Using ROOT with C++ to fill a histogram

Last updated on 2024-12-16 | Edit this page

Overview

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.

BASH

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.

CPP

#include "TH1F.h"

We create an output file to store the histogram in.

CPP

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

Define the histogram.

CPP

   // 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.

CPP

         // 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.

CPP

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

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

Source code for fill_histogram.cc

CPP

#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.

MAKEFILE

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) -o write_ROOT_file write_ROOT_file.o $(LDFLAGS)

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

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

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

And then compile and run it:

BASH

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.

BASH

root -l output.root

OUTPUT

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).

CPP

root [1] TBrowser b;

You should see the TBrowser pop up! If no browser pops up, check out the uscms.org page about the LPC to troubleshoot your X11 connection, or ask for help on Mattermost.

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

Quit ROOT by choosing the “Quit Root” option from Browser menu of the TBrowser window or by typing .q in the ROOT prompt.

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:

BASH

root -l tree.root

You can 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.

CPP

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

When you’re done, quit ROOT.

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.

BASH

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.

CPP

   // 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

CPP

{

  // 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.

BASH

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