tree1.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 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();

    
   #include "TROOT.h"
   #include "TFile.h"
   #include "TTree.h"
   #include "TBrowser.h"
   #include "TH2.h"
   #include "TRandom.h"
    
   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();
   }
    
   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();
   }
    
   void tree1() {
      tree1w();
      tree1r();
   }

Author

Rene Brun

Definition in file tree1.C.