ROOT

1
ROOT

• An object oriented HEP analysis framework.
• Day 3
• http//www-pat.fnal.gov/root/
• The ROOT system website is at http//root.cern.ch
  /

2
What we covered

• Day 1
• GUI
• Day 2
• More commands (CINT ACLiC)
• Functions and Fitting
• Tree Viewer

3
Class Schedule Day 3

• Building Root Trees
• Reading Root Trees
• Using Trees in Analysis
• The Draw method
• The MakeClass method
• Add your class to ROOT
• With the Interpreter
• With a compiler (Shared Library)
• With ACLiC

4
Building ROOT Trees

• Overview of
• ROOT Files
• Trees
• Branches
• 5 Steps to build a TTree
• Demonstration

5
ROOT Files (TFile)

• When a ROOT file is opened it becomes the
  current directory.
• Histograms and trees are automatically saved in
  the file.
• When the file is closed the histogram and tree
  objects associated with the file are deleted.
• Any object derived from TObject can be written to
  a ROOT file. It has to be added explicitly.

6
ROOT Trees (TTree)

• Storing large number of entries.
• Hierarchy of branches and leaves.
• Reading selective branches

7
Five Steps to Build a Tree

• Steps
• 1. Create a TFile
• 2. Create a TTree
• 3. Add TBranch to the TTree
• 4. Fill the tree
• 5. Write the file

8
Step 1 Create a TFile Object

• The TFile constructor
• file name (i.e. " AFile.root ")
• option NEW, CREATE, RECREATE, UPDATE, or READ
• file title
• compression level 0-9, defaults to 1.

TFile hfile new TFile("AFile.root","RECREATE
","Example")
9
Step 2 Create a TTree Object

• The TTree Constructor
• Tree Name (e.g. "myTree")
• Tree Title
• Maximum total size of buffers kept in memory when
  reading a TTree (defaults to 64 MB)

TTree tree new TTree("myTree","A ROOT tree")
10
Create a Tree with Folders

• TTree aliTree("aliTree", "/aliroot")

• First Parameter tree name
• Second Parameter /name of the top folder

11
Step 3 Adding a Branch(case 1)

• Branch name
• Class name
• Address of the pointer to the Object (descendant
  of TObject)
• Buffer size (default 32,000)
• Split level (default 99)

Event event new Event() myTree-gtBranch
("EventBranch","Event",event)
12
Splitting a Branch

• Setting the split level (default 99)

Split level 0
Split level 99
Example tree-gtBranch("EvBr","Event",ev,64000,0)
13
Adding Branches with a List of Variables

• Branch name
• Address the address of the first item of a
  structure.
• Leaflist all variable names and types
• Order the variables according to their size

Example TBranch b tree-gtBranch
("Ev_Branch",event, "ntrack/Insegnvtexflag/i
temp/F")
14
Adding Branches with a TClonesArray

• Branch name
• Address of a pointer to a TClonesArray
• Buffer size
• Split level

Example tree-gtBranch( "Track_B",Track,64000)
15
List and Folder Branches

• Branch(TList list, buffer, split)
• Creates one branch for each list element
• TObject
• TClonesArray
• Will add a split level parameter
• Branch("folder-name", buffer, split)
• Creates one branch per folder

16
Step 4 Fill the Tree

• Create a for loop
• Assign values to the event object
• Call the Fill method for the tree
• myTree-gtFill()

17
Step 5 Write the File

• The TFileWrite()
• Writes Histograms and Trees
• Write is needed to write file header
• hfile-gtWrite()

18
Demo 5 steps to build a Tree

• BuildTreeDemo.C
• create "AFile.root"
• 2nd Type of Branch, crated with a class name
  and split.
• .X BuildTreeDemo.C
• One tree called "T"
• One branch for eachdata member of Event.
• recursive split (see Track)

19
Summary (Building ROOT Trees)

• Overview of
• ROOT Files
• Trees
• Branches
• 5 Steps to build a TTree
• Demo

20
Reading a TTree

• How to Read a Tree
• Reading Simple variables
• Example reading Selected Branches
• Example reading an Object Branch
• Trees and Friends

21
Looking at the Tree

• TTreePrint() Shows the branchesTFile
  f("AFile.root")myTree-gtPrint() gt print.txt
• TTreeScan("leaf""leaf".)myTree-gtScan("fNseg
  fNtrack") gt scan.txt
• myTree-gtScan("fEventHdr.fDatefNtrack")

22
How To Read TTree

• ROOTSYS/tutorials/tree1.C
• Reading a simple tree
• 1. Open the TFile
• TFile f("tree1.root")
• 2. Get the TTree
• TTree t1
• (TTree)f.FindObject("t1")

23
How to Read A TTree

• 3. Create a variable to hold the data
• Float_t px, py, pz
• 4. Associate a branch with a variable
• SetBranchAddress("name", address)
• t1-gtSetBranchAddress("px", px)
• t1-gtSetBranchAddress("py", py)
• t1-gtSetBranchAddress("pz", pz)

24
GetEntry

• 5. Read one Entry in the TTree
• t1-gtGetEntry(0) // first entry
• root 20 px
• (Float_t)(-1.10227906703948970e00)
• root 21 py
• (Float_t)(-1.79938960075378420e00)
• root 22 pz
• (Float_t)4.45282220840454100e00

25
Demo Reading Branches

• DemoreadTree1.C
• Read selected branches
• Fill two histograms

26
Reading an Object Branch

• Print the first entry with less than 587 tracks
• Find the entry using the fNtrack sub-branch
• Once found, read the entire entry

ROOTSYS/tutorials/tree4.C
27
Friends of Trees

• Adding Branches
• Often the tree is read only
• Risk of Damaging existing tree
• Add a Friend
• Unrestricted Access to the Friend's data

28
Adding a Friend to a TTree

• AddFriend("treeName", "fileName")
• tree.AddFriend("ft1", "ff.root")
• Friends with Trees of the same name
• tree.AddFriend("tree1 tree","ff.root")

29
Accessing Friends

• Access
• treeName.branchName.leafname
• Example
• Int_t px
• t-gtSetBranchAddress("t2.px")
• Or
• t-gtScan("t2.px.px") //unique
• t-gtScan("px")
• Alsot-gtPrint("all")

30
The Friends List

• Number of Entries of a Friend must be greater or
  equal

To access the list of Friends TTreeGetListOfFri
ends() Persistent tree-gtWrite()
31
Summary Reading Trees

• How to Read a Tree
• Reading Simple variables
• Example reading Selected Branches
• Example reading an Object Branch
• Trees and their Friends

32
Trees in Analysis

• Using TTreeDraw()
• Using MakeClass
• TChains

33
Using Trees in Analysis

• The TTreeDraw()
• Parameters
• 1. expressions for x,y,z

myTree-gtDraw("ntrack") myTree-gtDraw("sqrt(ntrack)
ntrack")
34
Using Trees in Analysis (cont.)
The TTreeDraw() Parameters 2. selection 3.
draw option 4. number of entries
myTree-gtDraw("sqrt(ntrack) ntrack", "temp gt
20.8")myTree -gtDraw("sqrt(ntrack)
ntrack", "temp gt20.8","surf2")
35
Using Trees in Analysis (cont.)

• If the Branch was created with an object and was
  not split we can still use the Draw() method.
• myTree-gtDraw("event.GetNtrack()")
• event branch name
• GetNtrack() a method of the
  object on the branch.

36
Histograms and Lists

• The TTreeDraw() parameters continued
• - creating a histogram
• myTree -gtDraw(" ntrack gtgt myHisto") myHisto-gtD
  raw()
• - saving an event list myTree -gtDraw("gtgt
  myList","ntrackgt0") myList-gtPrint("all")
• - using an event list myTree -gtSetEventList(myLis
  t) myTree -gtDraw("ntrack")

37
TTree Contents

• After executing the Draw command, we can get
  information about the TTree
• GetSelectedRows()
• Returns the number of entries accepted by the
  selection expression.
• GetV1(), GetV2(), GetV3()
• returns a pointer to the float array of the
  first, second, or third variable (x,y,z)
• GetW()

38
Introducing MakeClass

• Draw() is powerful and quick.
• What if you would like to plot the masses of all
  oppositely charged pairs of tracks? You need a
  loop over all events, find all pairs of tracks,
  and calculate the required quantities.
• ROOT provides MakeClass to do this

39
Using MakeClass

• Scenario We would like to do selective
  plotting. For simplicity we choose to plot only
  the first 100 tracks of each entry.
• We have a ROOT file with a tree with one branch
  which has leaves of type "Event". The designer
  has made the class definition available in the
  shared library libEvent.so and given you the
  header file Event.h.

40
Event.h

• Event has
• a TClonesArray of Tracks
• GetNtrack() method
• much more
• Track has
• a GetPx() method
• much more ...

41
Using MakeClass()

• 1. Load the shared library
• root 0.L libEvent.so
• 2. Load the root file
• root 1 TFile f new TFile
• ("EventOB.root")
• 3. Call MakeClass root 2 T-gtMakeClass("MyClass"
  )
• - creates MyClass.C and MyClass.h- where does T
  come from?

42
Using MakeClass()

• MyClass.h and MyClass.C
• MyClass.h
• contains the class definition of "MyClass"
• MyTree.C
• contains the class implementation of "MyClass"

43
Loading and Using MyClass.C

• Load the macro and create a MyClass object
• root 0.L libEvent.so
• root 1.L MyClass.C
• root 2 MyClass m new MyClass ()

44
GetEntry()

• MyClassGetEntry()
• root 3 m-gtGetEntry(1)
• root 4 m-gtevent-gtGetNtrack()
• (Int_t)597
• root 5 m-gtGetEntry(2)
• root 6 m-gtevent-gtGetNtrack()
• (Int_t)606

45
Loop()

• MyClassLoop()
• root 6 m-gtLoop()
• Bytes read 48492
• Bytes read 48413
• Bytes read 48255
• Bytes read 48413
• Bytes read 48255
• Bytes read 48176
• ...

46
Demo - Expanding Loop()

• Modifying MyClassLoop()
• 1. Create a Track object
• Track track 0
• 2. Create two histograms
• TH1F myHisto new TH1F(
• "myHisto","fPx",100,-5,5)
• TH1F smallHisto new TH1F(
• "small","fPx 100",100,-5,5)

47
Expanding Loop() (cont.)

• 3. In Event loop, get the event branch
• fChain-gtGetEntry(i)
• 4. And get the number of tracks
• n_Tracks event-gtGetNtrack()
• 6. Add track loop
• for (Int_t j 0 j lt n_Tracks j)
• track (Track) event-gtGetTracks()-gtAt(j)

48
Expanding Loop() (cont.)

• Fill the first histogram with Px
• myHisto-gtFill(track-gtGetPx())
• Add an if statement for the first 100 tracks
• if (j lt 100)
• smallHisto-gtFill(track-gtGetPx())
• Outside of the Event loop, draw the histograms
• myHisto-gtDraw()
• smallHisto-gtDraw("Same")

49
Expanding Loop() (cont.)

• .L libEvent.so
• .L MyClass.C
• MyClass m new MyClass()
• m-gtLoop()

50
Chains

• Scenario
• Perform an analysis using multiple ROOT files.
  All files are of the same structure and have the
  same tree.

51
Chains (cont.)

• TChainAdd()
• root 3 TChain chain("T")
• root 4 chain.Add("Event.root")
• root 5 chain.Draw("fTracks.fPx")
• root 6 myCanvas-gtcd(2)
• root 7 chain.Add("Event50.root")
• root 8 chain.Draw("fTracks.fPx")

52
Chains (cont.)

• TChainGetListOf
• To see the files that are chained
• chain.GetListOfFiles()-gtPrint()
• List the branches and leaves of the chain.
• chain.GetListOfBranches()-gtPrint()
• chain.GetListOfLeaves()-gtPrint()
• TChainMerge()
• To merge the files in a chain and write them to a
  new file
• chain.Merge("all.root")

53
Demo Building Chains

• gROOT-gtLoadMacro("ROOTSYS/test/libEvent.so")
• TChain chain("T")
• chain-gtAdd("EventOB.root")
• chain-gtAdd("EventOB40.root")
• chain-gtAdd("EventOB50.root")
• chain-gtGetListOfFiles()-gtPrint()
• chain-gtMakeClass("ChainClass")

54
Summary Trees in Analysis

• From the command line using TTreeDraw()
• Using MakeClass and Loop()
• Using Chains

55
Adding Your Own Class

• From the Interpreter
• No I/O
• As a Shared Library
• Full Functionality
• With ACLiC
• Full Functionality

56
A Class From the Interpreter
Define the Class in a Macro IClass.Csee notes
below Load the Class by Loading the Macroroot
0 .L IClass.C Instantiate an IClassroot 1
IClass ic new IClass()
57
A Class From the Interpreter
Use the IClass root 2 ic-gtSetX(3) root 3
ic-gtSetY(500) root 4 ic-gtPrint() fX 3, fY
500 Cool, but can't save it. We need the Write()
method from TObject. root 5 ic-gtWrite() Error
Can't call IClassWrite()
58
A Class From a Shared Library
Step 1 define your class as a descendent of
TObject and write the implementation. SClass.h in
clude ltiostream.hgt include "TObject.h" class
SClass public TObject

59
A Class From a Shared Library

• Step 2
• ClassDef(ClassName,ClassVersionID)
• At the end of the class definition
• ClassImp(ClassName)
• At the beginning of the implementation file

60
ClassDef and ClassImp

• ClassDef and ClassImp needed for Object I/O
• These macros can automatically create
• Streamer method needed for writing to ROOT files
  and Trees.
• ShowMembers()
• gtgt operator overload

61
The LinkDef file

• Step 3
• create a LinkDef.h file
• ifdef __CINT__
• pragma link off all globals
• pragma link off all classes
• pragma link off all functions
• pragma link C class SClass
• endif

62
The LinkDef Options

• "-" do not generate a streamer.
• pragma link C class SClass-
• Use for objects with customized streamers
• "!" do not generate the operator gtgt
• pragma link C class SClass-!
• Use for classes not inheriting from TObject.
• "" use the byte count check
• pragma link C class SClass
• !! In ROOT 3 the "" turns on the new ROOT IO.

63
Makefile and rootcint

• Step 4
• Write a Makefile and call rootcint to add your
  class to the dictionary
• SClassDict.cxx SClass.h LinkDef.h
• (ROOTSYS)/bin/rootcint -f SClassDict.cxx -c
  SClass.h LinkDef.h

64
rootcint

• LinkDef.h must be the last argument on the
  rootcint command line.
• The LinkDef file name MUST contain the string
• LinkDef.h or linkdef.h, i.e. NA49_LinkDef.h is
  fine just like, mylinkdef.h.

65
Compile and Load

• Compile the class using the Makefile
• gmake f Makefile.sgikcc
• Load the shared library
• root 0 .L SClass.so
• root 1 SClass sc new SClass()
• root 2 TFile f new TFile("Afile.root",
  "UPDATE")
• root 3 sc-gtWrite()

66
Adding Your Class With ACLiC

• Step 1 define your class
• include "TObject.h"
• class ABC public TObject
• public
• Float_t a,b,c,p
• ABC()a(0),b(0),c(0),p(0)
• ClassDef(ABC,1)
• if !defined(__CINT__) // conditional
• ClassImp(ABC)
• endif

67
Adding Your Class With ACLiC

• Step 2 Load the ABC class in the script.
• Check if ABC is already loaded
• if (!TClassTableGetDict("ABC"))
• gSystem-gtCompileMacro("ABCClass.C")
• Use the Class
• ABC v new ABC
• v-gtp (sqrt((v-gta v-gta)
• (v-gtb v-gtb)(v-gtc v-gtc)))

68
Adding Your Class With ACLiC

• Step 3 Run ABCWriteClass.C
• root0 .X ABCWriteClass.C

69
Summary How Add your own Class

• From the Interpreter
• As a Shared Library
• With ACLiC

70
Wrap Up

• Questions ?
• Feedback Forms
• Vote on importance of subject (no recounts)
• http//www-pat.fnal.gov/root/class/survey/vote.htm
  l
• More information
• http//www-pat.fnal.gov/root/
• http//root.cern.ch
• roottalk_at_root.cern.ch
• about-root_at_fnal.gov
• http//ods.fnal.gov/ods/root-eval

71
ROOT User's Workshop

• Fermilab
• June 13-15, 2001
• http//patwww/root/root2001/

72
Solutions to the Exercises

• http//patwww.fnal.gov/root/class/solutions.htm