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Data accumulation interface in ROOT

Root class headers

To accumulate statistics using ROOT in a standalone application one have to add headers

// ROOT Class Headers --
#include "root/TFile.h"
#include "root/TH1D.h"
#include "root/TH2D.h"
#include "root/TTree.h"
#include "root/TBranch.h"
#include "root/TRandom.h" // For random number generators.

Histograms

A minimal interface for accumulation of histograms in ROOT is shown below.

// Open output file
TFile* pfile = new TFile("file.root", "RECREATE", "Created for you by RootManager" );

// Create histograms
TH1D *pHis1 = new TH1D("pHis1","My comment to TH1D", 100, 0.5, 100.5);
TH2D *pHis2 = new TH2D("pHis2","My comment to TH1D", 100, 0.5, 100.5,
                                                     200, 0.5, 200.5);
// Fill histograms in each event
pHis1 -> Fill( value, [weight] );
pHis2 -> Fill( x, y, [weight] );

// Write histograms in file in the very end
pHis1 -> Write();
pHis2 -> Write();

// Close file
pfile -> Close();

This interface allows to

• open file file.root in mode RECREATE.
• create 1d histogram with name pHis1, title "My comment to TH1D", with 100 bins in the range from 0.5 to 100.5; histogram is addressed by their pointer *pHis1.
• create 2d histogram, which is very similar to 1d, but, in addition, defines a second axis with 200 bins in the range from 0.5 to 200.5.
• fill histograms in each event; we assume that parameters value, x, y, and optional weight are defined.
• save histogram(s) in file by the command like pHis1 -> Write(); at the end of job.
• and close file with accumulated histograms.

NTuple

Assuming that file is already open,

// Define some structures for TTree
typedef struct {float x,y,z;} POINT;
static POINT point;
float new_v;

// Create TTree
TTree * ptree = new TTree("ptree", "My comment to TTree");

// Create branch(es)
TBranch *pbranch = ptree->Branch("new_v", &new_v, "new_v/F");
// or create branch without pointer
                   ptree->Branch("point",&point,"x:y:z");

// Fill data structures for each event
      new_v   = gRandom->Gaus(0, 1);
      point.x = gRandom->Gaus(1, 1);
      point.y = gRandom->Gaus(2, 1);
      point.z = gRandom->Gaus(3, 1);

// Add an event record to the tree
      ptree->Fill();

// Write the tree in the file
ptree -> Write();

These commands allow to

• create the "Tree" object with name ptree, title "My comment to TTree", and pointer *ptree.
• create the "Branch" object, representing a single variable of data structure, with name new_v, address of the data structure &new_v, and the variable name/type new_v/F.
• fill variables for each event, using commands like new_v = gRandom->Gaus(0, 1);.
• save all the parameters in the Tree using command ptree->Fill();.
• save the Tree in ROOT file by the command ptree -> Write();.

Code examples

Accumulation of data and saving in ROOT file

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