First Steps: Histograms

Overview
The Lesson
Summary

Overview

Creating, updating and displaying histograms in real time. ROOT's random number facility.

The Lesson

Start by reading about TH2F in the ROOT Crib which describes the 2d histogram of floating point numbers. Now we will put it into practice. You may find it helpful to open a second browser window looking at the TH2F documentation as we proceed.

Create the file:-

hist.cxx containing:- { #include <math.h> // Reset the stack. gROOT->Reset(); // Create a TCanvas. TCanvas* my_canvas = new TCanvas("my_canvas","Histogram Example",200,10,600,400); // Create a 1D histogram, set its fill colour, and draw it on the current TPad TH1F *hist = new TH1F("hist","Exponential distribution",100,0.,5.); hist->SetFillColor(3); hist->Draw(); // Prepare the random number generator. gRandom->SetSeed(); // Loop generating data according to an exponetial. Float_t data; Int_t dummy; for ( Int_t i=0; i<10000; i++) { data = - log( gRandom->Rndm(dummy) ); hist->Fill(data); if ( i%100 == 0 ) { my_canvas->Modified(); my_canvas->Update(); } } } Cut and paste if you must! You should be able to understand most of this macro. gRandom is a global pointer to a TRandom object that produces various random numbers. You can look it up in the ROOT class library:-

TRandom

You can find further information in How to Use the histogram classes . For example, changing the line:-

hist->Draw(); to:- hist->Draw("LEGO"); produces a lego plot. Once it has been drawn you can rotate it by dragging the mouse inside the plot.

For another simple example of histogram use (on which our simple example was based) see the ROOT tutorial Filling several histograms and some graphics options

Summary

In ROOT the histogram concept encapsulates both the concept of a histogram as an abstract data representation and its graphical presentation, something that does not please some purists! However, for beginners, wanting to collect and display data in histogram form, its good as all the facilities are collected together in a single inheritance tree of classes.
ROOT has a variety of 1D, 2D and 3D histogram classes. Facilities exist to collect, combine and display them in a variety of formats For a good overview see How to Use the histogram classes .
As with all graphical objects, the TPad that displays them does so by holding a link to the real histogram object. There are two important consequences:-
- Updates to the histogram will be reflected on screen (although you have to send the TPad its Modified() and Update() messages to do this).
- If you modify the histogram interactively (not covered in these lessons) then you are really changing the underlying histogram object.
In those cases where you need to break the link between the TPad and the underlying object it is possible to clone (copy) the object and display the clone.

Go Back to the The First Steps Top Page

If you have any comments about this page please send them to Nick West