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Visualizing Data with ROOT

Visualizing Data with ROOT. Joe Foster University of Manchester. 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)

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Visualizing Data with ROOT

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  1. Visualizing Data with ROOT Joe Foster University of Manchester Joe Foster

  2. 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->Draw(). • Selections. • 2D Plots, • Drawing options • Saving + Printing plots • ROOT macros Joe Foster

  3. 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!). Joe Foster

  4. Where to Get Information • http://root.cern.ch/ • User’s 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. Joe Foster

  5. 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: TH1FMyHist; Class Object • Data is held in ‘member variables’. • Everything is done by ‘member functions’ or ‘methods’. • See ROOT Class Index. Joe Foster

  6. 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. Joe Foster

  7. 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 aTH1F constructor function TH1F objectcalled with histogram parameters New object is stored on the heap & persists when the calling function exits. (warning: memory leaks!) • Rememberdelete h1; Joe Foster

  8. 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] 2+2 (const int)4 root [2] acos(-1) (const double)3.14159265358979312e+00 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. Joe Foster

  9. Basic Classes: TFile, TDirectory Class TFile: public Tdirectory • Open a ROOT file: TFile* ntF = new TFile("ModTests050418.root"); • Close it: ntF->Close() • You can have > 1 file open. Change focus to another open file: File2->cd() Joe Foster

  10. Basic Classes: TFile, TDirectory Class TFile: public Tdirectory • List file contents: root [6]ntF->ls() ‘->’ calls methods for pointers to objects. TFile** ModTests050418.root TreeFile TFile* ModTests050418.root TreeFile KEY: TTree tms;1 Module Production Status KEY: TH1I hintstart;1 Total Modules Started KEY: TH1I hintbond;1 Total Modules Bonded … • Get the ntuple from the file so you can use it: root [7] TTree* tms0 = (TTree*) ntF->Get("tms") Joe Foster

  11. 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->Draw("E”) • The "E” option draws error bars. Joe Foster

  12. Basic Classes: TCanvas • Graphical output goes into a TCanvas object, usually called ‘c1’ by default: h1->Draw("E") • This opens c1 automatically. • You can subdivide the canvas and put different plots in each area: c1->Divide(1,2) c1->cd(1) h1->Draw() … • Set log or linear axes from the canvas: c1->SetLogy(1) // Turns log y axis on. c1->SetLogy(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. Joe Foster

  13. 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. Joe Foster

  14. Running ROOT • To run ROOT on the linux cluster : • Have X11 forwarding enabled for ssh • Open xterm window. % ssh you@linux.hep.man.ac.uk Linux % ssh -X -Y you@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 * * * ******************************************* Joe Foster

  15. 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. Joe Foster

  16. Exploring ROOT files. CINT Commands • Open a file: TFile* myf = new TFile("MyFile.root") • Some useful TDirectory commands: myf->pwd() myf->ls() myf->Close() • Get a Tree and find information: TTree* truth0 = (TTree*) myf->Get("Truth0") truth0->GetEntries() truth0->Print() Joe Foster

  17. Making plots: TTree->Draw(). • Draw b-quark eta distribution: truth0->Draw("Bot_eta") • You can draw calculated formulae: truth0->Draw("Bot_phi[1] - Bot_phi[0]") • Trees can store arrays as well as simple variables. • Formulae can include almost any valid C++ code. truth0->Draw("sqrt((Top_phi[1]-Top_phi[0])*(Top_phi[1]-Top_phi[0]) + (Top_eta[1]-Top_eta[0])*(Top_eta[1]-Top_eta[0]) )") Joe Foster

  18. TTree->Draw(): Selections, Weights • You can add cuts to Draw() commands. Any expression that evaluates to 0 or 1 works: truth0->Draw("W_phi[1] - W_phi[0]", "W_N==2") truth0->Draw("Top_phi-W_phi", "Top_charge*W_charge>0") • Entries can be weighted: truth0->Draw("W_phi[1] - W_phi[0]", "eventWeightMCatNLO*(W_N==2)") • Entries with total weight = 0 are cut. Joe Foster

  19. TTree->Draw(). 2D Plots. Draw Options • 2D plots can reveal information missing from 1D: truth0->Draw("Top_phi:W_phi", "Top_charge*W_charge>0") • Display options can be added from Draw(): truth0->Draw("Top_phi:W_phi", "Top_charge*W_charge>0","box") • Draw options are described in the Class Index web page entry for THistPainter::Paint . Joe Foster

  20. Saving + Printing plots • You can save the result of Ttree Draw() in a histogram and adjust its appearance: Truth0->Draw("W_p_T/1000.0>>TruthPt(50,0.0,500.0)", "W_N>0"); TH1F* TruthPt = (TH1F*) gDirectory->Get("TruthPt"); TruthPt->SetTitle("Truth W Pt"); TruthPt->SetXTitle("Pt"); • Display and print it: TruthPt->Draw(); C1->Print("Truth_W_Pt.gif", "gif"); • See the Tpad Print() command for printing options. Joe Foster

  21. 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++. Joe Foster

  22. ROOT Macros: Example1 • File midyfAll.cpp: • #include <TROOT.h> • #include <TH1F.h> • #include <TTree.h> • #include <TFile.h> • void midyfAll(char* infile) { • TFile* ff = new TFile(infile); • TTree* mod = (TTree*) ff->Get("mod"); • gStyle->SetOptStat(1); • mod->Draw("midyf-midyfNom>>Midyf(16,-0.008,0.008)", "mxy.Test>0"); • TH1F* Midyf = (TH1F*) gDirectory->Get("Midyf"); • Midyf->SetFillColor(8); • Midyf->SetTitle("Midyf - Nominal (mm)"); • Midyf->Draw(); • } Joe Foster

  23. ROOT Macros: Example2 • This macro executes as if you had typed in the commands in CINT: { TChain* truth0 = new TChain("Truth0"); truth0->Add("AcerMCttbar.011.AANT0._*.root"); } • Note: no function name, just { }. Joe Foster

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