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ROOT and using ROOT prompt

Overview

Teaching: min
Exercises: min
Questions

  • How to use a ROOT file and the ROOT prompt ?

Objectives

  • Open a ROOT file

  • Use ROOT prompt

Interactive C++

One of the main features of ROOT is the possibility to use C++ interactively thanks to the C++ interpreter Cling. Cling lets you use C++ just like Python either from the prompt or in scripts.

The ROOT prompt

By just typing root in the terminal you will enter the ROOT prompt. Like the Python prompt, the ROOT prompt is well suited to fast investigations.

$ root
root [0] 1+1
(int) 2

If you pass a file as argument to root, the file will be opened when entering the prompt and put in the variable _file0. ROOT typically comes with support for reading files remotely via HTTP (and XRootD), which we will use for the following example:

No support for remote files?

Although unlikely, your ROOT build may not be configured to support remote file access. In this case, you can just download the file with curl -O https://root.cern/files/tmva_class_example.root and point to your local file. No other changes required!

$ root tmva_class_example.root

root [0]
Attaching file mva_class_example.root as _file0...
(TFile *) 0x555f82beca10

root [1] _file0->ls() // Show content of the file, all objects are accessible via the prompt!
TWebFile**              https://root.cern/files/tmva_class_example.root
 TWebFile*              https://root.cern/files/tmva_class_example.root
  KEY: TTree    TreeS;1 TreeS
  KEY: TTree    TreeB;1 TreeB

root [2] TreeS->GetEntries() // Number of events in the dataset

root [3] TreeS->Print() // Show dataset structure
******************************************************************************
*Tree    :TreeS     : TreeS                                                  *
*Entries :     6000 : Total =           98896 bytes  File  Size =      89768 *
*        :          : Tree compression factor =   1.00                       *
******************************************************************************
*Br    0 :var1      : var1/F                                                 *
*Entries :     6000 : Total  Size=      24641 bytes  One basket in memory    *
*Baskets :        0 : Basket Size=      32000 bytes  Compression=   1.00     *
*............................................................................*
*Br    1 :var2      : var2/F                                                 *
*Entries :     6000 : Total  Size=      24641 bytes  One basket in memory    *
*Baskets :        0 : Basket Size=      32000 bytes  Compression=   1.00     *
*............................................................................*
*Br    2 :var3      : var3/F                                                 *
*Entries :     6000 : Total  Size=      24641 bytes  One basket in memory    *
*Baskets :        0 : Basket Size=      32000 bytes  Compression=   1.00     *
*............................................................................*
*Br    3 :var4      : var4/F                                                 *
*Entries :     6000 : Total  Size=      24641 bytes  One basket in memory    *
*Baskets :        0 : Basket Size=      32000 bytes  Compression=   1.00     *
*............................................................................*

root [4] TreeS->Draw("var1") // Draw a histogram of the variable var1

Investigating data in ROOT files

You have already seen the usage of TTree::Draw in the previous section. Such quick investigations of data in ROOT files are typical usecases which most analysts encounter on a daily basis. In the following you can learn about different ways to approach this task!

Manually plotting with TTree::Draw

For quick studies on the raw data in a TTree on the command line, you can use TTree::Draw to make simple visualizations:

$ root https://root.cern/files/tmva_class_example.root

root [0]
Attaching file https://root.cern/files/tmva_class_example.root as _file0...
(TFile *) 0x558d7b54aa50
root [1] TreeS->Draw("var1") // just draw var1
Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
root [2] TreeS->Draw("var1", "var2 > var1", "SAME") // draw var1 with the selection var2 > var1
(long long) 3222

The TBrowser

More convenient is using ROOT’s tool for browsing ROOT files, the TBrowser. You can spawn the GUI directly from the ROOT prompt as shown below.

$ root https://root.cern/files/tmva_class_example.root

root [0]
Attaching file https://root.cern/files/tmva_class_example.root as _file0...
(TFile *) 0x557892a0ef10
root [1] TBrowser b
(TBrowser &) Name: Browser Title: ROOT Object Browser

The rootbrowse executable

For convenience, ROOT provides the executable rootbrowse, which lets you open a TBrowser directly from the command line and display the files given as arguments!

$ rootbrowse https://root.cern/files/tmva_class_example.root

Other ROOT executables

There are many small helpers shipped with ROOT, which let you operate on data quickly from the command line and solve typical day-to-day tasks with ROOT files.

List of ROOT executables

$ rootls https://root.cern/files/tmva_class_example.root
TreeB  TreeS

$ rootls -l https://root.cern/files/tmva_class_example.root
TTree  Jan 19 14:25 2009 TreeB  "TreeB"
TTree  Jan 19 14:25 2009 TreeS  "TreeS"

$ rootls -t https://root.cern/files/tmva_class_example.root
TTree  Jan 19 14:25 2009 TreeB  "TreeB"
  var1    "var1/F"    0
  var2    "var2/F"    0
  var3    "var3/F"    0
  var4    "var4/F"    0
  weight  "weight/F"  0
  Cluster INCLUSIVE ranges:
   - # 0: [0, 5998]
   - # 1: [5999, 5999]
  The total number of clusters is 2
TTree  Jan 19 14:25 2009 TreeS  "TreeS"
  var1  "var1/F"  0
  var2  "var2/F"  0
  var3  "var3/F"  0
  var4  "var4/F"  0
  Cluster INCLUSIVE ranges:
   - # 0: [0, 5998]
   - # 1: [5999, 5999]
  The total number of clusters is 2

Try it by yourself!

Feel free to investigate the tools presented here!

Key Points

  • Learn hoe to open a ROOT file.

  • Learn to use the ROOT prompt.

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