ntpl015_processor_join.C

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/// \file
/// \ingroup tutorial_ntuple
/// \notebook
/// Demonstrate the RNTupleProcessor for horizontal compositions (joins) of RNTuples
///
/// \macro_image
/// \macro_code
///
/// \date November 2024
/// \author The ROOT Team
 
// NOTE: The RNTupleProcessor and related classes are experimental at this point.
// Functionality and interface are still subject to changes.
 
#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleWriter.hxx>
#include <ROOT/RNTupleProcessor.hxx>
 
#include <TCanvas.h>
#include <TH1F.h>
#include <TRandom.h>
 
// Import classes from the `Experimental` namespace for the time being.
using ROOT::Experimental::RNTupleOpenSpec;
using ROOT::Experimental::RNTupleProcessor;
 
const std::string kPrimaryNTupleName = "mainNTuple";
const std::string kMainNTuplePath = "main_ntuple.root";
const std::string kAuxNTupleName = "auxNTuple";
const std::string kAuxNTuplePath = "aux_ntuple.root";
 
// Number of events to generate for the auxiliary ntuple. The primary ntuple will have a fifth of this number.
constexpr int kNEvents = 10000;
 
void WritePrimary(std::string_view ntupleName, std::string_view ntupleFileName)
{
   auto model = ROOT::RNTupleModel::Create();
 
   auto fldI = model->MakeField<std::uint32_t>("i");
   auto fldVpx = model->MakeField<float>("vpx");
 
   auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), ntupleName, ntupleFileName);
 
   // The primary ntuple only contains a subset of the entries present in the auxiliary ntuple.
   for (int i = 0; i < kNEvents; i += 5) {
      *fldI = i;
      *fldVpx = gRandom->Gaus();
 
      writer->Fill();
   }
 
   std::cout << "Wrote " << writer->GetNEntries() << " to the primary RNTuple" << std::endl;
}
 
void WriteAux(std::string_view ntupleName, std::string_view ntupleFileName)
{
   auto model = ROOT::RNTupleModel::Create();
 
   auto fldI = model->MakeField<std::uint32_t>("i");
   auto fldVpy = model->MakeField<float>("vpy");
 
   auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), ntupleName, ntupleFileName);
 
   for (int i = 0; i < kNEvents; ++i) {
      *fldI = i;
      *fldVpy = gRandom->Gaus();
 
      writer->Fill();
   }
 
   std::cout << "Wrote " << writer->GetNEntries() << " to the auxiliary RNTuple" << std::endl;
}
 
void Read()
{
   auto c = new TCanvas("c", "RNTupleJoinProcessor Example", 200, 10, 700, 500);
   TH1F hPy("h", "This is the px + py distribution", 100, -4, 4);
   hPy.SetFillColor(48);
 
   // The first specified ntuple is the primary ntuple and will be used to drive the processor loop. The subsequent
   // list of ntuples (in this case, only one) are auxiliary and will be joined with the entries from the primary
   // ntuple. We specify field "i" as the join field. This field, which should be present in all ntuples specified is
   // used to identify which entries belong together. Multiple join fields can be specified, in which case the
   // combination of field values is used. It is possible to specify up to 4 join fields. Providing an empty list of
   // join fields signals to the processor that all entries are aligned.
   auto processor =
      RNTupleProcessor::CreateJoin({kPrimaryNTupleName, kMainNTuplePath}, {kAuxNTupleName, kAuxNTuplePath}, {"i"});
 
   // Access to the processor's fields is done by first requesting them through RNTupleProcessor::RequestField(). The
   // returned value can be used to read the current entry's value for that particular field. Fields from the primary
   // ntuple are requested by their original name.
   auto px = processor->RequestField<float>("vpx");
   // Fields from auxiliary ntuples are requested by prepending the name of the auxiliary ntuple.
   auto py = processor->RequestField<float>(kAuxNTupleName + ".vpy");
 
   // The iterator value is the index of the current entry being processed. In this example, we don't use it.
   for (auto _ : *processor) {
      hPy.Fill(*px + *py);
   }
 
   std::cout << "Processed a total of " << processor->GetNEntriesProcessed() << " entries" << std::endl;
 
   hPy.DrawCopy();
}
 
void ntpl015_processor_join()
{
   WritePrimary(kPrimaryNTupleName, kMainNTuplePath);
   WriteAux(kAuxNTupleName, kAuxNTuplePath);
 
   Read();
}