tree104_tree.C File Reference

Detailed Description

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 tree104_tree.C

2. Using the automatic compiler interface

   .x tree104_tree.C++

3. .L tree104_tree.C  or .L tree104_tree.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 tree104_tree.C
   read_tree();

    
   #include "TROOT.h"
   #include "TFile.h"
   #include "TTree.h"
   #include "TBrowser.h"
   #include "TH2.h"
   #include "TRandom.h"
    
   void tree104_write()
   {
      // create a Tree file tree104.root
    
      // create the file, the Tree and a few branches
      TFile f("tree104.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();
   }
    
   void tree104_read()
   {
      // read the Tree generated by tree1w and fill two histograms
    
      Float_t px, py, pz;
      Double_t random;
      Int_t ev;
      // note that we create the TFile and TTree objects on the heap!
      // because we want to keep these objects alive when we leave this function.
      auto f = TFile::Open("tree104.root");
      auto t1 = f->Get<TTree>("t1");
      t1->SetBranchAddress("px", &px);
      t1->SetBranchAddress("py", &py);
      t1->SetBranchAddress("pz", &pz);
      t1->SetBranchAddress("random", &random);
      t1->SetBranchAddress("ev", &ev);
    
      // create two histograms
      auto hpx   = new TH1F("hpx", "px distribution", 100, -3, 3);
      auto 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();
   }
    
   void tree104_tree()
   {
      tree104_write();
      tree104_read();
   }

Author

Rene Brun

Definition in file tree104_tree.C.