ntpl002_vector.C File Reference

Detailed Description

Write and read STL vectors with RNTuple.

Adapted from the hvector tree tutorial.

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#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReader.hxx>
#include <ROOT/RNTupleWriter.hxx>
 
#include <TCanvas.h>
#include <TH1F.h>
#include <TRandom.h>
#include <TSystem.h>
 
#include <cstdio>
#include <iostream>
#include <memory>
#include <vector>
#include <utility>
 
// Where to store the ntuple of this example
constexpr char const* kNTupleFileName = "ntpl002_vector.root";
 
// Update the histogram GUI every so many fills
constexpr int kUpdateGuiFreq = 1000;
 
// Number of events to generate
constexpr int kNEvents = 25000;
 
// Generate kNEvents with vectors in kNTupleFileName
void Write()
{
   // We create a unique pointer to an empty data model
   auto model = ROOT::RNTupleModel::Create();
 
   // Creating fields of std::vector is the same as creating fields of simple types.  As a result, we get
   // shared pointers of the given type
   std::shared_ptr<std::vector<float>> fldVpx = model->MakeField<std::vector<float>>("vpx");
   auto fldVpy   = model->MakeField<std::vector<float>>("vpy");
   auto fldVpz   = model->MakeField<std::vector<float>>("vpz");
   auto fldVrand = model->MakeField<std::vector<float>>("vrand");
 
   // We hand-over the data model to a newly created ntuple of name "F", stored in kNTupleFileName
   // In return, we get a unique pointer to an ntuple that we can fill
   auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "F", kNTupleFileName);
 
   TH1F hpx("hpx", "This is the px distribution", 100, -4, 4);
   hpx.SetFillColor(48);
 
   auto c1 = new TCanvas("c1", "Dynamic Filling Example", 200, 10, 700, 500);
 
   gRandom->SetSeed();
   for (int i = 0; i < kNEvents; i++) {
      int npx = static_cast<int>(gRandom->Rndm(1) * 15);
 
      fldVpx->clear();
      fldVpy->clear();
      fldVpz->clear();
      fldVrand->clear();
 
      // Set the field data for the current event
      for (int j = 0; j < npx; ++j) {
         float px, py, pz;
         gRandom->Rannor(px, py);
         pz = px*px + py*py;
         auto random = gRandom->Rndm(1);
 
         hpx.Fill(px);
 
         fldVpx->emplace_back(px);
         fldVpy->emplace_back(py);
         fldVpz->emplace_back(pz);
         fldVrand->emplace_back(random);
      }
 
      // Gui updates
      if (i && (i % kUpdateGuiFreq) == 0) {
         if (i == kUpdateGuiFreq) hpx.Draw();
         c1->Modified();
         c1->Update();
         if (gSystem->ProcessEvents())
            break;
      }
 
      writer->Fill();
   }
 
   hpx.DrawCopy();
 
   // The ntuple unique pointer goes out of scope here.  On destruction, the ntuple flushes unwritten data to disk
   // and closes the attached ROOT file.
}
 
 
// For all of the events, histogram only one of the written vectors
void Read()
{
   // Get a unique pointer to an empty RNTuple model
   auto model = ROOT::RNTupleModel::Create();
 
   // We only define the fields that are needed for reading
   auto fldVpx = model->MakeField<std::vector<float>>("vpx");
 
   // Create an ntuple without imposing a specific data model.  We could generate the data model from the ntuple
   // but here we prefer the view because we only want to access a single field
   auto reader = ROOT::RNTupleReader::Open(std::move(model), "F", kNTupleFileName);
 
   // Quick overview of the ntuple's key meta-data
   reader->PrintInfo();
 
   std::cout << "Entry number 42 in JSON format:" << std::endl;
   reader->Show(41);
 
   TCanvas *c2 = new TCanvas("c2", "Dynamic Filling Example", 200, 10, 700, 500);
   TH1F h("h", "This is the px distribution", 100, -4, 4);
   h.SetFillColor(48);
 
   // Iterate through all the events using i as event number and as an index for accessing the view
   for (auto entryId : *reader) {
      reader->LoadEntry(entryId);
 
      for (auto px : *fldVpx) {
         h.Fill(px);
      }
 
      if (entryId && (entryId % kUpdateGuiFreq) == 0) {
         if (entryId == kUpdateGuiFreq) h.Draw();
         c2->Modified();
         c2->Update();
         if (gSystem->ProcessEvents())
            break;
      }
   }
 
   // Prevent the histogram from disappearing
   h.DrawCopy();
}
 
 
void ntpl002_vector()
{
   Write();
   Read();
}

Date

April 2019

Author

The ROOT Team

Definition in file ntpl002_vector.C.