C++ and ROOT

Introduction

Overview

Teaching: 5 min
Exercises: 0 min
Questions

  • What is C++?

  • What is ROOT?

  • What is the point of these exercises?

Objectives

  • Learn a bit about C++ and how to compile C++ code.

  • Learn how to use ROOT to write and read from files, using the C++ libraries.

  • learn how to use ROOT to investigate data and create simple histograms

  • Explore the ROOT python libraries.

Despite the order of the words in the title of this lesson, let’s talk about ROOT first!

What is ROOT?

From the ROOT website

ROOT is a framework for data processing, born at CERN, at the heart of the research on high-energy physics. Every day, thousands of physicists use ROOT applications to analyze their data or to perform simulations.

In short, ROOT is an overarching toolkit that defines a file structure, methods to analyze particle physics data, visualization tools, and is the backbone of many widely used statistical analysis tool kits, such as RooFit and RooStats. You don’t need to use ROOT for your own analysis, but you will have to have some familiarity with it when you are first accessing the open data.

OK, that sounds cool. So what’s the deal with C++?

ROOT and C++

In the mid-80’s, C++ extended the very popular C programming language, primarily with the introduction of object-oriented programming (OOP). This programming paradigm was adopted by many particle physics experiments in the 1990’s and when ROOT was written, it was written in C++. While there are now python hooks to call the ROOT functions, the underlying code is all in C++.

Because C++ is a compiled code, it is usually much faster than a scripted language like python, though that is changing with modern python tools. Still, since much of the original analysis and access code was written in C++ and calling the C++ ROOT libraries, it’s good to know some of the basics of ROOT, in a C++ context.

Most CMS analysts interface with ROOT using python scripts and you may find yourself using a similar workflow. Later on in this lesson, we’ll walk you through some very basic python scripts and point you toward more in-depth tutorials, for those who want to go further.

You still have choices!

Just to emphasize, you really only need to use ROOT and C++ at the very earliest stages of an analysis, when accessing the data and using some of the CMS-provided tools. However, downstream in your analysis, you are welcome to use whatever tools and file formats you choose.

Key Points

  • C++ has a reputation for being intimidating, but there are only a few things you need to learn to edit the open data code for your own uses.

  • You can use the ROOT toolkit using both C++ and python.

  • Some ROOT code is written in C++ to access the datafiles.

  • People will often use simpler C++ scripts or python scripts to analyze reduced datasets.

Lightning overview of C++

Overview

Teaching: 5 min
Exercises: 10 min
Questions

  • How do I write and execute C++ code?

Objectives

  • To write a hello-world C++ program

Setting up your CMSSW area

If you completed the lessons on virtual machines or Docker you should already have a working CMSSW area.

Make sure you change directories to the CMSSW_5_3_32/src area; for instance, in Docker:

cd /home/cmsusr/CMSSW_5_3_32/src

Note that we are not really “installing” CMSSW but setting up an environment for it. CMSSW was already installed. This is why every time you open a new shell you will have to issue the cmsenv command, which is just a script that runs to set some environmental variables for your working area:

cmsenv

Your first C/C++ program

Let’s start with writing a simple hello world program in C. First we’ll edit the source code with an editor of your choice.

Let’s create a new file called hello_world.cc, using your preferred editor. If you’re using nano, you’ll type

nano hello_world.cc

or if you’re using vi, you’ll type

vi hello_world.cc

The first thing we need to do, is include some standard libraries. These libraries allow us to access the C and C++ commands to print to the screen (stdout and stderr) as well as other basic function.

At the very beginning of your file, add these three lines

#include<cstdlib>
#include<cstdio>
#include<iostream>

The first library, cstdlib, you will see in almost every C++ program as it has many of the very basic functions, including those to allocate and free up memory, or even just exit the program.

The second library, cstdio, contains the basic C functions to print to screen, like printf.

The third library, iostream, contains C++ functions to print to screen or write to files.

Usually people will use one or the other of the C or C++ printing functions, but for pedagogical purposes, we show you both.

Every C++ program must have a main function. So let’s define it here. The scope of this function is defined by curly brackets { }. So let’s add

int main() {


    return 0;
}

The int at the beginning tells us that this function will be returning an integer value. At the end of the main function we have return 0, which usually means the function has run successfully to completion.

Warning!

Note that at the end of return 0, we have a semicolon ;, which is how C/C++ programs terminate lines. If you’re used to programming in python or any other language that does not use a similar terminator, this can be tough to remember. If you get errors when you compile, check the error statements for the lack of ; in any of your lines!

For this function, we are not passing in any arguments so we just have the empty ( ) after the main.

This function would compile, but it doesn’t do anything. Let’s print some text to screen. Before the return 0; line, let’s add these three lines.

    printf("Hello world! This uses the ANSI C 'printf' statement\n");

    std::cout << "Hello world! This uses the C++ 'iostream' library to direct output to standard out." << std::endl;

    std::cerr << "Hello world! This uses the C++ 'iostream' library to direct output to standard error." << std::endl;

The text itself, should explain what they are doing. If you want to learn more about standard error and standard output, you can read more on Wikipedia.

OK! Your full hello_world.cc should look like this.

Full source code file for hello_world.cc

#include<cstdlib>
#include<cstdio>
#include<iostream>


int main() {

    printf("Hello world! This uses the ANSI C 'printf' statement\n");

    std::cout << "Hello world! This uses the C++ 'iostream' library to direct output to standard out." << std::endl;

    std::cerr << "Hello world! This uses the C++ 'iostream' library to direct output to standard error." << std::endl;

    return 0;

}

This won’t do anything yet though! We need to compile the code, which means turning this into machine code. To do this, we’ll use the GNU C++ compiler, g++. Once you have saved your file and exited out of your editor, you can type this in your shell (make sure you’re in the same directory as your hello_world.cc source code file!).

g++ hello_world.cc -o hello_world

This compiles your code to an executable called hello_world. You can now run this by typing the following on the shell command line, after which you’ll see the subsequent output.

./hello_world

Hello world! This uses the ANSI C 'printf' statement
Hello world! This uses the C++ 'iostream' library to direct output to standard out.
Hello world! This uses the C++ 'iostream' library to direct output to standard error.

When you are working with the Open Data, you will be looping over events and may find yourself making selections based on certain physics criteria. To that end, you may want to familiarize yourself with the C++ syntax for loops and conditionals.

Key Points

  • We must compile our C++ code before we can execute it.

Using ROOT with C++ to write and read a file

Overview

Teaching: 20 min
Exercises: 40 min
Questions

  • Why do I need to use ROOT?

  • How do I use ROOT with C++?

Objectives

  • Write a ROOT file using compiled C++ code.

  • Read a ROOT file using compiled C++ code.

Why ROOT?

HEP data can be challenging! Not just to analyze but to store! The data don’t lend themselves to neat rows in a spreadsheet. One event might have 3 muons and the next event might have none. One event might have 2 jets and the next event might have 20. What to do???

The ROOT toolkit provides a file format that can allow for efficient storage of this type of data with heterogenous entries in each event. It also provides a pretty complete analysis environment with specialized libraries and visualization packages. Until recently, you had to install the entire ROOT package just to read a file. The software provided by CMS to read the open data relies on some minimal knowledge of ROOT to access. From there, you can write out more ROOT files for further analysis or dump the data (or some subset of the data) to a format of your choosing.

Interfacing with ROOT

ROOT is a toolkit. That is, it is a set of functions and libraries that can be utilized in a variety of languages and workflows. It was originally written in C++ and lends itself nicely to being used in standard, compiled C++ code.

However, analysts wanted something more interactive, and so the ROOT team developed CINT, a C++ interpreter. This gave users an iteractive environment where they could type of C++ code one line at a time and have it executed immediately. This gave rise to C++ scripts that many analysts use and in fact the sample ROOT tutorals are almost exclusively written as these C++ scripts (with a .C file extension). Because they are written to run in CINT, they usually do not need the standard C++ include statements that you will see in the examples below.

With the rise of the popularity of python, a set of Python-C++ bindings were written and eventually officially supported by the ROOT development team, called PyROOT. Many analysts currently write the code which plots or fits their code using PyROOT, and we will show you some examples later in this exercise.

What won’t you learn here

ROOT is an incredibly powerful toolkit and has a lot in it. It is heavily used by most nuclear and particle physics experiments running today. As such, a full overview is beyond the scope of this minimal tutorial!

This tutorial will not teach you how to

OK, where can I learn that stuff?

There are some great resources and tutorials out there for going further.

ROOT terminology

To store these datasets, ROOT uses an object called TTree (ROOT objects are often prefixed by a T).

Each variable on the TTree, for example the transverse momentum of a muon, is stored in its own TBranch.

Write to a file

Let’s open a file using our preferred editor. We’ll call this file write_ROOT_file.cc. If we’re using vi as our editor, we would type

vi write_ROOT_file.cc

As in our last example, we first include some header files, both the standard C++ ones and some new ROOT-specific ones.

#include<cstdio>
#include<cstdlib>
#include<iostream>

#include "TROOT.h"
#include "TTree.h"
#include "TFile.h"
#include "TRandom.h"

Note the inclusion of TRandom.h, which we’ll be using to generate some random data for our test file.

Next, we’ll create our main function and start it off by defining our ROOT file object. We’ll also include some explanatory comments, which in the C++ syntax are preceded by two slashes, //.

int main() {

    // Create a ROOT file, f.
    // The first argument, "tree.root" is the name of the file.
    // The second argument, "recreate", will recreate the file, even if it already exists.
    TFile f("tree.root","recreate");

    return 0;
}

Now we define the TTree object which will hold all of our variables and the data they represent.

This line comes after the TFile creation, but before the return 0 statement at the end of the main function. Subsequent edits will also follow the previous edit but come before return 0 statement.

    // A TTree object called t1.
    // The first argument is the name of the object as stored by ROOT.
    // The second argument is a short descriptor.
    TTree t1("t1","A simple Tree with simple variables");

For this example, we’ll assume we’re recording the missing transverse energy, which means there is only one value recorded for each event.

We’ll also record the energy and momentum (transverse momentum, eta, phi) for jets, where there could be between 0 and 5 jets in each event.

This means we will define some C++ variables that will be used in the program. We do this before we define the TBranches in the TTree.

When we define the variables, we use ROOT’s Float_t and Int_t types, which are analogous to float and int but are less dependent on the underlying computer OS and architecture.

   Float_t met; // Missing energy in the transverse direction.

   Int_t njets; // Necessary to keep track of the number of jets

   // We'll define these assuming we will not write information for
   // more than 16 jets. We'll have to check for this in the code otherwise
   // it could crash!
   Float_t pt[16];
   Float_t eta[16];
   Float_t phi[16];

We now define the TBranch for the met variable.

    // The first argument is ROOT's internal name of the variable.
    // The second argument is the *address* of the actual variable we defined above
    // The third argument defines the *type* of the variable to be stored, and the "F"
    // at the end signifies that this is a float
    t1.Branch("met",&met,"met/F");

Next we define the TBranches for each of the other variables, but the syntax is slightly different as these are acting as arrays with a varying number of entries for each event.

    // First we define njets where the syntax is the same as before,
    // except we take care to identify this as an integer with the final
    // /I designation
    t1.Branch("njets",&njets,"njets/I");

    // We can now define the other variables, but we use a slightly different
    // syntax for the third argument to identify the variable that will be used
    // to count the number of entries per event
    t1.Branch("pt",&pt,"pt[njets]/F");
    t1.Branch("eta",&eta,"eta[njets]/F");
    t1.Branch("phi",&phi,"phi[njets]/F");

OK, we’ve defined where everything will be stored! Let’s now generate 1000 mock events.


    Int_t nevents = 1000;

    for (Int_t i=0;i<nevents;i++) {

        // Generate random number between 10-60 (arbitrary)
        met = 50*gRandom->Rndm() + 10;

        // Generate random number between 0-5, inclusive
        njets = gRandom->Integer(6);

        for (Int_t j=0;j<njets;j++) {
            pt[j] =  100*gRandom->Rndm();
            eta[j] =  6*gRandom->Rndm();
            phi[j] =  6.28*gRandom->Rndm() - 3.14;
        }

        // After each event we need to *fill* the TTree
        t1.Fill();
    }

    // After we've run over all the events, we "change directory" (cd) to the file object
    // and write the tree to it.
    // We can also print the tree, just as a visual identifier to ourselves that
    // the program ran to completion.
    f.cd();
    t1.Write();
    t1.Print();

The full write_ROOT_file.cc should now look like this

Full source code file for write_ROOT_file.cc



#include<cstdio>
#include<cstdlib>
#include<iostream>

#include "TROOT.h"
#include "TTree.h"
#include "TFile.h"
#include "TRandom.h"

int main() {

   // Create a ROOT file, f.
   // The first argument, "tree.root" is the name of the file.
   // The second argument, "recreate", will recreate the file, even if it already exists.
   TFile f("tree.root","recreate");

   // A TTree object called t1.
   // The first argument is the name of the object as stored by ROOT.
   // The second argument is a short descriptor.
   TTree t1("t1","A simple Tree with simple variables");

   Float_t met; // Missing energy in the transverse direction.

   Int_t njets; // Necessary to keep track of the number of jets

   // We'll define these assuming we will not write information for
   // more than 16 jets. We'll have to check for this in the code otherwise
   // it could crash!
   Float_t pt[16];
   Float_t eta[16];
   Float_t phi[16];

   // The first argument is ROOT's internal name of the variable.
   // The second argument is the *address* of the actual variable we defined above
   // The third argument defines the *type* of the variable to be stored, and the "F"
   // at the end signifies that this is a float
   t1.Branch("met",&met,"met/F");

   // First we define njets where the syntax is the same as before,
   // except we take care to identify this as an integer with the final
   // /I designation
   t1.Branch("njets",&njets,"njets/I");

   // We can now define the other variables, but we use a slightly different
   // syntax for the third argument to identify the variable that will be used
   // to count the number of entries per event
   t1.Branch("pt",&pt,"pt[njets]/F");
   t1.Branch("eta",&eta,"eta[njets]/F");
   t1.Branch("phi",&phi,"phi[njets]/F");

   Int_t nevents = 1000;

   for (Int_t i=0;i<nevents;i++) {

       // Generate random number between 10-60 (arbitrary)
       met = 50*gRandom->Rndm() + 10;

       // Generate random number between 0-5, inclusive
       njets = gRandom->Integer(6);

       for (Int_t j=0;j<njets;j++) {
           pt[j] =  100*gRandom->Rndm();
           eta[j] =  6*gRandom->Rndm();
           phi[j] =  6.28*gRandom->Rndm() - 3.14;
       }

       // After each event we need to *fill* the TTree
       t1.Fill();
   }

   // After we've run over all the events, we "change directory" to the file object
   // and write the tree to it.
   // We can also print the tree, just as a visual identifier to ourselves that
   // the program ran to completion.
   f.cd();
   t1.Write();
   t1.Print();
   
   return 0;
}

Because we need to compile this in such a way that it links to the ROOT libraries, we will use a Makefile to simplify the build process.

Create a new file called Makefile in the same directory as write_ROOT_file.cc and add the following to the file. You’ll most likely do this with the editor of your choice.

CC=g++

CFLAGS=-c -g -Wall `root-config --cflags`

LDFLAGS=`root-config --glibs`

all: write_ROOT_file

write_ROOT_file: write_ROOT_file.cc
    $(CC) $(CFLAGS) -o write_ROOT_file.o write_ROOT_file.cc
    $(CC) $(LDFLAGS) -o write_ROOT_file write_ROOT_file.o

Warning! Tabs are important in Makefiles!

Makefiles have been around a long time and are used for many projects, not just C/C++ code. While other build tools are slowly supplanting them (e.g. CMake), Makefiles are a pretty tried and true standard and it is worth taking time at some point and learning more about them.

One frustrating thing though can be a Makefile’s reliance on tabs for specific purposes. In the example above, the following lines are preceeded by a tab and not four (4) spaces.

   $(CC) $(CFLAGS) -o write_ROOT_file.o write_ROOT_file.cc
   $(CC) $(LDFLAGS) -o write_ROOT_file write_ROOT_file.o

If your Makefile has spaces at those points instead of a tab, make will not work for you and you will get an error.

You can now compile and run your compiled program from the command line!

make write_ROOT_file
./write_ROOT_file

Output from write_ROOT_file

******************************************************************************
*Tree    :t1        : A simple Tree with simple variables                    *
*Entries :     1000 : Total =           51536 bytes  File  Size =      36858 *
*        :          : Tree compression factor =   1.35                       *
******************************************************************************
*Br    0 :met       : met/F                                                  *
*Entries :     1000 : Total  Size=       4542 bytes  File Size  =       3641 *
*Baskets :        1 : Basket Size=      32000 bytes  Compression=   1.12     *
*............................................................................*
*Br    1 :njets     : njets/I                                                *
*Entries :     1000 : Total  Size=       4552 bytes  File Size  =        841 *
*Baskets :        1 : Basket Size=      32000 bytes  Compression=   4.84     *
*............................................................................*
*Br    2 :pt        : pt[njets]/F                                            *
*Entries :     1000 : Total  Size=      14084 bytes  File Size  =      10445 *
*Baskets :        1 : Basket Size=      32000 bytes  Compression=   1.29     *
*............................................................................*
*Br    3 :eta       : eta[njets]/F                                           *
*Entries :     1000 : Total  Size=      14089 bytes  File Size  =      10424 *
*Baskets :        1 : Basket Size=      32000 bytes  Compression=   1.30     *
*............................................................................*
*Br    4 :phi       : phi[njets]/F                                           *
*Entries :     1000 : Total  Size=      14089 bytes  File Size  =      10758 *
*Baskets :        1 : Basket Size=      32000 bytes  Compression=   1.26     *
*............................................................................*

Your numbers may be slightly different because of the random numbers that are generated.

Huzzah! You’ve successfully written your first ROOT file!

Will I have to make my Open Data analysis code?

Yes, you will! However, you won’t actually call make, nor will you need to write your own Makefile. Instead, the CMS software uses a configuration and build system called SCRAM.

So instead of typing make, you’ll find yourself typing scram. However, it serves the same purpose by compiling and linking your code for you.

Read a ROOT file

Let’s try to read this file in now. We won’t do much with but we’ll try to understand the process necessary to read in all the data and loop over this event-by-event.

We’ll start with the basic include statements and the main program.

#include<cstdio>
#include<cstdlib>
#include<iostream>

#include "TROOT.h"
#include "TTree.h"
#include "TFile.h"
#include "TRandom.h"

int main() {

    return 0;
}

In the main function, we’ll define the input file.

   // Here's the input file
   // Without the 'recreate' argument, ROOT will assume this file exists to be read in.
   TFile f("tree.root");

We’ll make use of the built-in member functions to TFile to pull out the TTree named t1. There’s a few other things to note.

First, we’re going to assign it to a local variable named input_tree. This is to emphasize that t1 is just a string that refers to the name of the object stored in the TFile and that we can assign it to any variable name, not just one named t1.

The second thing to note is that we are going to create a pointer to input_tree, which makes some of the memory management easier. This means that we precede our variable name with an asterix *, we have to cast the object pulled out of the TFile as a TTree pointer (TTree*), and subsequent uses of input_tree will access data members and member functions with the -> operator rather than a period ..

If you want to learn more about pointers, there are many, many, resources out there.

   // We will now "Get" the tree from the file and assign it to
   // a new local variable.
   TTree *input_tree = (TTree*)f.Get("t1");

Just as we did in the write_ROOT_file.cc example, we will define some local variables. These variables will actually get “filled” by the ROOT file when we loop over the events.

   Float_t met;
   Int_t njets;
   Float_t pt[16];
   Float_t eta[16];
   Float_t phi[16];

We’ll now assign these local variables to specific TBranches in input_tree. Note that we’ll be using the address of each local variable when we precede the variable name with an ampersand &.

   // Assign these variables to specific branch addresses
   input_tree->SetBranchAddress("met",&met);
   input_tree->SetBranchAddress("njets",&njets);
   input_tree->SetBranchAddress("pt",&pt);
   input_tree->SetBranchAddress("eta",&eta);
   input_tree->SetBranchAddress("phi",&phi);

We’re ready now to loop over events! Each time we call input_tree->GetEntry(i), it pulls the ith values out of input_tree and “fills” the local variables with those values.

  for (Int_t i=0;i<nevents;i++) {

       // Get the values for the i`th event and fill all our local variables
       // that were assigned to TBranches
       input_tree->GetEntry(i);

       // Print the number of jets in this event
       printf("%d\n",njets);

       // Print out the momentum for each jet in this event
       for (Int_t j=0;j<njets;j++) {
           printf("%f,%f,%f\n",pt[j], eta[j], phi[j]);
       }
   }

The final version of your read_ROOT_file.cc should look like

Full source code file for read_ROOT_file.cc

#include<cstdio>
#include<cstdlib>
#include<iostream>

#include "TROOT.h"
#include "TTree.h"
#include "TFile.h"
#include "TRandom.h"

int main() {

  // Here's the input file
  // Without the 'recreate' argument, ROOT will assume this file exists to be read in.
  TFile f("tree.root");

  // We will now "Get" the tree from the file and assign it to
  // a new local variable.
  TTree *input_tree = (TTree*)f.Get("t1");

  Float_t met; // Missing energy in the transverse direction.

  Int_t njets; // Necessary to keep track of the number of jets

  // We'll define these assuming we will not write information for
  // more than 16 jets. We'll have to check for this in the code otherwise
  // it could crash!
  Float_t pt[16];
  Float_t eta[16];
  Float_t phi[16];

  // Assign these variables to specific branch addresses
  input_tree->SetBranchAddress("met",&met);
  input_tree->SetBranchAddress("njets",&njets);
  input_tree->SetBranchAddress("pt",&pt);
  input_tree->SetBranchAddress("eta",&eta);
  input_tree->SetBranchAddress("phi",&phi);

  // Get the number of events in the file
  Int_t nevents = input_tree->GetEntries();

  for (Int_t i=0;i<nevents;i++) {

      // Get the values for the i`th event and fill all our local variables
      // that were assigned to TBranches
      input_tree->GetEntry(i);

      // Print the number of jets in this event
      printf("%d\n",njets);

      // Print out the momentum for each jet in this event
      for (Int_t j=0;j<njets;j++) {
          printf("%f,%f,%f\n",pt[j], eta[j], phi[j]);
      }
  }

  return 0;
}

Now we need to modify our Makefile to compile this code. We edit it so that it looks like this.

CC=g++

CFLAGS=-c -g -Wall `root-config --cflags`

LDFLAGS=`root-config --glibs`

all: write_ROOT_file read_ROOT_file

write_ROOT_file: write_ROOT_file.cc
    $(CC) $(CFLAGS) -o write_ROOT_file.o write_ROOT_file.cc
    $(CC) $(LDFLAGS) -o write_ROOT_file write_ROOT_file.o

read_ROOT_file: read_ROOT_file.cc
    $(CC) $(CFLAGS) -o read_ROOT_file.o read_ROOT_file.cc
    $(CC) $(LDFLAGS) -o read_ROOT_file read_ROOT_file.o

clean:
    rm -f ./*~ ./*.o ./write_ROOT_file
    rm -f ./*~ ./*.o ./read_ROOT_file

We can now compile and run the code!

make read_ROOT_file
./read_ROOT_file

We get a lot of output! However it should look something like the following, keeping in mind your numbers will be different because of the random numbers that make up the values.

Output of read_ROOT_file

1
85.105431,5.602912,0.501085
1
18.954712,4.375443,-1.546321
1
39.784435,5.165263,2.592412
3
80.748314,0.387768,1.786288
52.971573,3.939434,2.484405
12.969198,3.115963,0.910543
3
93.604256,0.737315,-0.647755
86.382034,3.493269,-1.573663
68.181541,3.658454,-1.206015
3
96.990395,5.839735,3.046098
79.096542,4.515290,0.039709
83.234497,4.990829,2.586360
4
60.880657,1.233623,-2.837789
25.723198,4.751074,2.355202
20.403908,4.656353,-2.171340
18.961079,1.425917,2.016828

Awesome! You’ve now written and read in a very simple ROOT file! There is obviously much more that can be done, but this should give you the basics of interfacing with ROOT TFiles and TTrees.

You’ll see some version of this code when using analyzers to run over the open data code. At that point, you can write out subsets of the data to new ROOT files or even simply dump the data to a text or .csv file.

In the next section, we’ll take a quick look at how to read in a file and make a few histograms, still using the C++ syntax.

Key Points

  • ROOT defines the file format in which all of the CMS Open Data is stored.

  • These files can be accessed quickly using C++ code and the relevant information can be dumped out into other formats.

Using ROOT with C++ to fill a histogram

Overview

Teaching: 20 min
Exercises: 40 min
Questions

  • Is there more than reading and writing files that can be done with ROOT?

  • How do I run a ROOT script?

Objectives

  • Learn to fill a histogram and save it to a file.

  • Learn to run a simple ROOT script

Filling a histogram

ROOT can easily fill a histogram as you are looping over individual events. Let’s try creating and filling a histogram with the transverse momentum values. We’ll start with the read_ROOT_file.cc code we wrote in the previous episode and copy what we have to a new file, fill_histogram.cc.

cp read_ROOT_file.cc fill_histogram.cc

Into this file, we’ll add some lines at some key spots. For now, we’ll go through those lines of code individually, and then show you the completed file at the end to see where they went.

First we need to include the header file for the ROOT TH1F class.

#include "TH1F.h"

We create an output file to store the histogram in.

   // Let's make an output file which we'll use to save our
   // histogram
   TFile fout("output.root","recreate");

Define the histogram.

   // We define an histogram for the transverse momentum of the jets
   // The arguments are as follow
   // * Internal name of the histogram
   // * Title that will be used if the histogram is plotted
   // * Number of bins
   // * Low edge of the lowest bin
   // * High edge of the highest bin
   TH1F h1("h1","jet pT (GeV/c)",50,0,150);

And then inside the event loop, we fill the histogram each time we get a new value for the transverse momentum.

         // Fill the histogram with each value of pT
         h1.Fill(pt[j]);

Before we leave the function, we “change directory” to the output file, write the histogram to the file, and then close the output file.

   fout.cd();
   h1.Write();
   fout.Close();

The final version of fill_histogram.cc will look like this.

Source code for fill_histogram.cc

#include<cstdio>
#include<cstdlib>
#include<iostream>

#include "TROOT.h"
#include "TTree.h"
#include "TFile.h"
#include "TRandom.h"
#include "TH1F.h"

int main() {

  // Here's the input file
  // Without the 'recreate' argument, ROOT will assume this file exists to be read in.
  TFile f("tree.root");

  // Let's make an output file which we'll use to save our
  // histogram
  TFile fout("output.root","recreate");

  // We define an histogram for the transverse momentum of the jets
  // The arguments are as follow
  // * Internal name of the histogram
  // * Title that will be used if the histogram is plotted
  // * Number of bins
  // * Low edge of the lowest bin
  // * High edge of the highest bin
  TH1F h1("h1","jet pT (GeV/c)",50,0,150);

  // We will now "Get" the tree from the file and assign it to
  // a new local variable.
  TTree *input_tree = (TTree*)f.Get("t1");

  Float_t met; // Missing energy in the transverse direction.

  Int_t njets; // Necessary to keep track of the number of jets
  // We'll define these assuming we will not write information for
  // more than 16 jets. We'll have to check for this in the code otherwise
  // it could crash!
  Float_t pt[16];
  Float_t eta[16];
  Float_t phi[16];

  // Assign these variables to specific branch addresses
  input_tree->SetBranchAddress("met",&met);
  input_tree->SetBranchAddress("njets",&njets);
  input_tree->SetBranchAddress("pt",&pt);
  input_tree->SetBranchAddress("eta",&eta);
  input_tree->SetBranchAddress("phi",&phi);

  // Get the number of events in the file
  Int_t nevents = input_tree->GetEntries();

  for (Int_t i=0;i<nevents;i++) {

      // Get the values for the i`th event and fill all our local variables
      // that were assigned to TBranches
      input_tree->GetEntry(i);

      // Print the number of jets in this event
      printf("%d\n",njets);

      // Print out the momentum for each jet in this event
      for (Int_t j=0;j<njets;j++) {
          printf("%f,%f,%f\n",pt[j], eta[j], phi[j]);

          // Fill the histogram with each value of pT
          h1.Fill(pt[j]);
      }
  }

  fout.cd();
  h1.Write();
  fout.Close();

  return 0;
}

We will modify our Makefile accordingly.

CC=g++

CFLAGS=-c -g -Wall `root-config --cflags`

LDFLAGS=`root-config --glibs`

all: write_ROOT_file read_ROOT_file fill_histogram

write_ROOT_file: write_ROOT_file.cc
    $(CC) $(CFLAGS) -o write_ROOT_file.o write_ROOT_file.cc
    $(CC) $(LDFLAGS) -o write_ROOT_file write_ROOT_file.o

read_ROOT_file: read_ROOT_file.cc
    $(CC) $(CFLAGS) -o read_ROOT_file.o read_ROOT_file.cc
    $(CC) $(LDFLAGS) -o read_ROOT_file read_ROOT_file.o

fill_histogram: fill_histogram.cc
    $(CC) $(CFLAGS) -o fill_histogram.o fill_histogram.cc
    $(CC) $(LDFLAGS) -o fill_histogram fill_histogram.o

clean:
    rm -f ./*~ ./*.o ./write_ROOT_file
    rm -f ./*~ ./*.o ./read_ROOT_file
    rm -f ./*~ ./*.o ./fill_histogram

And then compile and run it!

make fill_histogram
./fill_histogram

The output on the screen should not look different. However, if you list the contents of the directory, you’ll see a new file, output.root!

To inspect this new ROOT file, we’ll launch CINT for the first time and create a TBrowser object.

On the command line, run the following to launch CINT and attach our new ROOT file.

root -l output.root

root [0]
Attaching file output.root as _file0...
root [1]

You can either type C++/ROOT commands or launch a TBrowser, which is a graphical tool to inspect ROOT files. Inside this CINT environment, type the following (without the root [1], as that is just the ROOT/CINT prompt).

root [1] TBrowser b;

You should see the TBrowser pop up!

TBrowser

If we double click on output.root, in the left-hand menu and then the h1;1 that appears below it, we should see the following plot appear!

Inspecting the ROOT file contents

Work assignment: investigating data in ROOT files

In the previous episode you generated a file called tree.root. It has some variables which were stored in a TTree called t1. Let’s explore the variables contained in this tree by using one of the methods available for TTree objects. You can find out more about these methods directly from the ROOT TTree class documentation.

Open the tree.root file with ROOT:

root -l tree.root

Now, dump the content of the t1 tree with the method Print. Note that, by opening the file, the ROOT tree in there is automatically loaded.

root [0]
Attaching file tree.root as _file0...
root [1] t1->Print()

Please copy the output this statement generates and paste it into the corresponding section in our assignment form; remember you must sign in and click on the submit button in order to save your work. You can go back to edit the form at any time.

Using a ROOT script

We could also loop over all the events, create and save the histogram, but also draw the histogram onto a TCanvas object and have it pop up, all from a ROOT script and the CINT.

First, let’s copy over our C++ source code into a C++ script.

cp fill_histogram.cc fill_histogram_SCRIPT.C

Next we’ll remove the headers at the beginning and even get rid of the int main designation, though we keep the curly brackets.

We’ll also define a TCanvas object on which we’ll plot our histogram. After we do that, we “change directory” to the canvas and draw our histogram. We can even save it to a .png file.

   // Declare a TCanvas with the following arguments
   // * Internal name of the TCanvas object
   // * Title to be displayed when it is drawn
   // * Width of the canvas
   // * Height of the canvas
   TCanvas *c1 = new TCanvas("c1", "Canvas on which to display our histogram", 800, 400);

   c1->cd(0);
   h1.Draw();
   c1->SaveAs("h_pt.png");

Your fill_histogram_SCRIPT.C should look like this.

Source code for fill_histogram_SCRIPT.C

{

  // Here's the input file
  // Without the 'recreate' argument, ROOT will assume this file exists to be read in.
  TFile f("tree.root");

  // Let's make an output file which we'll use to save our
  // histogram
  TFile fout("output.root","recreate");

  // We define an histogram for the transverse momentum of the jets
  // The arguments are as follow
  // * Internal name of the histogram
  // * Title that will be used if the histogram is plotted
  // * Number of bins
  // * Low edge of the lowest bin
  // * High edge of the highest bin
  TH1F h1("h1","jet pT (GeV/c)",50,0,150);

  // We will now "Get" the tree from the file and assign it to
  // a new local variable.
  TTree *input_tree = (TTree*)f.Get("t1");

  Float_t met; // Missing energy in the transverse direction.

  Int_t njets; // Necessary to keep track of the number of jets

  // We'll define these assuming we will not write information for
  // more than 16 jets. We'll have to check for this in the code otherwise
  // it could crash!
  Float_t pt[16];
  Float_t eta[16];
  Float_t phi[16];

     // Assign these variables to specific branch addresses
  input_tree->SetBranchAddress("met",&met);
  input_tree->SetBranchAddress("njets",&njets);
  input_tree->SetBranchAddress("pt",&pt);
  input_tree->SetBranchAddress("eta",&eta);
  input_tree->SetBranchAddress("phi",&phi);

  // Get the number of events in the file
  Int_t nevents = input_tree->GetEntries();

  for (Int_t i=0;i<nevents;i++) {

      // Get the values for the i`th event and fill all our local variables
      // that were assigned to TBranches
      input_tree->GetEntry(i);

      // Print the number of jets in this event
      printf("%d\n",njets);

      // Print out the momentum for each jet in this event
      for (Int_t j=0;j<njets;j++) {
          printf("%f,%f,%f\n",pt[j], eta[j], phi[j]);

          // Fill the histogram with each value of pT
          h1.Fill(pt[j]);
      }
  }

  // Declare a TCanvas with the following arguments
  // * Internal name of the TCanvas object
  // * Title to be displayed when it is drawn
  // * Width of the canvas
  // * Height of the canvas
  TCanvas *c1 = new TCanvas("c1", "Canvas on which to display our histogram", 800, 400);

  c1->cd(0);
  h1.Draw();
  c1->SaveAs("h_pt.png");

  fout.cd();
  h1.Write();
  fout.Close();

}

To run this, you need only type the following on the command line.

root -l fill_histogram_SCRIPT.C

You’ll be popped into the CINT environment and you should see the following plot pop up!

TBrowser

Key Points

  • You can quickly inspect your data using just ROOT

  • A simple ROOT script is often all you need for diagnostic work

Using ROOT with python

Overview

Teaching: 0 min
Exercises: 0 min
Questions

  • Can I call ROOT from python?

Objectives

  • Find resources

PyROOT

The PyROOT project started with Wim Lavrijsen in the late `00s and became very popular, paralleling the rise of more general python tools within the community.

If you want to learn how to use PyROOT, you can go through some individual examples here, or a more guided tutorial here.

Feel free to challenge yourself to rewrite the previous C++ code using PyROOT!

Key Points

  • PyROOT is a complete interface to the ROOT libraries