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Introduction and useful links Root Wikipedia, users-guide (5.26), Reference Guide, HowTo's, Tutorials, Root Talk, user forums. Root ROOT is an object-oriented program and library developed by CERN for the visualization and data processing. It provides containers for data analysis.

Navigating Root

• CINT/ROOT C/C++ Interpreter
• TBrowser

Starting Root

• CINT/ROOT C/C++ Interpreter:

>root --help
Usage: root [-l] [-b] [-n] [-q] [dir] [[file:]data.root] [file1.C ... fileN.C]
Options:
  -b : run in batch mode without graphics
  -n : do not execute logon and logoff macros as specified in .rootrc
  -q : exit after processing command line macro files
  -l : do not show splash screen
 dir : if dir is a valid directory cd to it before executing

  -?      : print usage
  -h      : print usage
  --help  : print usage
  -config : print ./configure options

Step Up Corner:

• Examples how to call a root macro from the command line:

>root test.C
>root -lbq test.C\(4\)
>root -lb test.C\(8,15\)
>root -l test.C\(\"myfile.root\",2342\)

Compiling a root macro

• CINT/ROOT C/C++ Interpreter:

root -l
root [0] gSystem->CompileMacro("test_macro.C");
root [1] test()

root -l
root [0] gSystem->Load("test_macro_C.so"); // loading the shared library we
created earlier
root [1] test()

Using an ntuple or tree

• CINT/ROOT C/C++ Interpreter:

root -l
TFile *mfile = new TFile("filename.root");
TTree *tree = (TTree*)mfile->Get("ntuple");
tree->Draw("x");

• Generating a class based on your tree:

root -l
>TFile *mfile = new TFile("filename.root");
>TTree *tree = (TTree*)mfile->Get("ntuple");
>tree->MakeClass();

MakeClass will create a source and header file for your class based on the ntuple. This is definitely the way to go for more complicated data analysis.

Step Up Corner:

• Examples using a tree:

>tree->AddFriend("Hits",mfile);

>tree->Draw("x","weight");
>tree->Draw("x-z","y","y>1623"); // (expression, weight, condition) 
>tree->Draw("(x-z) > histogram","y","(y>4) && (z<8)"); // (expression, weight, condition) 

Color and Markers:

Adding a legend and title to your plots

• Example of using a TLatex:

    /////////////////////////////
    ///     TLatex Title      ///
    /////////////////////////////

    char title[200];
    sprintf(title,"Average Number of Events");
    TLatex *tex1 = new TLatex(1,50,title);
    tex1->SetTextSize(0.055);
    tex1->SetTextColor(1);
    tex1->Draw("same");

• Example of using a TLegend:

   ////////////
   // Legend //
   ////////////

   char legend1[100];
   char legend2[100];
   char legend3[100];

   sprintf(legend1, "Simulation Background");
   sprintf(legend2, "Atmospheric Neutrino Simulation");
   sprintf(legend3, "Data");

   TLegend *le = new TLegend(.55, 0.76, .89, .90);
   le->AddEntry(h1_bkg ,legend1, "F");
   le->AddEntry(h1_sig ,legend2, "F");
   le->AddEntry(h1_data,legend3,"LP");

   le->SetFillColor(0);
   le->Draw("same");

How to scale your histogram correctly

• Example:

   ////////////////////////
   // resizing histogram //
   ////////////////////////

    float xmax = h1_sim->GetXaxis()->GetXmax();
    float xmin = h1_sim->GetXaxis()->GetXmin();
    float ymax = h1_sim->GetMaximum();
    float ymin = h1_sim->GetMinimum();
    if(h1_data->GetMaximum()         > ymax) ymax = h1_data->GetMaximum();
    if(h1_data->GetMinimum()         < ymin) ymin = h1_data->GetMinimum();
    if(ymin >0 ) ymin =0;

    ymax = (ymax+sqrt(ymax)) *1.2; // increase y axis maximum by sqrt of its value (simple stat error for error bad) and 20% to avoid the distribution being squeezed into the histogram and so that the error bars can show up

    h1_sim->GetYaxis()->SetRangeUser(0,ymax);

    printf("Yaxis [%f, %f]\n",ymin,ymax);

    // Redraw for clean plot //

    h1_sim->Draw();
    h1_data->Draw("same");

Example scripts:

• Please find examples here

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pyroot

• Presentation from Jim on pyroot
• How to use the pyroot interpreter