How To Use ROOT-- some features

Ann-Cecilie Larsen & Alexander Bürger

15 Januar 2009

OCL group meeting

   

What is ROOT?

Object-Oriented framework for data analysis data acquisition ...

developed at CERN implemented in C++ C++ interpreter CINT

allows to run C++ code without compilation

   

Why C++?

helps with object oriented programming (OOP) simplifies maintenance of large projects means

encapsulation (data access control) inheritance (generalization hierarchy) polymorphism (different implementation in inheritance tree)

“based” on C could do OOP in C, BUT

no compiler support, more complicated syntax far more work, more typing, more errors

   

animal

mammal

dog

fish

...

very general: something“abstract”: does not actually exist

very specific,not “abstract”, can exist

arrow= “is a”

Classes 'in Nature'

class: describes set of properties and 'functions' color of fur, weight, bark(), eat()

object: instance of a class with a specific set of properties white fur, 40kg

Romeo

cow 'instance' withspecific property values

   

Classes in C++

a class is a set of object properties methods/functions

constructor&destructor variables

objects are instances of their class

// derived class definitionclass Histogram {public:    void  Fill(float value);    int   GetNbins();    virtual float GetBinContent(int bin);protected:    float* GetBins() const { return bins; }private:    int    nbins;    float* bins;};...// method definitionfloat Histogram::GetBinContent(int bin){    return bins[bin];}...// create an instanceHistogram* h1 = new Histogram(...);cout << h1­>GetBinContent(5) << endl;delete h1;

encapsulation

   

TObject

TNamed

TH1

TVirtualPad

TCanvas

...

...

...

very general: something“abstract”: does not actually exist

something with a namestill “abstract”

histogram with bins, axes, ...not “abstract”, can exist

arrow=“derived from”

Inheritance

a derived class inherits all methods/functions from the base class may override some methods

e.g. TObject::Draw is overridden by TH1::Draw

   

Derived Classes

a derived class can override (replace)

methods/functions add variables and

methods access proteced and public variables

which version is called? “derived” if virtual “named” otherwise

// derived class definitionclass SafeHistogram : public Histogram {public:    float GetBinContent(int bin); // virtual    int GetNbins(); // not virtual};...// method definitionfloat SafeHistogram::GetBinContent(int bin){    if( bin>=0 && bin<GetNbins() )       return Histogram::GetBinContent(bin);    else       return 0;}int SafeHistogram::GetNbins(){    if( GetBins() )        return Histogram::GetNbins();    return ­1;}...// create an instanceHistogram* h1 = new SafeHistogram(...);cout << h1­>GetBinContent(5) << endl;delete h1;

polymorphism

inheritance

   

Syntactic Sugar -- “Typing help”

less typing, easier to read, fewer errors

examples array access stream

extraction/insertion name overloading

distinguish functions by signature (name + types)

implicit type conversion

a[i][j]        // instead of *(*(a + i) + j)h1­>GetNbins() // instead of (*h1).GetNbins()

#include <fstream>

ifstream input_file(“/tmp/plot.dat”);float x, y;input_file >> x >> y;

// definition: istream& istream::operator>>(float& x);

void do_something(int x);    // integer argumentvoid do_something(double x); // double argumentvoid do_something(string s); // string

const char* text = “OCL”;do_something(text); // creates temporary                    // string objectfloat f = 15.1;do_something(f); // conversion may be tricky!

   

Templates and the STL templates

“generic classes” help avoid preprocessor

macros

STL – standard template library implements often-

needed things examples

sorted collections binary search

documentation e.g.http://www.litho.ucalgary.ca/opt/SPRO/stdlib/

// bad:#define MAX(a,b) ((a>b)?a:b)MAX(“here”,”there”); // compares pointersMAX(f(1),f(2));      // calls f three times

// bettertemplate<typename T>T& max(T& a, T& b){    if( a > b )        return a;    else        return b;}max(f(1),f(2));      // calls f two times                     // automatically figures Tmax<double>(10, 15.1); // int and double

template <class T>class vector {public:    unsigned size() const { ... }};

#include <map>#include <vector>

std::map<int,std::string> mymap;cout << mymap[5] << endl; // operator[] overload// const T& map<...>::operator[](int) const;

std::vector<int> my_values;my_values.push_back(5);std::cout << my_values.size() << std::endl;

   

Where to Get Help with ROOT?

Homepage: http://root.cern.ch/ documentation tutorials user forum ...

ROOT User's Guide (PDF from homepage) from people using ROOT

MacOS finder: swshareroottutorials

   

Installing ROOT on MacOS

easiest with fink shows i if installed

otherwise install withfink install root5in a terminal

   

Starting and Stopping

start: “root”in a terminal

stop: “.q”

   

Running Scripts

.x mymacro.C+ or.x mymacro.C++ compiled mode

.x mymacro.C interpreted mode

   

Script Example// CINT automatically uses “using namespace std”, some 

“#include”s, ...{ gROOT­>Reset();           // reset global variables gROOT­>SetStyle("Plain"); // no colors/fill areas gStyle­>SetOptTitle(0); // no histogram title gStyle­>SetOptTitle(1); gStyle.SetOptStat(0);     // don't show statistics gStyle.SetFillColor(0);   // no fill in the canvas gStyle.SetPadBorderMode(0); // no pad borders

 // deleting histogram from last macro run ­­ avoid memory leak

 m = (TH2F*)gROOT­>FindObject("h"); if(m) m­>Delete();

 // open input files ifstream rho44file("rho44.cnt.web"), 

rho46file("rho46.cnt.web"); // create output files ofstream levdens44Ti("rho44Ti_EB"), 

levdens46Ti("rho46Ti_EB");

 // declare arrays float rho44[44],rhoerr44[44],energy44[44]; float rho46[87],rhoerr46[87],energy46[87]; float energyerr[430] = {0.};

 // Read in from input, and write to output file levdens44Ti << "E[MeV]\trho[MeV­1]rhoerr[MeV­1]" << endl; int i=0; while(rho44file){ float x,y,z;  int j;  rho44file >> j >> energy44[i] >> rho44[i] >> rhoerr44[i];  levdens44Ti << energy44[i] << '\t' << rho44[i] << '\t'              << rhoerr44[i] << endl;  i++; }

 levdens46Ti << "E[MeV]\trho[MeV­1]\trhoerr[MeV­1]" << endl; i=0; while(rho46file){  int j;  rho46file >> j >> energy46[i] >> rho46[i] >> rhoerr46[i];  levdens46Ti << energy46[i] << '\t' << rho46[i] << '\t'              << rhoerr46[i] << endl;  i++; }

 // close output files levdens44Ti.close(); levdens46Ti.close();

 // create a canvas to plot upon TCanvas *c1 = new TCanvas("c1","Level densities, 

Ti",600,400);

 // create 2D histogram (used here only to get axes) TH2F *h = new TH2F("h","",10,0,12,50,0.3,56e3);

 // create graphs TGraphErrors *rho44ti = new    TGraphErrors(38,energy44,rho44,energyerr,rhoerr44); TGraphErrors *rho46ti = new    TGraphErrors(75,energy46,rho46,energyerr,rhoerr46);

 // start plotting c1.SetLogy();              // logscale on y axis c1.SetLeftMargin(0.14); h.GetXaxis().CenterTitle(); h.GetXaxis().SetTitle("Excitation energy E (MeV)"); h.GetYaxis().CenterTitle(); h.GetYaxis().SetTitleOffset(1.4); h.GetYaxis().SetTitle("Level density #rho (E) (MeV^{­1})"); h.Draw(); // draw the histogram (here only to get the axes)

   

Script Example (2) rho44ti­>SetMarkerStyle(20); rho44ti­>SetMarkerSize(0.8); rho44ti­>SetMarkerColor(kBlue); rho44ti­>SetLineColor(kBlue); rho44ti­>Draw("P"); // draw as points with line    rho46ti­>SetMarkerStyle(24);    rho46ti­>SetMarkerSize(0.8); rho46ti­>Draw("P");

 // create legend to visualize where the data points belong TLegend *leg = new TLegend(0.15,0.60,0.6,0.75); leg.SetBorderSize(0); leg­>AddEntry(rho44ti,"Oslo data, ^{44}Ti (norm. E&B)","P"); leg­>AddEntry(rho46ti,"Oslo data, ^{46}Ti (norm. E&B)","P"); leg­>Draw();

 // add some LaTeX text TLatex t; t.SetTextSize(0.05); t.DrawLatex(/*X=*/1.502,             /*y=*/1.632e+04,             "^{44}Ti and ^{46}Ti level density #rho");

 c1­>Update(); // actually draw something before “printing” it c1­>Print("example.eps"); // “print” as .eps c1­>Print("example.pdf"); // “print” as .pdf} // END of script