Tree 1

This example is a variant of hsimple.C but using a TTree instead of a TNtuple. It shows:

  • how to fill a Tree with a few simple variables.
  • how to read this Tree
  • how to browse and analyze the Tree via the TBrowser and TTreeViewer This example can be run in many different ways:
  1. Using the Cling interpreter
    .x tree1.C
  2. Using the automatic compiler interface
    .x tree1.C++
  3. .L tree1.C  or .L tree1.C++
    tree1()
    One can also run the write and read parts in two separate sessions. For example following one of the sessions above, one can start the session:
    .L tree1.C
    tree1r();

Author: Rene Brun
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Monday, May 25, 2020 at 04:46 AM.

In [1]:
%%cpp -d
#include "TROOT.h"
#include "TFile.h"
#include "TTree.h"
#include "TBrowser.h"
#include "TH2.h"
#include "TRandom.h"

A helper function is created:

In [2]:
%%cpp -d
void tree1w()
{
   //create a Tree file tree1.root

   //create the file, the Tree and a few branches
   TFile f("tree1.root","recreate");
   TTree t1("t1","a simple Tree with simple variables");
   Float_t px, py, pz;
   Double_t random;
   Int_t ev;
   t1.Branch("px",&px,"px/F");
   t1.Branch("py",&py,"py/F");
   t1.Branch("pz",&pz,"pz/F");
   t1.Branch("random",&random,"random/D");
   t1.Branch("ev",&ev,"ev/I");

   //fill the tree
   for (Int_t i=0;i<10000;i++) {
     gRandom->Rannor(px,py);
     pz = px*px + py*py;
     random = gRandom->Rndm();
     ev = i;
     t1.Fill();
  }

  //save the Tree header. The file will be automatically closed
  //when going out of the function scope
  t1.Write();
}

A helper function is created:

In [3]:
%%cpp -d
void tree1r()
{
   //read the Tree generated by tree1w and fill two histograms

   //note that we use "new" to create the TFile and TTree objects !
   //because we want to keep these objects alive when we leave this function.
   TFile *f = new TFile("tree1.root");
   TTree *t1 = (TTree*)f->Get("t1");
   Float_t px, py, pz;
   Double_t random;
   Int_t ev;
   t1->SetBranchAddress("px",&px);
   t1->SetBranchAddress("py",&py);
   t1->SetBranchAddress("pz",&pz);
   t1->SetBranchAddress("random",&random);
   t1->SetBranchAddress("ev",&ev);

   //create two histograms
   TH1F *hpx   = new TH1F("hpx","px distribution",100,-3,3);
   TH2F *hpxpy = new TH2F("hpxpy","py vs px",30,-3,3,30,-3,3);

   //read all entries and fill the histograms
   Long64_t nentries = t1->GetEntries();
   for (Long64_t i=0;i<nentries;i++) {
     t1->GetEntry(i);
     hpx->Fill(px);
     hpxpy->Fill(px,py);
  }

  //we do not close the file. We want to keep the generated histograms
  //we open a browser and the TreeViewer
  if (gROOT->IsBatch()) return;
  new TBrowser();
  t1->StartViewer();
  // in the browser, click on "ROOT Files", then on "tree1.root".
  //     you can click on the histogram icons in the right panel to draw them.
  // in the TreeViewer, follow the instructions in the Help button.

  // Allow to use the TTree after the end of the function.
  t1->ResetBranchAddresses();
}
In [4]:
tree1w();
tree1r();