Visualizing%20Data%20with%20ROOT

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Visualizing Data with ROOT

• Joe Foster
• University of Manchester

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Contents

• What is ROOT? Why use it?
• Where to get information.
• A little C. CINT the C interpreter.
• Basic Classes
• ROOT files (TFile) directories (TDirectory)
• Histograms (TH1, TH2, TH3)
• Canvases (TCanvas)
• Trees or ntuples (TTree)
• Running ROOT.
• Exploring ROOT files. Using TBrowser.
• Making plots TTree-gtDraw().
• Selections.
• 2D Plots,
• Drawing options
• Saving Printing plots
• ROOT macros

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What is ROOT and why use it?

• What An Object Oriented Data Analysis
  Framework
• OO brings scalable, maintainable code.
• Data analysis
• Visualization 1D, 2D, 3D plots.
• Function evaluation Fitting.
• Either CINT interpreter or compiled C.
• Efficient data input/output storage format.
• Link to SQL databases.
• Network access to data (xrootd).
• Parallel processing (PROOF).
• Why (See above)
• More flexible than spreadsheets.
• Widely used in HEP (ATLAS, D0, BaBar )
• Fairly easy to learn (Physicists are smart!).

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Where to Get Information

• http//root.cern.ch/
• Users Guide (pdf)
• Tutorials, including BaBar FNAL
• Reference Guide. List of all the classes
  member functions.
• In ROOT, do ? for list of CINT commands.
• Colleagues
• Sometimes save hours of searching reading.
• Know about ROOT culture of your experiment.

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A Little C Classes Objects.

• Everything is made from classes.
• A class is an abstract data type.
• Instances of classes are objects.
• Example - Declaring a histogram object
• TH1F MyHist
• Class
• Object
• Data is held in member variables.
• Everything is done by member functions or
  methods.
• See ROOT Class Index.

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A Little C Inheritance

• Specialized classes can inherit properties from
  more general parent classes
• class TH1F public TH1, public TArrayF
• All TH1F objects are also TH1s and TArrayFs.
• TH1F inherits Draw() method from TH1.
• Explore the ROOT class hierarchy in the Class
  Index web pages.
• Some parent classes are never instantiated.

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A Little C Constructor Methods

• When an object is created, its constructor is
  run.
• Usual way of initializing objects.
• Example create a 1D histogram
• TH1F h1 new TH1F("MyHist","My
  Title",100,0,4.4)
• Pointer to a TH1F constructor function
• TH1F object called with histogram parameters
• New object is stored on the heap persists when
  
• the calling function exits. (warning memory
  leaks!)
• Remember delete h1

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CINT the C interpreter

• Command line interpreter.
• Syntax is mostly(!) the same as C.
• Develop code interactively, then save as macros.
• Any C expression is evaluated immediately
• root 1 22
• (const int)4
• root 2 acos(-1)
• (const double)3.14159265358979312e00
• Root 3 .x MyProg.cxx load execute MyProg
• root 4 .q quit ROOT
• CINT commands are prefixed with a . No at
  end of line.
• A simple debugger lets you step through a
  program, set breakpoints, etc. Do ? in ROOT
  to see the commands.

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Basic Classes TFile, TDirectory

• Class TFile public Tdirectory
• Open a ROOT file
• TFile ntF new TFile("ModTests050418.root")
• Close it
• ntF-gtClose()
• You can have gt 1 file open. Change focus to
  another open file
• File2-gtcd()

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Basic Classes TFile, TDirectory

• Class TFile public Tdirectory
• List file contents
• root 6 ntF-gtls() -gt calls methods for
• pointers to objects.
• TFile ModTests050418.root TreeFile
• TFile ModTests050418.root TreeFile
• KEY TTree tms1 Module Production Status
  
• KEY TH1I hintstart1 Total Modules
  Started
• KEY TH1I hintbond1 Total Modules
  Bonded
• Get the ntuple from the file so you can use it
• root 7 TTree tms0 (TTree) ntF-gtGet("tms")

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Basic Classes Histograms

• 1D, 2D, 3D histograms (TH1, TH2, TH3).
• In each case, options for 1 byte, integer, float,
  double per channel.
• Book a histogram by declaring it, supplying
  nchans, xlow, xup, etc as parameters to
  constructor method
• TH1F h1 new TH1F("MyHist","MyTitle",100,0.0,4.4
  )
• Draw it
• h1-gtDraw("E)
• The "E option draws error bars.

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Basic Classes TCanvas

• Graphical output goes into a TCanvas object,
  usually called c1 by default
• h1-gtDraw("E")
• This opens c1 automatically.
• You can subdivide the canvas and put different
  plots in each area
• c1-gtDivide(1,2)
• c1-gtcd(1)
• h1-gtDraw()
• Set log or linear axes from the canvas
• c1-gtSetLogy(1) // Turns log y axis on.
• c1-gtSetLogy(0) // Turns it off.
• There is also a graphical editor. Switch it on
  from Options View menus.
• Once the plot is to your liking, save it from the
  File menu on c1.

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Basic Classes TTree

• A Tree is like an ntuple which stores any kind of
  object, not just floating point numbers.
• Efficient storage format - save disk space with
  large amounts of data.
• Fast access methods - quickly scan the whole
  Tree.
• Produce 1D, 2D, 3D plots directly from the Tree.
  
• Plot results of calculations on stored data.
• Complex selections of which data to plot.
• Save plots as histograms.
• Loop over arrays stored in the rows.
• Add variables from other Trees with AddFriend().
• Extend effective length of a Tree with a Chain of
  Trees.

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Running ROOT

• To run ROOT on the linux cluster
• Have X11 forwarding enabled for ssh
• Open xterm window.
• ssh you_at_linux.hep.man.ac.uk Linux
• ssh -X -Y you_at_linux.hep.man.ac.uk Mac
• cd YourDataDirectory
• root
• W E L C O M E to R O O T
• Version 5.12/00 10 July 2006
• You are welcome to visit our Web site
• http//root.cern.ch

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Exploring ROOT files. Using TBrowser.

• TBrowser is a graphical interface for exploring
  ROOT files and Directories. You can display
  stored histograms, and make simple plots from
  Trees.
• To start a TBrowser
• Declare a TBrowser object in an xterm window
• TBrowser tb
• Wait patiently while it starts.

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Exploring ROOT files. CINT Commands

• Open a file
• TFile myf new TFile("MyFile.root")
• Some useful TDirectory commands
• myf-gtpwd()
• myf-gtls()
• myf-gtClose()
• Get a Tree and find information
• TTree truth0 (TTree) myf-gtGet("Truth0")
• truth0-gtGetEntries()
• truth0-gtPrint()

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Making plots TTree-gtDraw().

• Draw b-quark eta distribution
• truth0-gtDraw("Bot_eta")
• You can draw calculated formulae
• truth0-gtDraw("Bot_phi1 - Bot_phi0")
• Trees can store arrays as well as simple
  variables.
• Formulae can include almost any valid C code.
• truth0-gtDraw("sqrt((Top_phi1-Top_phi0)(Top_ph
  i1-Top_phi0) (Top_eta1-Top_eta0)(Top_e
  ta1-Top_eta0) )")

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TTree-gtDraw() Selections, Weights

• You can add cuts to Draw() commands. Any
  expression that evaluates to 0 or 1 works
• truth0-gtDraw("W_phi1 - W_phi0", "W_N2")
• truth0-gtDraw("Top_phi-W_phi", "Top_chargeW_charge
  gt0")
• Entries can be weighted
• truth0-gtDraw("W_phi1 - W_phi0",
  "eventWeightMCatNLO(W_N2)")
• Entries with total weight 0 are cut.

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TTree-gtDraw(). 2D Plots. Draw Options

• 2D plots can reveal information missing from 1D
• truth0-gtDraw("Top_phiW_phi", "Top_chargeW_charge
  gt0")
• Display options can be added from Draw()
• truth0-gtDraw("Top_phiW_phi", "Top_chargeW_charge
  gt0","box")
• Draw options are described in the Class Index web
  page entry for THistPainterPaint .

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Saving Printing plots

• You can save the result of Ttree Draw() in a
  histogram and adjust its appearance
• Truth0-gtDraw("W_p_T/1000.0gtgtTruthPt(50,0.0,500.0)"
  , "W_Ngt0")
• TH1F TruthPt (TH1F) gDirectory-gtGet("TruthPt")
  
• TruthPt-gtSetTitle("Truth W Pt")
• TruthPt-gtSetXTitle("Pt")
• Display and print it
• TruthPt-gtDraw()
• C1-gtPrint("Truth_W_Pt.gif", "gif")
• See the Tpad Print() command for printing
  options.

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ROOT macros

• ROOT macros are C files that execute within
  ROOT.
• They can be built up from commands tried out in
  CINT.
• Remember to add '' at the ends of lines.
• Include the necessary 'include' directives to
  make it stand alone.
• Execute a macro in CINT
• .x MyMacro.cxx
• File extension should be '.cxx', '.cpp', or just
  '.C' in case it is just C and not C.

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ROOT Macros Example1

• File midyfAll.cpp
• include ltTROOT.hgt
• include ltTH1F.hgt
• include ltTTree.hgt
• include ltTFile.hgt
• void midyfAll(char infile)
• TFile ff new TFile(infile)
• TTree mod (TTree) ff-gtGet("mod")
• gStyle-gtSetOptStat(1)
• mod-gtDraw("midyf-midyfNomgtgtMidyf(16,-0.008,0.008)
  ", "mxy.Testgt0")
• TH1F Midyf (TH1F) gDirectory-gtGet("Midyf")
• Midyf-gtSetFillColor(8)
• Midyf-gtSetTitle("Midyf - Nominal (mm)")
• Midyf-gtDraw()

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ROOT Macros Example2

• This macro executes as if you had typed in the
  commands in CINT
• TChain truth0 new TChain("Truth0")
• truth0-gtAdd("AcerMCttbar.011.AANT0._.root")
• Note no function name, just .