Working with Trees in ROOT

Welcome to hands-on session dedicated on working with the Trees in ROOT. First will start with an exercise on the I/O of ROOT by storing and reading an histogram from a file

Exercise 1: Writing and Reading Histogram from a file

Exercise 1: Writing and Reading histogram from a file

Open a file then create a simple histogram, for example an histogram generated with exponential distribution. Fit it and write it in a file. Why the ROOT Canvas does not show the histogram ?

Use TFile::Open to open the file or just create a TFile object. Call TH1::Write to write the histogram in the file after having filled it.

#include "TFile.h"
#include "TH1.h"

void histogramWrite() { 

   TFile f("histogram.root","RECREATE");

   TH1D * h1 = new TH1D("h1","h1",100,0,10); 
   for (int i = 0; i < 10000; ++i) 
      h1->Fill(gRandom->Exp(5) ); 



Now read the histogram from the file and plot it.

Create a file object (or call TFile::Open) and then TFile::Get

void histogramRead() { 

   TFile * file = new TFile("histogram.root");

   TH1 * h1 = 0;
   // you can also use nut you need to cast if you compile the code
   //TH1 * h1 = (TH1*) file->Get("h1");


Pointing hand You can also use the TBrowser to open the file and display the histogram.

Pointing hand What is going to happen if you delete the file after having retrieved the histogram from the file ?

Exercise 2: Writing and Reading a ROOT n-tuple

Create a ROOT ntuple, a class TNtuple containing 4 variables (for example x,y,z,t). Fill the ntuple with data (for example 10000 events) where x is generated according to a uniform distribution, y a gaussian and z an exponential and t a Landau distribution. Write also the tuple in the file.

Create and fill the tuple as explained in slide 21.

#include "TRandom.h"
#include "TFile.h"
#include "TNtuple.h"

void exampleNtuple() { 

   TNtuple data("ntuple","Example N-tuple","x:y:z:t");

// fill it with random data
   for (int i = 0; i<10000; ++i) {
      float x = gRandom->Uniform(-10,10); 
      float y = gRandom->Gaus(0,5);
      float z = gRandom->Exp(10);
      float t = gRandom->Landau(0,2);
// write in a file
   TFile f("ntuple_data.root","RECREATE");


Afterwards having saved the file, re-open the file and get the ntuple. Plot each single variable of the tuple and also one variable versus another one (for example x versus y).

Open the file, get the tuple object and call the Draw method (see for example slides 22 and 23).

void exampleNtupleDraw() { 

   TFile f("ntuple_data.root");

   TNtuple *ntuple=0;

   // to add a selection cut and a graphic option

Then loop on the tuples, and print every 100 entries on the screen

Open the file, get the tuple object and loop on the entries as in slide 24

void exampleNtupleRead() {

   TFile f("ntuple_data.root");

   TNtuple *ntuple=0;

   // loop on the ntuple entries                                                                                              
   for (int i = 0; i < ntuple->GetEntries(); ++i) {

      float * raw_content = ntuple->GetArgs();
      float x = raw_content[0];
      float y = raw_content[1];
      float z = raw_content[2];
      float t = raw_content[3];

      if (i%100) std::cout << x << "  " << y << "   " << z << "  " << t  << std::endl;
// write in a file                                                                                                            



Exercise 3: Creating a ROOT Tree

Create a ROOT tree which contains an EventData object. The Event data object is defined in the EventData.h attached file. The macro creating the tree and filling with them with the events is CreateTree.C. Look at the macro and try to understand. Run the macro to create and write the tree in the file.

Pointing hand Re-make exercise 2 (the writing part), but instead of using the TNtuple class, use the TTree class. The way to do is to describe the variables when calling TTree::Branch. For example for defining a floating variable x in the TTree you do: tree.Branch("x",&x,"x/F");. See the example at page 38 of the booklet ROOT Guide for Beginners.

Exercise 4: Reading a ROOT Tree

Read the tree from the file and by using the TBrowser or TTree::Draw plot for example the momentum of all the particle or the event size.

Now read the Tree with a macro and calculate the sum of all event sizes.

  • Open the file using its file name in TFile::Open() and get the Tree. Remember to check if the file pointer is not null. If it is null means the file is not existing.
  • Get then a pointer to the tree.
  • Connect a Tree Branch with the Data Member.We have to somehow connect the branch we want to read with the variables used to actually store the data by calling TTree::SetBranchAddress().
  • Load the TTree data. For the analysis example we need to access the events' size, which is stored in the variable eventSize. But the TTree first needs to load the data for each event it contains. For that call TBranch::GetEntry(entry) in a loop, passing the TTree entry number from the loop index to GetEntry(). Again TBranch is the class name, but you obviously need to call it on an object. To know how many entries the tree contains, simply call TTree::GetEntries(). The branch is stored in eventSizeBranch.
  • In the same loop, compute the total size of all events (simply add the current event size to the total size)
  • Without the call to GetEntry(), the variables will not contain data. GetEntry() loads the data into the variables connected with the tree by the call to SetBranchAddress().
  • Access the Analysis Result. At the end of the loop, print the sum of all event sizes. This sum shows you the real power of a TTree: even though you can analyze large amounts of data (our example tree with 22MB is tiny!) ROOT needs just a few MB of your RAM, no matter how many events you analyze. Imagine what it would be like if you had to load all data into memory, e.g. using a simple vector<EventData>

#include "TFile.h"
#include "TTree.h"
#include "TBranch.h"

void AnalyzeTree()
   // Variables used to store the data
   Int_t     totalSize = 0;        // Sum of data size (in bytes) of all events
   Int_t     eventSize = 0;        // Size of the current event
   TBranch  *eventSizeBranch = 0;  // Pointer to the event.fEventsize branch

   // open the file
   TFile *f = TFile::Open("eventdata.root");
   if (f == 0) {
      // if we cannot open the file, print an error message and return immediatly
      printf("Error: cannot open eventdata.root!\n");
   // get a pointer to the tree
   TTree *tree = (TTree *)f->Get("EventTree");

   // To use SetBranchAddress() with simple types (e.g. double, int)
   // instead of objects (e.g. std::vector<Particle>).

   // Connect the branch "fEventSize" with the variable 
   // eventSize that we want to contain the data.
   // While we are at it, ask the tree to save the branch 
   // in eventSizeBranch
   tree->SetBranchAddress("fEventSize", &eventSize, &eventSizeBranch);


   // First, get the total number of entries
   Long64_t nentries = tree->GetEntries();
   // then loop over all of them
   for (Long64_t i=0;i<nentries;i++) {
      // Load the data for TTree entry number "i" from branch 
      // fEventSize into the connected variable (eventSize):
      // compute the total size of all events
      totalSize += eventSize;

   Int_t sizeInMB = totalSize/1024/1024;
   printf("Total size of all events: %d MB\n", sizeInMB);

You can also download the macro AnalyzeTree.C.

Exercise 5: Chaining ROOT Trees.

Use the macro you have used to create a TNtuple (or a TTree) in exercise 2 and run few times, but changing always the name of the file where the tree is stored. Use then the TChain class to merge the tree and draw some of the variables of the tree.

Here is for example the macro to create several trees (We use the Tree class for showing how a TTree is built with simple variables)
#include "TRandom.h"
#include "TFile.h"
#include "TTree.h"

void exampleTTree(const char * filename= "tree.root") {

   TTree data("tree","Example TTree");
   double x, y, z, t;

// fill it with random data                                                                                                   
   for (int i = 0; i<10000; ++i) {
      x = gRandom->Uniform(-10,10);
      y = gRandom->Gaus(0,5);
      z = gRandom->Exp(10);
      t = gRandom->Landau(0,2);

// write in a file                                                                                                            
   TFile f(filename,"RECREATE");


This are the few lines to create the TChain, that you can run directly from the prompt. You can also use wildcard's to chain many files

TChain chain("tree");

Exercise 6: Using Tree Friends

Make a Tree with three variables (x,u) and you fill with some random variables that you prefer. Read from the file the Tree used in exercise 5 and add as a friend to this tree. Plot the x variable of the first tree versus the x variable of the second one.

Here is the macro to create a second tree, containing x, u:
#include "TRandom.h"
#include "TFile.h"
#include "TTree.h"

void exampleTTree2() {

   TTree data("tree_2","Example TTree");
   double x, u;

// fill it with random data                                                                                                   
   for (int i = 0; i<10000; ++i) {
      x = gRandom->Exp(100);
      u = gRandom->Uniform(0,10);

// write in a file                                                                                                            
   TFile f("tree_2.root","RECREATE");


Here are the lines of codes to Draw the x of the first tree versus the x of the second tree with a selection depending on u and t. You can run these lines from the ROOT prompt.

TFile f("tree.root");   // to get  the first tree
tree->Draw("x:tree_2.x","t<100 && tree_2.u<6","COLZ");

Exercise 7: Using the TSelector class for analysing a TTree

Create your own Selector for the simple (x,y,z,t) TTree made in Exercise 5. Use TTree::MakeSelector to create your own Selector class. Inside the code of your Selector do the following:

  • book an histogram in the initialisation routine, for one of the variable of the tree (e.g. the variable t )
  • fill the histogram in the Process function
  • draw the histogram in the Terminate function

Here is what you need to do, after having opened the file with the tree

The file MySelector.h and MySelector.C will be created. Add in MySelector.h, inside the class MySelector, a new data member, the histogram you want to create,

TH1D * h_t;

Edit then the file MySelector.C and add in MySelector::SlaveBegin the booking of the histogram.

h_t = new TH1D("h_t","t",100,0,100);

In MySelector::Process the filling of the histogram after calling TSelector::GetEntry()


In MySelector::Terminate the drawing of the histogram.

After having saved the file run the selection by doing (for example from the ROOT prompt):

TFile f("tree.root");

See the attached file MySelector.h and MySelector.C.
Topic attachments
I Attachment History Action Size Date Who Comment
C source code filec AnalyzeTree.C r2 r1 manage 1.5 K 2013-02-27 - 15:38 LorenzoMoneta Macro to read and analyse the tree
C source code filec CreateTree.C r1 manage 1.4 K 2013-02-26 - 22:48 LorenzoMoneta Macro to create and write the tree in the file
Header fileh EventData.h r2 r1 manage 0.9 K 2013-02-27 - 12:24 LorenzoMoneta File defining the EventData class which will be written in the tree
C source code filec MySelector.C r1 manage 3.3 K 2013-02-27 - 00:33 LorenzoMoneta MySelector files to run Exercise 6
Header fileh MySelector.h r1 manage 2.9 K 2013-02-27 - 00:33 LorenzoMoneta MySelector files to run Exercise 6
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Topic revision: r8 - 2013-03-04 - LorenzoMoneta
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