Tree 1¶

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();

Author: Rene Brun
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Monday, September 20, 2021 at 03:08 PM.

In [1]:
%%cpp -d
#include "TROOT.h"
#include "TFile.h"
#include "TTree.h"
#include "TBrowser.h"
#include "TH2.h"
#include "TRandom.h"


A helper function is created:

In [2]:
%%cpp -d
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();
}


A helper function is created:

In [3]:
%%cpp -d
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;

//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.

tree1w();