Tutorial I: Reading ROOT Input (1)

Preparation

Create ROOT files for testing purposes

For testing purposes, there is a script to fill a TTree in a ROOT file with Gaussian random numbers.

generateCorrelatedGaussians.py
generateCorrelatedGaussians.py -t gaussians1 gaussians2 gaussians3 -o gaussians3.root
generateCorrelatedGaussians.py --n-events 1000 -o gaussians1000.root

Execute it to get an example file for plotting. This tutorial is based on the output of these commands (gaussians.root, gaussians3.root and gaussians100.root).

Inspect ROOT files

There are several ways to inspect the contend of a ROOT file. This works for arbitrary ROOT files.

• TBrowser in interactive ROOT sessions

   tBrowser.sh gaussians.root

• Print objects (and types) within ROOT files in the shell

   get_root_file_content.py gaussians.root [--log-level debug]

  This script is in many cases the fastest option to see what objects a ROOT file contains. By using the debug logging option, the TObject.Print functions are called.

• Print numbers of entries of all trees in a ROOT file

   get_entries.py gaussians.root

• Print contents of all trees in a ROOT file

   print_trees.py gaussians.root

  The size of the output corresponds to the size of the trees. Therefore this script is rather recommended for small trees.

• Print binning of all histograms in a ROOT file

   get_binning.py gaussians.root

  This does not give an output for the file gaussians.root, because it does not contain any histogram.

Similar tools can be easily added. Get inspired by looking at the code of these scripts.

Plotting

The examples here are based on the testing files generated above.

The default input module of HarryPlotter (InputRoot) is able to read in data from ROOT trees as well as every other drawable ROOT object (e.g. histograms, graphs and functions). Two arguments are important to specify the input: -i and -f. The first takes the input ROOT file(s) and the second the folder(s)/path(s) inside the ROOT file pointing either to a TDirectory containing drawable objects or to a TTree object. This part of the tutorial focusses the options related to a single input. Explanations about multiple inputs are given in part 2.

Trees as input

In this section, all plots are done with the PlotRoot backend with the additional option --plot-modules PlotRoot.

Fill a histogram with the values of a single branches

• 1D

   harry.py -i gaussians3.root -f gaussians1 -x var0

  

• 2D

   harry.py -i gaussians3.root -f gaussians1 -x var0 -y var1

  

Weighted histograms

• 1D

   harry.py -i gaussians3.root -f gaussians1 -x var0 -w var1

  

• 2D

   harry.py -i gaussians3.root -f gaussians1 -x var0 -y var1 -w var2

  

The full functionallity of TTree.Draw

The functionallity of the InputRoot module of HarryPlotter is completely based on the very versatile functions TChain.Add and TTree.Draw in ROOT. First the input tree is filled with calls of

TChain.Add(<file>/<folder>)

where <file>/<folder> are the arguments of -i/-f. Then the histograms are filled using the functionallity of

TTree.Draw([[<z>:]<y>:]<x> >>name <binning>, <weight>, <tree draw option>)

where the arguments -x/-y/-z, --x-bins/--y-bins/--z-bins, -w and --tree-draw-options are used. This means, that these arguments support all options documented in the link above. The calls of these two functions are printed with debug options --log-level debug. For example,

harry.py -i gaussians3.root -f gaussians1 -x var0 --log-level debug

gives the output

Reading from ntuple gaussians3.root/gaussians1 ...
ROOT.TTree.Draw("var0>>histogram_1b95afa2adc1683092da6ef154c4bbdf(25,,)", "1.0", " GOFF")

Some examples:

• Plotting formulas evaluated with tree variables

  The syntax of formulas is described in the documenation of the TFormula class.

   harry.py -i gaussians3.root -f gaussians1 -x "abs((var0+var1) * var2)"
   harry.py -i gaussians3.root -f gaussians1 -x "(Entry$ % 2) * var0"

  

• Cuts/Selections

   harry.py -i gaussians3.root -f gaussians1 -x var0 -y var1 -w "(var0 < 5) * (var1 > -20)"
   harry.py -i gaussians3.root -f gaussians1 -x var0 -w "Entry$ < 100"

  

• Profile Plots

   harry.py -i gaussians3.root -f gaussians1 -x var0 -y var1 --tree-draw-options profs
   harry.py -i gaussians3.root -f gaussians1 -x var0 -y var1 -z var2 --tree-draw-options profs

  

• Graphs

   harry.py -h | grep " --tree-draw-options" -A 21
   harry.py -i gaussians3.root -f gaussians1 -x Entry$ -y var0 --tree-draw-options TGraph -w "Entry$ < 20" --plot-modules PlotRoot -m PL
   harry.py -i gaussians3.root -f gaussians1 -x Entry$:0.3 -y "var0:sqrt(abs(var0))" --tree-draw-options TGraphErrors -w "Entry$ < 20" --plot-modules PlotRoot -m P