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Reference Guide
h1analysis.C File Reference

Detailed Description

View in nbviewer Open in SWAN Example of analysis class for the H1 data.

This file uses 4 large data sets from the H1 collaboration at DESY Hamburg. One can access these data sets (277 MBytes) from the standard Root web site at: https://root.cern.ch/files/h1/ The Physics plots below generated by this example cannot be produced when using smaller data sets.

There are several ways to analyze data stored in a Root Tree

  • Using TTree::Draw: This is very convenient and efficient for small tasks. A TTree::Draw call produces one histogram at the time. The histogram is automatically generated. The selection expression may be specified in the command line.
  • Using the TTreeViewer: This is a graphical interface to TTree::Draw with the same functionality.
  • Using the code generated by TTree::MakeClass: In this case, the user creates an instance of the analysis class. They have the control over the event loop and he can generate an unlimited number of histograms.
  • Using the code generated by TTree::MakeSelector. Like for the code generated by TTree::MakeClass, the user can do complex analysis. However, they cannot control the event loop. The event loop is controlled by TTree::Process called by the user. This solution is illustrated by the current code. The advantage of this method is that it can be run in a parallel environment using PROOF (the Parallel Root Facility).

A chain of 4 files (originally converted from PAW ntuples) is used to illustrate the various ways to loop on Root data sets. Each data set contains a Root Tree named "h42" The class definition in h1analysis.h has been generated automatically by the Root utility TTree::MakeSelector using one of the files with the following statement:


This produces two files: h1analysis.h and h1analysis.C (skeleton of this file) The h1analysis class is derived from the Root class TSelector.

The following members functions are called by the TTree::Process functions.

  • Begin(): Called every time a loop on the tree starts. A convenient place to create your histograms.
  • Notify(): This function is called at the first entry of a new Tree in a chain.
  • Process(): Called to analyze each entry.
  • Terminate(): Called at the end of a loop on a TTree. A convenient place to draw/fit your histograms.

To use this file, try the following sessions

Root > gROOT->Time(); /// will show RT & CPU time per command
#define gROOT
Definition: TROOT.h:404
const char * Root
Definition: TXMLSetup.cxx:47

Case A: Create a TChain with the 4 H1 data files

The chain can be created by executed the short macro h1chain.C below:

TChain chain("h42");
chain.Add("$H1/dstarmb.root"); /// 21330730 bytes 21920 events
chain.Add("$H1/dstarp1a.root"); /// 71464503 bytes 73243 events
chain.Add("$H1/dstarp1b.root"); /// 83827959 bytes 85597 events
chain.Add("$H1/dstarp2.root"); /// 100675234 bytes 103053 events
/// where $H1 is a system symbol pointing to the H1 data directory.
A chain is a collection of files containing TTree objects.
Definition: TChain.h:33

Case B: Loop on all events

Root > chain.Process("h1analysis.C")

Case C: Same as B, but in addition fill the entry list with selected entries.

The entry list is saved to a file "elist.root" by the Terminate function. To see the list of selected events, you can do elist->Print("all"). The selection function has selected 7525 events out of the 283813 events in the chain of files. (2.65 per cent)

Root > chain.Process("h1analysis.C","fillList")

Case D: Process only entries in the entry list

The entry list is read from the file in elist.root generated by step C

Root > chain.Process("h1analysis.C","useList")

Case E: The above steps have been executed via the interpreter.

You can repeat the steps B, C and D using the script compiler by replacing "h1analysis.C" by "h1analysis.C+" or "h1analysis.C++" in a new session (see F).

Case F: Create the chain as in A, then execute

Root > chain.Process("h1analysis.C+","useList")

The same analysis can be run on PROOF. For a quick try start a PROOF-Lite session

This class controls a Parallel ROOT Facility, PROOF, cluster.
Definition: TProof.h:316
static TProof * Open(const char *url=0, const char *conffile=0, const char *confdir=0, Int_t loglevel=0)
Start a PROOF session on a specific cluster.
Definition: TProof.cxx:11579

create (if not already done) the chain by executing the 'h1chain.C' macro mentioned above, and then tell ROOT to use PROOF to process the chain:

Root > chain.SetProof()

You can then repeat step B above. Step C can also be executed in PROOF. However, step D cannot be executed in PROOF as in the local session (i.e. just passing option 'useList'): to use the entry list you have to

Case G: Load first in the session the list form the file

Root > TFile f("elist.root")
Root > TEntryList *elist = (TEntryList *) f.Get("elist")
#define f(i)
Definition: RSha256.hxx:104
A List of entry numbers in a TTree or TChain.
Definition: TEntryList.h:26
A ROOT file is a suite of consecutive data records (TKey instances) with a well defined format.
Definition: TFile.h:54

set it on the chain:

Root > chain.SetEntryList(elist)

call Process as in step B. Of course this works also for local processing.

#include "h1analysis.h"
#include "TH2.h"
#include "TF1.h"
#include "TStyle.h"
#include "TBranch.h"
#include "TCanvas.h"
#include "TPaveStats.h"
#include "TLine.h"
#include "TMath.h"
const Double_t dxbin = (0.17-0.13)/40; // Bin-width
const Double_t sigma = 0.0012;
Double_t x = xx[0];
if (x <= 0.13957) return 0;
Double_t xp3 = (x-par[3])*(x-par[3]);
Double_t res = dxbin*(par[0]*TMath::Power(x-0.13957, par[1])
+ par[2] / 2.5066/par[4]*TMath::Exp(-xp3/2/par[4]/par[4]));
return res;
Double_t x = xx[0];
if (x <= 0.13957) return 0;
Double_t xp3 = (x-0.1454)*(x-0.1454);
Double_t res = dxbin*(par[0]*TMath::Power(x-0.13957, 0.25)
+ par[1] / 2.5066/sigma*TMath::Exp(-xp3/2/sigma/sigma));
return res;
void h1analysis::Begin(TTree * /*tree*/)
// function called before starting the event loop
// -it performs some cleanup
// -it creates histograms
// -it sets some initialisation for the entry list
// This is needed when re-processing the object
//print the option specified in the Process function.
TString option = GetOption();
Info("Begin", "starting h1analysis with process option: %s", option.Data());
//process cases with entry list
delete gDirectory->GetList()->FindObject("elist");
// case when one creates/fills the entry list
if (option.Contains("fillList")) {
elist = new TEntryList("elist", "H1 selection from Cut");
// Add to the input list for processing in PROOF, if needed
if (fInput) {
fInput->Add(new TNamed("fillList",""));
// We send a clone to avoid double deletes when importing the result
// This is needed to avoid warnings from output-to-members mapping
elist = 0;
Info("Begin", "creating an entry-list");
// case when one uses the entry list generated in a previous call
if (option.Contains("useList")) {
if (fInput) {
// In PROOF option "useList" is processed in SlaveBegin and we do not need
// to do anything here
} else {
TFile f("elist.root");
elist = (TEntryList*)f.Get("elist");
if (elist) elist->SetDirectory(0); //otherwise the file destructor will delete elist
// function called before starting the event loop
// -it performs some cleanup
// -it creates histograms
// -it sets some initialisation for the entry list
//initialize the Tree branch addresses
//print the option specified in the Process function.
TString option = GetOption();
"starting h1analysis with process option: %s (tree: %p)", option.Data(), tree);
//create histograms
hdmd = new TH1F("hdmd","dm_d",40,0.13,0.17);
h2 = new TH2F("h2","ptD0 vs dm_d",30,0.135,0.165,30,-3,6);
// Entry list stuff (re-parse option because on PROOF only SlaveBegin is called)
if (option.Contains("fillList")) {
// Get the list
if (fInput) {
if ((elist = (TEntryList *) fInput->FindObject("elist")))
// Need to clone to avoid problems when destroying the selector
if (elist)
if (fillList) Info("SlaveBegin", "creating an entry-list");
if (option.Contains("useList")) useList = kTRUE;
// entry is the entry number in the current Tree
// Selection function to select D* and D0.
//in case one entry list is given in input, the selection has already been done.
if (!useList) {
// Read only the necessary branches to select entries.
// return as soon as a bad entry is detected
// to read complete event, call fChain->GetTree()->GetEntry(entry)
b_md0_d->GetEntry(entry); if (TMath::Abs(md0_d-1.8646) >= 0.04) return kFALSE;
b_ptds_d->GetEntry(entry); if (ptds_d <= 2.5) return kFALSE;
b_etads_d->GetEntry(entry); if (TMath::Abs(etads_d) >= 1.5) return kFALSE;
b_ik->GetEntry(entry); ik--; //original ik used f77 convention starting at 1
b_ipi->GetEntry(entry); ipi--;
if (nhitrp[ik]*nhitrp[ipi] <= 1) return kFALSE;
if (rend[ik] -rstart[ik] <= 22) return kFALSE;
if (rend[ipi]-rstart[ipi] <= 22) return kFALSE;
b_nlhk->GetEntry(entry); if (nlhk[ik] <= 0.1) return kFALSE;
b_nlhpi->GetEntry(entry); if (nlhpi[ipi] <= 0.1) return kFALSE;
b_ipis->GetEntry(entry); ipis--; if (nlhpi[ipis] <= 0.1) return kFALSE;
b_njets->GetEntry(entry); if (njets < 1) return kFALSE;
// if option fillList, fill the entry list
if (fillList) elist->Enter(entry);
// to read complete event, call fChain->GetTree()->GetEntry(entry)
// read branches not processed in ProcessCut
b_dm_d->GetEntry(entry); //read branch holding dm_d
b_rpd0_t->GetEntry(entry); //read branch holding rpd0_t
b_ptd0_d->GetEntry(entry); //read branch holding ptd0_d
//fill some histograms
// Count the number of selected events
return kTRUE;
// nothing to be done
// function called at the end of the event loop
hdmd = dynamic_cast<TH1F*>(fOutput->FindObject("hdmd"));
h2 = dynamic_cast<TH2F*>(fOutput->FindObject("h2"));
if (hdmd == 0 || h2 == 0) {
Error("Terminate", "hdmd = %p , h2 = %p", hdmd, h2);
//create the canvas for the h1analysis fit
TCanvas *c1 = new TCanvas("c1","h1analysis analysis",10,10,800,600);
hdmd->GetXaxis()->SetTitle("m_{K#pi#pi} - m_{K#pi}[GeV/c^{2}]");
//fit histogram hdmd with function f5 using the log-likelihood option
if (gROOT->GetListOfFunctions()->FindObject("f5"))
delete gROOT->GetFunction("f5");
TF1 *f5 = new TF1("f5",fdm5,0.139,0.17,5);
f5->SetParameters(1000000, .25, 2000, .1454, .001);
//create the canvas for tau d0
TCanvas *c2 = new TCanvas("c2","tauD0",100,100,800,600);
// Project slices of 2-d histogram h2 along X , then fit each slice
// with function f2 and make a histogram for each fit parameter
// Note that the generated histograms are added to the list of objects
// in the current directory.
if (gROOT->GetListOfFunctions()->FindObject("f2"))
delete gROOT->GetFunction("f2");
TF1 *f2 = new TF1("f2",fdm2,0.139,0.17,2);
f2->SetParameters(10000, 10);
TH1D *h2_1 = (TH1D*)gDirectory->Get("h2_1");
TLine *line = new TLine(0,0,0,c2->GetUymax());
// Have the number of entries on the first histogram (to cross check when running
// with entry lists)
//save the entry list to a Root file if one was produced
if (fillList) {
if (!elist)
elist = dynamic_cast<TEntryList*>(fOutput->FindObject("elist"));
if (elist) {
Printf("Entry list 'elist' created:");
TFile efile("elist.root","recreate");
} else {
Error("Terminate", "entry list requested but not found in output");
// Notify the amount of processed events
if (!fInput) Info("Terminate", "processed %lld events", fProcessed);
const Bool_t kFALSE
Definition: RtypesCore.h:101
bool Bool_t
Definition: RtypesCore.h:63
double Double_t
Definition: RtypesCore.h:59
const Bool_t kTRUE
Definition: RtypesCore.h:100
#define gDirectory
Definition: TDirectory.h:327
void Printf(const char *fmt,...)
R__EXTERN TStyle * gStyle
Definition: TStyle.h:412
virtual void SetTitleOffset(Float_t offset=1)
Set distance between the axis and the axis title.
Definition: TAttAxis.cxx:293
virtual void SetMarkerStyle(Style_t mstyle=1)
Set the marker style.
Definition: TAttMarker.h:40
virtual Int_t GetEntry(Long64_t entry=0, Int_t getall=0)
Read all leaves of entry and return total number of bytes read.
Definition: TBranch.cxx:1644
The Canvas class.
Definition: TCanvas.h:23
virtual void SetDirectory(TDirectory *dir)
Add reference to directory dir. dir can be 0.
virtual Bool_t Enter(Long64_t entry, TTree *tree=0)
Add entry #entry to the list.
Definition: TEntryList.cxx:625
virtual void Print(const Option_t *option="") const
Print this list.
1-Dim function class
Definition: TF1.h:213
virtual void SetParameters(const Double_t *params)
Definition: TF1.h:644
1-D histogram with a double per channel (see TH1 documentation)}
Definition: TH1.h:618
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:575
TAxis * GetXaxis()
Get the behaviour adopted by the object about the statoverflows. See EStatOverflows for more informat...
Definition: TH1.h:320
TList * GetListOfFunctions() const
Definition: TH1.h:243
virtual TFitResultPtr Fit(const char *formula, Option_t *option="", Option_t *goption="", Double_t xmin=0, Double_t xmax=0)
Fit histogram with function fname.
Definition: TH1.cxx:3892
virtual Int_t Fill(Double_t x)
Increment bin with abscissa X by 1.
Definition: TH1.cxx:3350
virtual void Draw(Option_t *option="")
Draw this histogram with options.
Definition: TH1.cxx:3073
2-D histogram with a float per channel (see TH1 documentation)}
Definition: TH2.h:251
Int_t Fill(Double_t)
Invalid Fill method.
Definition: TH2.cxx:358
virtual void FitSlicesX(TF1 *f1=0, Int_t firstybin=0, Int_t lastybin=-1, Int_t cut=0, Option_t *option="QNR", TObjArray *arr=0)
Project slices along X in case of a 2-D histogram, then fit each slice with function f1 and make a hi...
Definition: TH2.cxx:979
TObject * FindObject(const char *name) const
Find object using its name.
Definition: THashList.cxx:262
A simple line.
Definition: TLine.h:22
virtual void Add(TObject *obj)
Definition: TList.h:87
virtual TObject * FindObject(const char *name) const
Find an object in this list using its name.
Definition: TList.cxx:578
The TNamed class is the base class for all named ROOT classes.
Definition: TNamed.h:29
virtual void SetTitle(const char *title="")
Set the title of the TNamed.
Definition: TNamed.cxx:164
virtual TObject * Clone(const char *newname="") const
Make a clone of an object using the Streamer facility.
Definition: TNamed.cxx:74
virtual Int_t Write(const char *name=0, Int_t option=0, Int_t bufsize=0)
Write this object to the current directory.
Definition: TObject.cxx:798
virtual void Error(const char *method, const char *msgfmt,...) const
Issue error message.
Definition: TObject.cxx:893
virtual void Draw(Option_t *option="")
Default Draw method for all objects.
Definition: TObject.cxx:197
virtual void Info(const char *method, const char *msgfmt,...) const
Issue info message.
Definition: TObject.cxx:867
The histogram statistics painter class.
Definition: TPaveStats.h:18
void SetOptStat(Int_t stat=1)
Set the stat option.
Definition: TPaveStats.cxx:306
TList * fInput
List of objects available during processing.
Definition: TSelector.h:41
TSelectorList * fOutput
! List of objects created during processing
Definition: TSelector.h:42
Long64_t fStatus
Selector status.
Definition: TSelector.h:37
virtual const char * GetOption() const
Definition: TSelector.h:57
Basic string class.
Definition: TString.h:136
const char * Data() const
Definition: TString.h:369
Bool_t Contains(const char *pat, ECaseCompare cmp=kExact) const
Definition: TString.h:624
void SetOptStat(Int_t stat=1)
The type of information printed in the histogram statistics box can be selected via the parameter mod...
Definition: TStyle.cxx:1589
void SetOptFit(Int_t fit=1)
The type of information about fit parameters printed in the histogram statistics box can be selected ...
Definition: TStyle.cxx:1541
A TTree represents a columnar dataset.
Definition: TTree.h:79
virtual void SetEntryList(TEntryList *list, Option_t *opt="")
Set an EntryList.
Definition: TTree.cxx:8933
TBranch * b_md0_d
Definition: h1analysis.h:265
TBranch * b_dm_d
Definition: h1analysis.h:254
Float_t rpd0_t
Definition: h1analysis.h:131
TBranch * b_nlhpi
Definition: h1analysis.h:320
TBranch * b_ipis
Definition: h1analysis.h:261
void Terminate()
Bool_t Process(Long64_t entry)
The Process() function is called for each entry in the tree (or possibly keyed object in the case of ...
TBranch * b_ipi
Definition: h1analysis.h:260
Float_t nlhpi[200]
Definition: h1analysis.h:166
Bool_t useList
Definition: h1analysis.h:30
Int_t nhitrp[200]
Definition: h1analysis.h:157
TBranch * b_ik
Definition: h1analysis.h:259
TBranch * b_rstart
Definition: h1analysis.h:315
TBranch * b_rend
Definition: h1analysis.h:316
TBranch * b_rpd0_t
Definition: h1analysis.h:285
Bool_t fillList
Definition: h1analysis.h:31
Float_t md0_d
Definition: h1analysis.h:111
void Init(TTree *tree)
Definition: h1analysis.h:390
TBranch * b_etads_d
Definition: h1analysis.h:253
TTree * fChain
Definition: h1analysis.h:35
TBranch * b_nhitrp
Definition: h1analysis.h:311
void Begin(TTree *tree)
Long64_t fProcessed
Definition: h1analysis.h:33
Int_t ik
Definition: h1analysis.h:105
TH1F * hdmd
Definition: h1analysis.h:27
Float_t dm_d
Definition: h1analysis.h:100
void Reset()
Definition: h1analysis.h:376
Float_t rend[200]
Definition: h1analysis.h:162
TBranch * b_nlhk
Definition: h1analysis.h:319
TBranch * b_ptds_d
Definition: h1analysis.h:252
TBranch * b_ntracks
Definition: h1analysis.h:303
Float_t rstart[200]
Definition: h1analysis.h:161
TBranch * b_njets
Definition: h1analysis.h:328
void SlaveTerminate()
Int_t ipis
Definition: h1analysis.h:107
Float_t ptd0_d
Definition: h1analysis.h:109
Int_t njets
Definition: h1analysis.h:174
Float_t nlhk[200]
Definition: h1analysis.h:165
void SlaveBegin(TTree *tree)
TEntryList * elist
Definition: h1analysis.h:32
Float_t etads_d
Definition: h1analysis.h:99
TH2F * h2
Definition: h1analysis.h:28
TBranch * b_ptd0_d
Definition: h1analysis.h:263
Float_t ptds_d
Definition: h1analysis.h:98
Int_t ipi
Definition: h1analysis.h:106
long long Long64_t
Definition: cpp_cppyy.h:13
TLine * line
Double_t fdm5(Double_t *xx, Double_t *par)
const Double_t sigma
const Double_t dxbin
Double_t fdm2(Double_t *xx, Double_t *par)
return c1
Definition: legend1.C:41
Double_t x[n]
Definition: legend1.C:17
return c2
Definition: legend2.C:14
Double_t Exp(Double_t x)
Definition: TMath.h:727
LongDouble_t Power(LongDouble_t x, LongDouble_t y)
Definition: TMath.h:735
Short_t Abs(Short_t d)
Definition: TMathBase.h:120
Definition: tree.py:1
Rene Brun

Definition in file h1analysis.C.