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Reference Guide
ntpl003_lhcbOpenData.C File Reference

Detailed Description

View in nbviewer Open in SWAN 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!
// Until C++ runtime modules are universally used, we explicitly load the ntuple library. Otherwise
// triggering autoloading from the use of templated types would require an exhaustive enumeration
// of "all" template instances in the LinkDef file.
#include <ROOT/RField.hxx>
#include <ROOT/RNTuple.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 RNTupleReader = ROOT::Experimental::RNTupleReader;
using RNTupleWriter = ROOT::Experimental::RNTupleWriter;
constexpr char const* kTreeFileName = "http://root.cern.ch/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
auto model = RNTupleModel::Create();
// 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()
// 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->GetName() << " [" << field->GetType() << "]" << 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
// 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
void *fieldDataPtr = model->GetDefaultEntry()->GetValue(l->GetName()).GetRawPtr();
tree->SetBranchAddress(b->GetName(), fieldDataPtr);
// The new ntuple takes ownership of the model
auto ntuple = RNTupleWriter::Recreate(std::move(model), "DecayTree", kNTupleFileName);
auto nEntries = tree->GetEntries();
for (decltype(nEntries) i = 0; i < nEntries; ++i) {
if (i && i % 100000 == 0)
std::cout << "Wrote " << i << " entries" << std::endl;
void ntpl003_lhcbOpenData()
// Create histogram of the flight distance
// We open the ntuple without specifiying 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);
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
#define b(i)
Definition: RSha256.hxx:100
#define f(i)
Definition: RSha256.hxx:104
#define c(i)
Definition: RSha256.hxx:101
#define h(i)
Definition: RSha256.hxx:106
Definition: Rtypes.h:472
A field translates read and write calls from/to underlying columns to/from tree values.
The RNTupleModel encapulates the schema of an ntuple.
An RNTuple that is used to read data from storage.
Definition: RNTuple.hxx:90
An RNTuple that gets filled with entries (data) and writes them to storage.
Definition: RNTuple.hxx:213
The Canvas class.
Definition: TCanvas.h:23
static TFile * Open(const char *name, Option_t *option="", const char *ftitle="", Int_t compress=ROOT::RCompressionSetting::EDefaults::kUseCompiledDefault, Int_t netopt=0)
Create / open a file.
Definition: TFile.cxx:3997
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:575
A TLeaf describes individual elements of a TBranch See TBranch structure in TTree.
Definition: TLeaf.h:57
TRangeDynCast is an adaptater class that allows the typed iteration through a TCollection.
Definition: TCollection.h:411
A TTree represents a columnar dataset.
Definition: TTree.h:79
constexpr const char * kNTupleFileName
Definition: tree.py:1
auto * l
Definition: textangle.C:4
April 2019
The ROOT Team

Definition in file ntpl003_lhcbOpenData.C.