tree4.C File Reference

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

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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 tree4.C    

• way2 using the Cling interpreter:

  .L tree4.C
  tree4()

• way3 using ACLIC:

  .L ../test/libEvent.so
  .x tree4.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 tree4.C
  tree4r();

  #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 tree4w()
  {
  
    //create a Tree file tree4.root
    TFile f("tree4.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 tree4r()
  {
    // check to see if the event class is in the dictionary
    // if it is not load the definition in libEvent.so
    if (!TClassTable::GetDict("Event")) {
      gSystem->Load("$ROOTSYS/test/libEvent");
    }
  
    // read the tree generated with tree4w
  
    //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("tree4.root");
    TTree *t4 = (TTree*)f->Get("t4");
  
    // create a pointer to an event object. This will be used
    // to read the branch values.
    Event *event = new Event();
  
    // get two branches and set the branch address
    TBranch *bntrack = t4->GetBranch("fNtrack");
    TBranch *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 tree4() {
     Event::Reset(); // Allow for re-run this script by cleaning static variables.
     tree4w();
     Event::Reset(); // Allow for re-run this script by cleaning static variables.
     tree4r();
  }

Author

Rene Brun

Definition in file tree4.C.