tree108_tree.C File Reference

Detailed Description

This example writes a tree with objects of the class Event.

It is a simplified version of $ROOTSYS/test/MainEvent.cxx to write the tree, and $ROOTSYS/test/eventb.C It shows:

• how to fill a Tree with an event class containing these data members:

  char           fType[20];
  Int_t          fNtrack;
  Int_t          fNseg;
  Int_t          fNvertex;
  UInt_t         fFlag;
  Float_t        fTemperature;
  EventHeader    fEvtHdr;
  TClonesArray  *fTracks;            //->
  TH1F          *fH;                 //->
  Int_t          fMeasures[10];
  Float_t        fMatrix[4][4];
  Float_t       *fClosestDistance;   //[fNvertex]

• the difference in splitting or not splitting a branch
• how to read selected branches of the tree, and print the first entry with less than 587 tracks.
• how to browse and analyze the Tree via the TBrowser and TTreeViewer

This example can be run in many different ways:

• way1 using the Cling interpreter:

  .x tree108_tree.C

• way2 using the Cling interpreter:

  .L tree108_tree.C
  tree108_tree()

• way3 using ACLIC:

  .L ../test/libEvent.so
  .x tree108_tree.C++

  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 tree108_tree.C
  tree108_read();

   
  #ifdef R__WIN32
  R__LOAD_LIBRARY($ROOTSYS/test/libEvent.dll)
  #else
  R__LOAD_LIBRARY($ROOTSYS/test/libEvent.so)
  #endif
   
  #include "TFile.h"
  #include "TTree.h"
  #include "TBrowser.h"
  #include "TH2.h"
  #include "TRandom.h"
  #include "TClassTable.h"
  #include "TSystem.h"
  #include "TROOT.h"
  #include "../test/Event.h"
   
  void tree108_write()
  {
     // create a Tree file tree108.root
     TFile f("tree108.root","RECREATE");
   
     // create a ROOT Tree
     TTree t4("t4","A Tree with Events");
   
     // create a pointer to an Event object
     Event *event = new Event();
   
     // create two branches, split one.
     t4.Branch("event_split", &event,16000,99);
     t4.Branch("event_not_split", &event,16000,0);
   
     // a local variable for the event type
     char etype[20];
   
     // fill the tree
     for (Int_t ev = 0; ev < 100; ev++) {
        Float_t sigmat, sigmas;
        gRandom->Rannor(sigmat, sigmas);
        Int_t ntrack = Int_t(600 + 600 * sigmat / 120.);
        Float_t random = gRandom->Rndm(1);
        sprintf(etype, "type%d", ev%5);
        event->SetType(etype);
        event->SetHeader(ev, 200, 960312, random);
        event->SetNseg(Int_t(10 * ntrack + 20 * sigmas));
        event->SetNvertex(Int_t(1 + 20 * gRandom->Rndm()));
        event->SetFlag(UInt_t(random + 0.5));
        event->SetTemperature(random + 20.);
   
        for (UChar_t m = 0; m < 10; m++) {
           event->SetMeasure(m, Int_t(gRandom->Gaus(m, m + 1)));
        }
   
        // fill the matrix
        for (UChar_t i0 = 0; i0 < 4; i0++) {
           for(UChar_t i1 = 0; i1 < 4; i1++) {
              event->SetMatrix(i0, i1, gRandom->Gaus(i0 * i1, 1));
           }
        }
   
        // Create and fill the Track objects
        for (Int_t t = 0; t < ntrack; t++) event->AddTrack(random);
   
        // Fill the tree
        t4.Fill();
   
        // Clear the event before reloading it
        event->Clear();
     }
     // Write the file header
     f.Write();
   
     // Print the tree contents
     t4.Print();
  }
   
   
  void tree108_read()
  {
     // read the tree generated with tree108_write
   
     // 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("tree108.root");
     auto t4 = f->Get<TTree>("t4");
   
     // create a pointer to an event object. This will be used
     // to read the branch values.
     auto event = new Event();
   
     // get two branches and set the branch address
     auto bntrack = t4->GetBranch("fNtrack");
     auto branch  = t4->GetBranch("event_split");
     branch->SetAddress(&event);
   
     Long64_t nevent = t4->GetEntries();
     Int_t nselected = 0;
     Int_t nb = 0;
     for (Long64_t i=0; i<nevent; i++) {
        // read branch "fNtrack"only
        bntrack->GetEntry(i);
   
        // reject events with more than 587 tracks
        if (event->GetNtrack() > 587)
           continue;
   
        // read complete accepted event in memory
        nb += t4->GetEntry(i);
        nselected++;
   
        // print the first accepted event
        if (nselected == 1)
           t4->Show();
   
        // clear tracks array
        event->Clear();
     }
   
     if (gROOT->IsBatch())
        return;
     new TBrowser();
     t4->StartViewer();
  }
   
  void tree108_tree()
  {
     Event::Reset(); // Allow for re-run this script by cleaning static variables.
     tree108_write();
     Event::Reset(); // Allow for re-run this script by cleaning static variables.
     tree108_read();
  }

Author

Rene Brun

Definition in file tree108_tree.C.