Logo ROOT   6.21/01
Reference Guide
tree4.C File Reference

Detailed Description

View in nbviewer Open in SWAN 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();
    R__LOAD_LIBRARY($ROOTSYS/test/libEvent.so)
    #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()
    {
    // 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.