tree104_tree.C

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/// \file
/// \ingroup tutorial_tree
/// \notebook -nodraw
/// 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();
/// ~~~
/// \macro_code
///
/// \author Rene Brun
 
#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();
}