Athena Developer Guide

Athena
Chapter 13 Framework services Version/Issue: 2.0.0
• Within the user code a series of numbers is required only once, i.e. not every event. In this
case the object is used locally and resources are released immediately after use. This example
is shown in Listing 13.5 .
Listing 13.5 Example of the use of the random number generator to fill a histogram with a Gaussian
distribution within a standard Athena algorithm
1: Rndm::Numbers gauss(randSvc(), Rndm::Gauss(0.5,0.2));
2: if ( gauss ) {
3: IHistogram1D* his = histoSvc()->book("/stat/2","Gaussian",40,0.,3.);
4: for ( long i = 0; i < 5000; i++ )
5: his->fill(gauss(), 1.0);
6: }
• One or several random numbers are required for the processing of every event. An example is
shown in Listing 13.6.
Listing 13.6 Example of the use of the random number generator within a standard Athena algorithm, for use
at every event. The wrapper to the generator is part of the Algorithm itself and must be initialized before being
used. Afterwards the usage is identical to the example described in Listing 13.5
1: #include "GaudiKernel/RndmGenerators.h"
2:
3: // Constructor
4: class myAlgorithm : public Algorithm {
5: Rndm::Numbers m_gaussDist;
6: ...
7: };
8:
9: // Initialisation
10: StatusCode myAlgorithm::initialize() {
11: ...
1: StatusCode sc=m_gaussDist.initialize(randSvc(), Rndm::Gauss(0.5,0.2));
2: if ( !status.isSuccess() ) {
3: // put error handling code here...
4: }
5: ...
6: }
There are a few points to be mentioned in order to ensure the reproducibility:
• Do not keep numbers across events. If you need a random number ask for it. Usually caching
does more harm than good. If there is a performance penalty, it is better to find a more generic
solution.
• Do not access the RndmEngine directly.
• Do not manipulate the engine. The random seeds should only be set by the framework on an
event by event basis.
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