ntpl001_staff.C File Reference

Detailed Description

Write and read tabular data with RNTuple.

Adapted from the cernbuild and cernstaff tree tutorials. Illustrates the type-safe ntuple model interface, which is used to define a data model that is in a second step taken by an ntuple reader or writer.

 
#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReader.hxx>
#include <ROOT/RNTupleWriter.hxx>
 
#include <TCanvas.h>
#include <TH1I.h>
#include <TROOT.h>
#include <TString.h>
 
#include <cassert>
#include <cstdio>
#include <fstream>
#include <iostream>
#include <memory>
#include <string>
#include <sstream>
#include <utility>
 
constexpr char const* kNTupleFileName = "ntpl001_staff.root";
 
void Ingest() {
   // The input file cernstaff.dat is a copy of the CERN staff data base from 1988
   ifstream fin(gROOT->GetTutorialDir() + "/io/tree/cernstaff.dat");
   assert(fin.is_open());
 
   // We create a unique pointer to an empty data model
   auto model = ROOT::RNTupleModel::Create();
 
   // To define the data model, we create fields with a given C++ type and name.  Fields are roughly TTree branches.
   // MakeField returns a shared pointer to a memory location that we can populate to fill the ntuple with data
   auto fldCategory = model->MakeField<int>("Category");
   auto fldFlag     = model->MakeField<unsigned int>("Flag");
   auto fldAge      = model->MakeField<int>("Age");
   auto fldService  = model->MakeField<int>("Service");
   auto fldChildren = model->MakeField<int>("Children");
   auto fldGrade    = model->MakeField<int>("Grade");
   auto fldStep     = model->MakeField<int>("Step");
   auto fldHrweek   = model->MakeField<int>("Hrweek");
   auto fldCost     = model->MakeField<int>("Cost");
   auto fldDivision = model->MakeField<std::string>("Division");
   auto fldNation   = model->MakeField<std::string>("Nation");
 
   // We hand-over the data model to a newly created ntuple of name "Staff", 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), "Staff", kNTupleFileName);
 
   std::string record;
   while (std::getline(fin, record)) {
      std::istringstream iss(record);
      iss >> *fldCategory >> *fldFlag >> *fldAge >> *fldService >> *fldChildren >> *fldGrade >> *fldStep >> *fldHrweek
          >> *fldCost >> *fldDivision >> *fldNation;
      writer->Fill();
   }
 
   // The ntuple unique pointer goes out of scope here.  On destruction, the ntuple flushes unwritten data to disk
   // and closes the attached ROOT file.
}
 
void Analyze() {
   // Get a unique pointer to an empty RNTuple model
   auto model = ROOT::RNTupleModel::Create();
 
   // We only define the fields that are needed for reading
   std::shared_ptr<int> fldAge = model->MakeField<int>("Age");
 
   // Create an ntuple and attach the read model to it
   auto reader = ROOT::RNTupleReader::Open(std::move(model), "Staff", kNTupleFileName);
 
   // Quick overview of the ntuple and list of fields.
   reader->PrintInfo();
 
   std::cout << "The first entry in JSON format:" << std::endl;
   reader->Show(0);
 
   auto c = new TCanvas("c", "", 200, 10, 700, 500);
   TH1I h("h", "Age Distribution CERN, 1988", 100, 0, 100);
   h.SetFillColor(48);
 
   for (auto entryId : *reader) {
      // Populate fldAge
      reader->LoadEntry(entryId);
      h.Fill(*fldAge);
   }
 
   h.DrawCopy();
}
 
void ntpl001_staff() {
   Ingest();
   Analyze();
}

Date

April 2019

Author

The ROOT Team

Definition in file ntpl001_staff.C.