_dataframe
## \notebook -draw
## Show how to work with non-flat data models, e.g. vectors of tracks.
##
## This tutorial shows the possibility to use data models which are more
## complex than flat ntuples with RDataFrame.
##
## \macro_code
## \macro_image
##
## \date May 2017
## \author Danilo Piparo (CERN)
 
import ROOT
 
# A simple helper function to fill a test tree: this makes the example stand-alone.
fill_tree_code = '''
using FourVector = ROOT::Math::XYZTVector;
using FourVectorVec = std::vector<FourVector>;
using CylFourVector = ROOT::Math::RhoEtaPhiVector;
 
// A simple helper function to fill a test tree: this makes the example
// stand-alone.
void fill_tree(const char *filename, const char *treeName)
{
   const double M = 0.13957; // set pi+ mass
   TRandom3 R(1);
 
   auto genTracks = [&](){
      FourVectorVec tracks;
      const auto nPart = R.Poisson(15);
      tracks.reserve(nPart);
      for (int j = 0; j < nPart; ++j) {
         const auto px = R.Gaus(0, 10);
         const auto py = R.Gaus(0, 10);
         const auto pt = sqrt(px * px + py * py);
         const auto eta = R.Uniform(-3, 3);
         const auto phi = R.Uniform(0.0, 2 * TMath::Pi());
         CylFourVector vcyl(pt, eta, phi);
         // set energy
         auto E = sqrt(vcyl.R() * vcyl.R() + M * M);
         // fill track vector
         tracks.emplace_back(vcyl.X(), vcyl.Y(), vcyl.Z(), E);
      }
      return tracks;
   };
 
   ROOT::RDataFrame d(64);
   d.Define("tracks", genTracks).Snapshot<FourVectorVec>(treeName, filename, {"tracks"});
}
'''
 
# We prepare an input tree to run on
fileName = "df002_dataModel_py.root"
treeName = "myTree"
ROOT.gInterpreter.Declare(fill_tree_code)
ROOT.fill_tree(fileName, treeName)
 
# We read the tree from the file and create a RDataFrame, a class that
# allows us to interact with the data contained in the tree.
d = ROOT.RDataFrame(treeName, fileName)
 
# Operating on branches which are collections of objects
# Here we deal with the simplest of the cuts: we decide to accept the event
# only if the number of tracks is greater than 8.
n_cut = 'tracks.size() > 8'
nentries = d.Filter(n_cut).Count();
 
print("%s events passed all filters" % nentries.GetValue())
 
# Another possibility consists in creating a new column containing the
# quantity we are interested in.
# In this example, we will cut on the number of tracks and plot their
# transverse momentum.
 
getPt_code ='''
using namespace ROOT::VecOps;
ROOT::RVecD getPt(const RVec<FourVector> &tracks)
{
   auto pt = [](const FourVector &v) { return v.pt(); };
   return Map(tracks, pt);
}
'''
ROOT.gInterpreter.Declare(getPt_code)
 
getPtWeights_code ='''
using namespace ROOT::VecOps;
ROOT::RVecD getPtWeights(const RVec<FourVector> &tracks)
{
   auto ptWeight = [](const FourVector &v) { return 1. / v.Pt(); };
   return Map(tracks, ptWeight);
};
'''
ROOT.gInterpreter.Declare(getPtWeights_code)
 
augmented_d = d.Define('tracks_n', '(int)tracks.size()') \
               .Filter('tracks_n > 2') \
               .Define('tracks_pts', 'getPt( tracks )') \
               .Define("tracks_pts_weights", 'getPtWeights( tracks )' )
 
# The histogram is initialised with a tuple containing the parameters of the
# histogram
trN = augmented_d.Histo1D(("", "", 40, -.5, 39.5), "tracks_n")
trPts = augmented_d.Histo1D("tracks_pts")
trWPts = augmented_d.Histo1D("tracks_pts", "tracks_pts_weights")
 
c1 = ROOT.TCanvas()
trN.Draw()
c1.SaveAs("df002_trN.png")
 
c2 = ROOT.TCanvas()
trPts.Draw()
c2.SaveAs("df002_trPts.png")
 
c3 = ROOT.TCanvas()
trWPts.Draw()
c2.SaveAs("df002_trWPts.png")
 
print("Saved figures to df002_*.png")