ROOT Cheat Sheet

1. Interactive work

Command	Description
`root -h`	show help for root command
`root [-l]`	open ROOT
`root [-l] file.root`	open ROOT and load a file
`.?`	show help in interactive ROOT
`.q[qq[qq[qq]]]`	close ROOT (more q's – more violently)
`.ls`	list present ROOT directory
`tree->Show(2)`	show 3rd (!) tree entry
`tree->Scan()`	print entries as a table
`tree->Scan("n:x:y:z", "x>2")`	specify variables to show and condition
`new TBrowser`	open TBrowser
`ntuple->StartViewer()`	open an analysis GUI

2. Scripts/programs

2.1 C++ macro

Running:

root [-l] [-b] [-q] macro.C[+[+]]

root [-l] [-b]
.x macro.C[+[+]]

Example:

void macro() {
    cout << "hello, world" << endl;
    TCanvas c("can1", "Canvas title", 800, 600);
    TF1 f("fun1", "sin(x)", -5, 5);
    f.Draw();
    c.SaveAs("sin.png");
}

2.2 C++ program

Running:

g++ program.C -o program[.exe] `root-config --cflags --glibs`
./program[.exe]

Example:

#include <iostream>
#include "TCanvas.h"
#include "TF1.h"
using namespace std;
int main() {
    cout << "hello, world" << endl;
    TCanvas c("can1", "Canvas title", 800, 600);
    TF1 f("fun1", "sin(x)", -5, 5);
    f.Draw();
    c.SaveAs("sin.png");
    return 0;
}

2.3 PyROOT script

#!/usr/bin/env python
import ROOT
print "hello, world"
c = ROOT.TCanvas("can1", "Canvas title", 800, 600)
f = ROOT.TF1("fun1", "sin(x)", -5, 5)
f.Draw()
c.SaveAs("sin.png")

3. Important classes

3.1 std::vector

Object-oriented array of elements. Not really ROOT, but very useful.

#include <vector>
std::vector<double> x;
x.push_back(10);    // add element
x.push_back(1.5);   // add element
x.push_back(1e2);   // add element
std::cout << x.at(1);  // get second element
std::cout << x[1];     // alternative way

3.2 TFile – reading

TFile f("input.root");  // open for reading
TH1F *h = (TH1F*)f.Get("hist1");
TTree *t = (TTree*)f.Get("data");
f.Close();

3.3 TFile – writing

TFile f("output.root", "recreate");  // open for writing
TH1F h("h", "title", 100, 0, 10);
h.Fill(10); h.Fill(12); h.Fill(15);
f.Write();
f.Close();

3.4 TTree

Class for storing data.

TFile f("input.root");
TTree *tree = (TTree*)f.Get("treename");
tree->Draw("x", "y>0");  // Plot x distribution for y > 0
tree->Draw("x:y", "n==1", "colz");  // Plot x vs y
tree->Draw("x>>hist(100,0,1)", "y>0");  // named histogram with defined binning

3.5 TH1F

One-dimensional histogram.

TH1F h("name", "title;x [mm];number of entries", 100, 0, 1);  // constructor
h.Fill(x);  // filling the histogram (repeated for each entry)
h.SetLineColor(2);
h.SetLineStyle(3);
h.SetLineWidth(1.5);
h.Scale(10);  // Multiply all bins by the same number
h.Integral();  // Calculate sum (!) of bins
h.GetBinContent(10);  // Get value of bin 10
h.SetBinContent(10, 0.4);  // Set value of bin 10
h.Draw("l");  // drawing

3.6 TH2F

Two-dimensional histogram.

TH2F h("name", "title;x [mm];number of entries", 100, 0, 1, 50, -1, 1);  // constructor
h.Fill(x, y);  // filling the histogram (repeated for each entry)
h.Draw("colz");  // drawing

3.7 TGraph

A graph of (x, y) points.

double x[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
double y[10] = {0, 1, 4, 9, 16, 25, 36, 49, 64, 81};
TGraph g(10, x, y);
g.Draw("APL");  // Axes, Points, Lines

3.8 TCanvas

Canvas on which plots can be drawn.

TCanvas c("c1", "Canvas 1", 1024, 768);  // constructor
h.Draw();  // drawing a histogram on the current canvas
c.SaveAs("output.png");  // saving to PNG file
c.Print("output.pdf[");  // opens a multi-page file
c.Print("output.pdf");   // writes a new page
c.Print("output.pdf]");  // closes the file
c.Divide(2, 3);  // divide canvas into 6 TPads
c.cd(2);  // from now on draw on 2nd sub-canvas
c.SetLogy();  // logarithmic scale on y axis

3.9 TLegend

Legend on the plot.

TLegend leg(0.7, 0.7, 0.9, 0.9);
leg.AddEntry(h1, "data", "p");  // h1 is a pointer
leg.AddEntry(&h2, "model", "l");  // h2 is an object

3.10 TLorentzVector

Useful for calculations in relativistic kinematics.

TLorentzVector p1, p2;
p1.SetPxPyPzE(5, 2, 7, 20);
p2.SetPtEtaPhiM(10, 2, 0.5, 2);
double m_inv = (p1 + p2).M();

3.11 TF1

One-dimensional function. For plotting and fitting.

TF1 f("fun1", "[0] + [1]*x + [2]*sin([3]*x)", -10, 10)
h.Fit(f);  // or h.Fit("fun1");

3.12 TRandom

TRandom gen;
std::cout << gen.Uniform(0, 12);
std::cout << gen.Gaus(2, 0.1);

MohamedElashri/ROOT_Cheatsheet.md