Example code which illustrates how to use ROOT trees and ntuples.

TTree represents a columnar dataset. Used for example by all LHC (Large Hadron Collider) experiments. Trees are optimized for reduced disk space and selecting, high-throughput columnar access with reduced memory usage.

In a nutshell:

auto file = TFile::Open("myfile.root", "recreate");
auto tree = new TTree("mytree", "A tree with my data");
...

Explore the examples below or go to TTree class reference.

List of all tutorials

Basic usage

These examples show how data can be stored to and read back from TTree.

Tutorial	Description
tree101_basic.C	Basic TTree usage.
tree102_basic.C	A variant of tree101_basic.C
tree103_tree.C	Simple Event class example
tree104_tree.C	A variant of hsimple.C but using a TTree instead of a TNtuple
tree105_tree.C	Illustrates how to make a tree from variables or arrays in a C struct - without a dictionary, by creating the branches for builtin types (int, float, double) and arrays explicitly
tree106_tree.C	The same as `tree105_tree.C`, but uses a class instead of a C-struct
tree107_tree.C	Example of a tree where branches are variable length arrays
tree108_tree.C	This example writes a tree with objects of the class `Event`
tree109_friend.C	Illustrates how to use tree friends
tree113_getval.C	Illustrates how to retrieve tree variables in arrays
tree114_circular.C	Example of a circular Tree. Circular Trees are interesting in online real time environments to store the results of the last maxEntries events

Copying Trees

These examples shows how to copy the content of trees.

Tutorial	Description
tree110_copy.C	Copy a subset of a tree to a new tree
tree111_copy.C	Copy a subset of a tree to a new tree, one branch in a separate file
tree112_copy.C	Copy a subset of a tree to a new tree, selecting entries

Collection Classes

These examples show how to write and read several collection classes (std::vector, TClonesArray,...) to trees.

Tutorial	Description
tree120_ntuple.C	Read a TNtuple from a file and performs a simple analysis
tree121_hvector.C	Write and read STL vectors in a tree
tree122_vector3.C	Write and read a `Vector3` class in a tree
tree123_clonesarray.C	Write and read a TClonesArray to a tree

More advanced examples

These examples shows a couple of more advanced examples using complex data.

Tutorial	Description
tree130_jets.C	Usage of a tree using the `JetEvent` class. The `JetEvent` class has several collections (TClonesArray) and other collections (TRefArray) referencing objects in the TClonesArrays
tree131_clones_event.C	Example to write & read a Tree built with a complex class inheritance tree. It demonstrates usage of inheritance and TClonesArrays

Graphics

These tutorials show how to generate complex graphics from TNtuple and/or TTree.

Tutorial	Description
tree140_spider.C	Example of a TSpider plot generated from a TNtuple (from hsimple.root)
tree141_parallelcoord.C	Illustrate the use of the TParallelCoord class (from a TNtuple)
tree142_parallelcoordtrans.C	It displays TParallelCoord from the same dataset twice. The first time without transparency and the second time with transparency
tree143_drawsparse.C	Convert a THnSparse to a TTree using efficient iteration through the THnSparse and draw it using TParallelCoord

More tutorials

A collection of other examples which showcases other usecases

Tutorial	Description
tree200_temperature.C	Illustrates how to use the highlight mode with trees, using a temperature dataset from Prague between 1775 and 2004
tree201_histograms.C	Save histograms in tree branches
tree202_benchmarks.C	Benchmark comparing row-wise and column-wise storage performance
tree500_cernbuild.C	Read data (CERN staff) from an ascii file and create a root file with a tree
tree501_cernstaff.C	Playing with a tree containing variables of type character
tree502_staff.C	Create a plot of the data in `cernstaff.root`

Files
file	csv2tntuple.py
	Build ROOT Ntuple from other source.

file	csv2tree_ReadStream.py
	This function provides an example of how one might massage a csv data file to read into a ROOT TTree via TTree::ReadStream.

file	hsimpleProxy.C
	Used by hsimpleProxyDriver.C.

file	hsimpleProxyDriver.C
	This is the driver of the hsimpleProxy example.

file	hsimpleReader.C
	TTreeReader simplest example.

file	imt_parTreeProcessing.C
	Illustrate the usage of the TTreeProcessorMT::Process method.

file	mp_fillNtuples.C
	Fill n-tuples in distinct workers.

file	mp_readNtuplesFillHistosAndFit.C
	Read n-tuples in distinct workers, fill histograms, merge them and fit.

file	mt_fillNtupleFromMultipleThreads.C
	Fill the same TNtuple from different threads.

file	mt_fillNtuples.C
	Fill n-tuples in distinct workers.

file	mt_readNtuplesFillHistosAndFit.C
	Read n-tuples in distinct workers, fill histograms, merge them and fit.

file	mtbb_fillNtuples.C
	Fill n-tuples in distinct workers.

file	ntuple1.py
	Ntuple drawing example.

file	printSizes.C
	This macro can be used to get aggregate information on the size take on disk or in memory by the various branches in a TTree.

file	staff.py
	example of macro to read data from an ascii file and create a root file with a Tree.

file	tree.C
	Display the Tree data structures.

file	tree101_basic.C
	Read data from an ascii file and create a root file with an histogram and an ntuple.

file	tree102_basic.C
	Create can ntuple reading data from an ascii file.

file	tree103_tree.C
	Simple Event class example

file	tree104_tree.C
	This example is a variant of hsimple.C but using a TTree instead of a TNtuple.

file	tree105_tree.C
	This example illustrates how to make a Tree from variables or arrays in a C struct - without a dictionary, by creating the branches for builtin types (int, float, double) and arrays explicitly.

file	tree106_tree.C
	This example is the same as tree105_tree.C, but uses a class instead of a C-struct.

file	tree107_tree.C
	Example of a Tree where branches are variable length arrays A second Tree is created and filled in parallel.

file	tree108_tree.C
	This example writes a tree with objects of the class Event.

file	tree109_friend.C
	Illustrates how to use Tree friends:

file	tree110_copy.C
	Copy a subset of a Tree to a new Tree

file	tree111_copy.C
	Copy a subset of a Tree to a new Tree, one branch in a separate file.

file	tree112_copy.C
	Example of Root macro to copy a subset of a Tree to a new Tree, selecting entries.

file	tree113_getval.C
	Illustrates how to retrieve TTree variables in arrays.

file	tree114_circular.C
	Example of a circular Tree

file	tree120_ntuple.C
	Simple tree analysis.

file	tree121_hvector.C
	Write and read STL vectors in a tree.

file	tree122_vector3.C
	Write and read a Vector3 class in a tree.

file	tree123_clonesarray.C
	How to write a TClonesArray to a TTree

file	tree130_jets.C
	Usage of a Tree using the JetEvent class.

file	tree131_clones_event.C
	Example to write & read a Tree built with a complex class inheritance tree.

file	tree140_spider.C
	TSpider example.

file	tree141_parallelcoord.C
	Script illustrating the use of the TParallelCoord class

file	tree142_parallelcoordtrans.C
	Use of transparency with \|\|-Coord.

file	tree143_drawsparse.C
	Convert a THnSparse to a TTree using efficient iteration through the THnSparse and draw a THnSparse using TParallelCoord.

file	tree200_temperature.C
	This tutorial illustrates how to use the highlight mode with trees.

file	tree201_histograms.C
	Save histograms in Tree branches

file	tree202_benchmarks.C
	Benchmark comparing row-wise and column-wise storage performance

file	tree500_cernbuild.C
	Read data (CERN staff) from an ascii file and create a root file with a Tree.

file	tree501_cernstaff.C
	Playing with a Tree containing variables of type character

file	tree502_staff.C
	Create a plot of the data in `cernstaff.root` To create `cernstaff.root`, execute tutorial `$ROOTSYS/tutorials/io/tree/tree500_cernbuild.C`