Introduction to ROOT

The information presented in this Workbook section is extended in the next section Analysis in ROOT II which covers fitting histograms in ROOT and introduces the ROOT Graphical User Interface, the Object Browser.

Contents

What is ROOT?

CINT I - the ROOT Command Line Language

Most familiar objects exist in ROOT with either capitalisation and a "_t" suffix or a "T" prefix, for example: Float_t, Double_t, ..., TClass, TFile, ...

Starting a New ROOT Session

To start a ROOT session at SLAC, run the script bbrroot (this only works after srtpath has been executed):

   > bbrroot 

On most computers, you would start root by typing "root." But at BaBar it is better to use the wrapper script "bbrroot" because it starts up the same version of ROOT as was used to produce your rootfile (in your original BetaMiniApp job, for example):

While ROOT is starting up a pop-up window with a picture of a naked lady with tree roots instead of legs, twirling a hoop over her head, may appear briefly, and on the terminal where you entered the startup command, the resulting output should look something like the following:

  *******************************************
  *                                         *
  *        W E L C O M E  to  R O O T       *
  *                                         *
  *   Version  5.14/00e     29 March 2007   *
  *                                         *
  *  You are welcome to visit our Web site  *
  *          http://root.cern.ch            *
  *                                         *
  *******************************************

FreeType Engine v2.1.9 used to render TrueType fonts.
Compiled on 31 March 2007 for linux with thread support.

CINT/ROOT C/C++ Interpreter version 5.16.16, November 24, 2006
Type ? for help. Commands must be C++ statements.
Enclose multiple statements between { }.
...using style 'Plain'
...found RELEASE directory
...found PARENT directory
...running in release 22.1.1

    For approved plots use: gROOT->SetStyle("BABAR");
    To add a BABAR label use: BABARLabel();
    To add a better-scaling BABAR label use: BABARSmartLabel();
    Type "BABARSmartLabel(-2);" for options

You can use ROOT as a quick calculator - just leave off the semi-colon at the end of the statement, e.g.

23+5

(const int)28

Now comes a step that you must know - how to exit ROOT at the end of a session!

   .q

Histograms in ROOT

TH1F *myHist=(TH1F*) gDirectory->Get("name of your histogram");

Making your first ROOT histogram

To declare a 1-d histogram which will be filled by giving it floating point numbers, for example, you would enter:

TH1F *hist_name = new TH1F("hist_name", "hist_title",num_bins, x_low, x_high);

Histograms in ROOT also have two bins for values that fall outside the binning values. The left-most bin (bin 0) is reserved for underflow (entries that are less than x_low) and the right-most (bin num_bins+1) bin is reserved for overflow (entries that are equal to or larger than x_high).

Booking 2- and 3-d histograms is analogous. An example of a 2-d histogram declaration is:

TH2D *h2 = new TH2D("h2", "histogram title",100, 0., 10., 18, 0., 180.);

To book your first histogram, start up a ROOT session and enter:

TH1F *hist_1 = new TH1F("hist_1", "My first histo", 100, 2, 200);

This is because you must now tell ROOT to draw your histogram. "Draw()" is one of the functions defined in the ROOT histogram class. To draw your new histogram, enter

hist_1.Draw();

   root [1] h1.Draw();
   <TCanvas::MakeDefCanvas>: created default TCanvas with name c1

This is an empty histogram with the title "My first histo".

Filling your new histogram

root [33] hist_1.Fill(50.0);
root [34] hist_1.Fill(120,2);
root [35] hist_1.Fill(130,2);
root [36] hist_1.Draw();

The result should look something like:

The Stats Box

gStyle->SetOptStat(mode);

gStyle->SetOptStat(11);
hist_1.Draw();

You can return to the default stats box settings with

gStyle->SetOptStat();

gStyle->SetOptStat(0);

Legends

  leg = new TLegend(0.6,0.7,0.89,0.89);  //coordinates are fractions
                                         //of pad dimensions
  leg->AddEntry(hist_1,"First histo","l");  // "l" means line
                                            // use "f" for a box
  leg->Draw();
  // oops we forgot the header (or "title") for the legend
  leg->SetHeader("The Legend Title");
  leg->Draw();

c1.Clear();

leg->SetTextSize(0.04);
// set size of text
leg->SetFillColor(0);
// Have a white background
leg->AddEntry(hist_1, "text 1", "p");
                                 // p shows points,
                                 // other options exist
                                 // (Check documentation)
leg->Draw();

A text box with lines of text is produced with

pt = new TPaveText(0.20,0.7,0.5,0.87, "NDC"); // NDC sets coords
                                              // relative to pad dimensions
pt->SetFillColor(0); // text is black on white
pt->SetTextSize(0.04); 
pt->SetTextAlign(12);

text = pt->AddText("Whatever you want");

pt->Draw();       //to draw your text object

To get larger labels, axis titles and Greek fonts onto a histogram:

gPad->SetLeftMargin(0.15); // experiment with the best value for you.
                              // Depends on font size:

gPad->SetBottomMargin(0.15);

hist_1->SetLabelSize(0.05,"y");

hist_1->GetYaxis()->SetTitleSize(0.05);
hist_1->SetLabelSize(0.05,"x");

hist_1->GetXaxis()->SetTitleSize(0.05);

hist_1->SetXTitle("`f[#circ]"); // degree symbol, just like PAW,
                               // and like LaTeX
                               // (using "#" instead of "\")
hist_1->SetYTitle("Entries / bin");
hist_1->Draw();

Changing Histogram Attributes

hist_1->SetLineColor(2);
hist_1->Draw();

Here are a few other commands that you can use to enhance the appearance of your histogram to give you an idea of some of the power of ROOT:

hist_1->GetXaxis()->SetTitle("Label of x axis");
hist_1->GetYaxis()->SetTitle("Label of y axis");

gROOT->SetStyle("Plain");      // Switches off the ROOT default style
gPad->UseCurrentStyle();       // this makes everything black and white,
                              // removing the default red border on images
gROOT->ForceStyle();          // forces the style chosen above to be used,
                              // not the style the rootfile was made with

Finally, to get a better colour set than the ROOT default:

gStyle->SetPalette(1);

enum EColor { kWhite, kBlack, kRed, kGreen, kBlue, kYellow,
kMagenta, kCyan };

Comparing Histograms

TH1F *hist_2 = new TH1F("hist_2", "Another histo", 100, 2, 300);

hist_2->Fill(20,10);
hist_2->Fill(50,4);
hist_2->Fill(3);

hist_2->Draw();

hist_1->Draw("histsame");

  c1->Clear();

  hist_1->SetLineColor(8)  // green
  hist_2->SetLineColor(4)  // blue

  hist_2.Draw();          //draw hist_2 first as it has a larger range
  hist_1.Draw("same");

  leg_hist = new TLegend(0.5,0.6,0.79,0.79);
  leg_hist->SetHeader("Some histograms");
  leg_hist->AddEntry(hist_1,"First histo","l");
  leg_hist->AddEntry(hist_2,"Second histo","l");
  leg_hist->Draw();

Note that unless you manually set the axis ranges, these will be set to those most appropriate to the first histogram which was drawn.

Copying Histograms

TH1F *hist_new=(TH1F*)hist_1->Clone();
hist_new->SetName("hist_new");

More Drawing Options

You should consult the official ROOT documentation for the many options for histogramming in ROOT. Here are some Draw options that you might like to experiment with for a 2-dimensional histogram:

h2->Draw("text");
h2->Draw("col"),
h2->Draw("colz");
h2->Draw("box");
h2->Draw("surf");

Including Error Bars in Histograms

You can plot errors on histograms (perhaps more appropriate to plot them on first to histograms!) by entering

hist_2.Draw("esame");

By default, errors are sqrt(entries). To get the error as sqrt(sum of weights), enter

hist_1->Sumw2()

Related Documents:

• Histograms - ROOT Site

Getting Help with ROOT

   .?

Note1: Cint is not aimed to be a 100%% ANSI/ISO compliant C/C++ language
 processor. It rather is a portable script language environment which
 is close enough to the standard C++.

Note2: Regulary check either of /tmp /usr/tmp /temp /windows/temp directory
 and remove temp-files which are accidentally left by cint.

Note3: Cint reads source file on-the-fly from the file system. Do not change
 the active source during cint run. Use -C option or C1 command otherwise.

Note4: In source code trace mode, cint sometimes displays extra-characters.
 This is harmless. Please ignore.

CINT/ROOT C/C++ interpreter interface.
All commands must be preceded by a . (dot), except
for the evaluation statement { } and the ?.
===========================================================================
             > [file]  : output redirection to [file]
             2> [file] : error redirection to [file]
             >& [file] : output&error redirection to [file]
Help:        ?         : help
...

hist_1.Print();

TH1.Print Name= hist_1, Entries= 3, Total sum= 5

hist_1.Print("all");

...
 fSumw[59]=0, x=117.83
 fSumw[60]=2, x=119.81
 fSumw[61]=0, x=121.79
 fSumw[62]=0, x=123.77
 fSumw[63]=0, x=125.75
 fSumw[64]=0, x=127.73
 fSumw[65]=2, x=129.71
 fSumw[66]=0, x=131.69
...

hist_1->Print("all"); > filename.txt

Saving Histograms as Image Files

   c1->SaveAs("myimage.eps");
   c1->SaveAs("myimage.ps");
   c1->SaveAs("myimage.gif");

Saving Source Code for Histograms

You can access and save that source code as a ROOT file which you can edit and later rerun (e.g. to change the titles, histogram attributes, etc). This is done by (assuming that your histogram is printed on canvas "c1"):

  c1->SaveAs("myimage.C");

Assuming you have saved your histogram as myimage.C, you can recreate the histogram in exactly the form that you saved it in a brand new ROOT session by entering:

.x myimage.C

Opening and Inspecting ROOT Histograms Saved in Files

To inspect your ROOT histogram enter:

   TFile f("myHistogram.root");

   f.ls();

TFile**         myHistogram.root        Created for you by RooTupleManager
 TFile*         myHistogram.root        Created for you by RooTupleManager
  KEY: TH1F     h1d1;1  MC reco abs mtm difference
  KEY: TH1F     h1d2;1  Reco track momentum
  KEY: TH1F     h1d3;1  Tracks per Event
  KEY: TH1F     h1d4;1  Momentum
  KEY: TH1F     h1d5;1  TagInspector Status

   h1d4.Draw();

h1d4.Fill(2,4);
h1d4.Draw();

Note that your Fill command actually affects only the histogram drawn on the canvas. This histogram is a copy of the one in the file. So the histogram in myHistogram.root remains unchanged.

Changing between files

TFile *f1=new TFile("first.root");
TFile *f2=new TFile("second.root");

If you want to perform actions on the first file you opened, for those functions which do not directly use the file's reference name (in ROOT, "f1"), you will need to change ROOT's focus to that file:

f1.cd();

ROOT Tutorials

• SLAC tutorials: 27 Feb 2002,    19 July 2000,    5 April 2000
• The ROOT System Home Page:   Tutorials,   more detailed How-Tos
• Fermilab's ROOT resources

ROOT, PAW and h2root

ROOT was created by the authors of PAW, and there are many similarities between the two systems. Within the BaBar one can create histograms in either ROOT or PAW formats, and it is easy to change code to alter the output between the two.

ROOT files contain histograms and trees (and, occasionally, other ROOT objects). A tree is (approximately) the equivalent of a PAW ntuple, albeit far more powerful: You can store objects (e.g. vectors) as well as numbers.

You can convert your PAW files to ROOT files using the ROOT utililty h2root. The original PAW file may contain histograms, ntuples and have subdirectories. PAW Ntuples become (simple) ROOT trees, and the subdirectory structure of PAW files is preserved. Once the you have issued the srtpath command (see Workbook: Quicktour) to set up the BaBar environment, you can run the wrapper script

  bbrh2root

to access this utility.

The syntax is:

   bbrh2root oldname.hbook newname.root

If you do not supply the second argument, then the new file will be called oldname.root. So if you have a directory full of .hbook files, you can just do "bbrh2root *" to change them all to .root files with the same names (except for the .root extension of course). How to access the trees and histograms in the new ROOT file was explained in previous sections.

One disadvantage of ROOT is that it will not run properly unless the display environment is properly set up. In PAW this was not always necessary, as PAW would produce histograms made of ascii characters for graphically-challenged computers. But ROOT needs a graphical display set up in order to work.

(Note: If you are working at SLAC, and your h2root command does not work, try logging into a Sun machine like flora and using the command there. This works for many people - I am not sure why.)

• Interactive ROOT Help - BaBar hypernews
• The ROOT System Home Page
• ROOT User's Guide
• ROOT Documentation, Hints and FAQ for BaBar
• PAW to ROOT commands conversion table

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Page maintained by Adam Edwards

Last modified: January 2008