ntpl003_lhcbOpenData.C File Reference

Detailed Description

Convert LHCb run 1 open data from a TTree to RNTuple.

This tutorial illustrates data conversion for a simple, tabular data model. For reading, the tutorial shows the use of an ntuple View, which selectively accesses specific fields. If a view is used for reading, there is no need to define the data model as an RNTupleModel first. The advantage of a view is that it directly accesses RNTuple's data buffers without making an additional memory copy.

 
// NOTE: The RNTuple classes are experimental at this point.
// Functionality, interface, and data format is still subject to changes.
// Do not use for real data!
 
#include <ROOT/RField.hxx>
#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReader.hxx>
#include <ROOT/RNTupleWriter.hxx>
 
#include <TBranch.h>
#include <TCanvas.h>
#include <TFile.h>
#include <TH1F.h>
#include <TLeaf.h>
#include <TTree.h>
 
#include <cassert>
#include <memory>
#include <vector>
 
// Import classes from experimental namespace for the time being
using RNTupleModel = ROOT::Experimental::RNTupleModel;
using RFieldBase = ROOT::Experimental::RFieldBase;
using RNTupleReader = ROOT::Experimental::RNTupleReader;
using RNTupleWriter = ROOT::Experimental::RNTupleWriter;
 
constexpr char const* kTreeFileName = "http://root.cern/files/LHCb/lhcb_B2HHH_MagnetUp.root";
constexpr char const* kNTupleFileName = "ntpl003_lhcbOpenData.root";
 
 
void Convert() {
   std::unique_ptr<TFile> f(TFile::Open(kTreeFileName));
   assert(f && ! f->IsZombie());
 
   // Get a unique pointer to an empty RNTuple model without a default entry
   auto model = RNTupleModel::CreateBare();
 
   // We create RNTuple fields based on the types found in the TTree
   // This simple approach only works for trees with simple branches and only one leaf per branch
   auto tree = f->Get<TTree>("DecayTree");
   for (auto b : TRangeDynCast<TBranch>(*tree->GetListOfBranches())) {
      // The dynamic cast to TBranch should never fail for GetListOfBranches()
      assert(b);
 
      // We assume every branch has a single leaf
      TLeaf *l = static_cast<TLeaf*>(b->GetListOfLeaves()->First());
 
      // Create an ntuple field with the same name and type than the tree branch
      auto field = RFieldBase::Create(l->GetName(), l->GetTypeName()).Unwrap();
      std::cout << "Convert leaf " << l->GetName() << " [" << l->GetTypeName() << "]"
                << " --> "
                << "field " << field->GetFieldName() << " [" << field->GetTypeName() << "]" << std::endl;
 
      // Hand over ownership of the field to the ntuple model.  This will also create a memory location attached
      // to the model's default entry, that will be used to place the data supposed to be written
      model->AddField(std::move(field));
   }
 
   // The new ntuple takes ownership of the model
   auto ntuple = RNTupleWriter::Recreate(std::move(model), "DecayTree", kNTupleFileName);
 
   auto entry = ntuple->GetModel().CreateEntry();
   for (auto b : TRangeDynCast<TBranch>(*tree->GetListOfBranches())) {
      auto l = static_cast<TLeaf *>(b->GetListOfLeaves()->First());
      // We connect the model's default entry's memory location for the new field to the branch, so that we can
      // fill the ntuple with the data read from the TTree
      std::shared_ptr<void> fieldDataPtr = entry->GetPtr<void>(l->GetName());
      tree->SetBranchAddress(b->GetName(), fieldDataPtr.get());
   }
 
   auto nEntries = tree->GetEntries();
   for (decltype(nEntries) i = 0; i < nEntries; ++i) {
      tree->GetEntry(i);
      ntuple->Fill(*entry);
 
      if (i && i % 100000 == 0)
         std::cout << "Wrote " << i << " entries" << std::endl;
   }
}
 
 
void ntpl003_lhcbOpenData()
{
   Convert();
 
   // Create histogram of the flight distance
 
   // We open the ntuple without specifying an explicit model first, but instead use a view on the field we are
   // interested in
   auto ntuple = RNTupleReader::Open("DecayTree", kNTupleFileName);
 
   // The view wraps a read-only double value and accesses directly the ntuple's data buffers
   auto viewFlightDistance = ntuple->GetView<double>("B_FlightDistance");
 
   auto c = new TCanvas("c", "B Flight Distance", 200, 10, 700, 500);
   TH1F h("h", "B Flight Distance", 200, 0, 140);
   h.SetFillColor(48);
 
   for (auto i : ntuple->GetEntryRange()) {
      // Note that we do not load an entry in this loop, i.e. we avoid the memory copy of loading the data into
      // the memory location given by the entry
      h.Fill(viewFlightDistance(i));
   }
 
   h.DrawCopy();
}

Date

April 2019

Author

The ROOT Team

Definition in file ntpl003_lhcbOpenData.C.