Accessing Event Data

1
Accessing Event Data
2
Objectives

• After completing this lesson, you should be able
  to
• Understand how objects are delivered to user
  algorithms.
• Retrieve objects from the event data store within
  an algorithm.
• Operate on objects collections.

3
Loading from Data Store
Unsuccessful ifrequested object is not present
Algorithm
(1) Retrieve object
4
StoreGatethe Athena Transient Store

• Objects of arbitrary type can be added to SG
• no more need to inherit from DataObject or
  ContainedObject (it does not hurt either)
• works with STL and STL-like containers (including
  ObjectVector). Can support custom ones (HepMC)
• Type-based hierarchy, type-safe access
• no need to specify (and propagate!) object
  "paths"
• object type is primary key, can retrieve all
  collections or the default one
• compiler checks type of retrieved objects
• optional "secondary key" (not only strings)
  allows to identify a specific data object

5
ObjectVectorltTgt
Atlfast uses typedefined ObjectVectorltTgt
collection objects ReconstructedParticleCollecti
on, CellCollection, ClusterCollection.
ObjectVectorltReconstructedParticlegt
ObjectVectorltCellgt
ObjectVectorltClustergt

• What you fill in are pointers to T
• Iteration like STL vector
• ObjectVectorltMCParticlegt pobjColl...Object
  VectorltMCParticlegtiterator iMCPfor(
  iMCPpobjColl-gtbegin()
• iMCP ! pobjColl-gtend() iMCP ) log
  ltlt MSGINFO
• ltlt (iMCP)-gtparticleID().id()

iMCP" is an iterator to a pointer to a
MCParticle Athena stores pointers to things in
its collections, therefore (iMCP) is a pointer
to a MCParticle
6
TDS Caveats

• The TDS expects full ownership.
• Do not destroy existing objects! Theyre not
  yours!
• Deleting recorded objects typically results in an
  access violation!
• StoreGate is responsible for that.
• The delete operator is invoked automatically at
  the end of each event.
• Almost const access
• You can't modify an object you retrieve
• compiler error!
• you can, when recording an object, allow
  downstream algorithms to modify it
• use sparingly!

7
StoreGate TDS access

• include StoreGate/StoreGateSvc.h
• Declare StoreGateSvc pointer
• private data member in your header file
• private
• StoreGateSvc m_sgSvc
• Get the StoreGate service
• StatusCode sc service(StoreGateSvc, m_sgSvc)
• if (sc.isFailure( )) print error message
• return StatusCodeFAILURE

8
Record RetrieveTo key or not to key

• To record an object/container
• StatusCode sc m_sgSvc?record(ptr_obj/cont
• , m_Obj/ContKey)
• if ( sc ! StatusCodeSUCCESS) print some
  error message

ptr_obj/cont is pointer to the object/containerm_
Obj/ContKey is the optional key given to your
object/container for keyed access

• To retrieve this object
• const DataHandleltObject/Containergt obj/cont
• StatusCode sc m_sgSvc?retrieve(obj/cont
• , m_Obj/ContKey)

Use the DataHandleltTgt as a pointer/iterator to
Tm_Obj/ContKey is the given ltoptionalgt key while
recording this object/container
9
Hands On Access Object Collections

• In AnalysisSkeleton.h
• Forward declare StoreGate service
• class StoreGateSvc
• Inside Atlfast namespace declare
• class ReconstructedParticle
• class Cell
• class Cluster
• Declare pointer to StoreGate service
• StoreGateSvc m_sgSvc
• declare additional locations
• stdstring m_cellLocation
• stdstring m_clusterLocation
• In the jobOptions, specify the key of objects to
  be retrieved
• AnalysisSkeleton.CellLocation
  /Event/AtlfastCells
• AnalysisSkeleton.ClusterLocation/Event/AtlfastCl
  usters

10
Hands On Access Object Collections

• In AnalysisSkeleton.cxx
• include StoreGate/StoreGateSvc.h
• Declare additional properties to the framework
• declareProperty(CellLocation, m_cellLocation)
• declareProperty(ClusterLocation,
  m_clusterLocation)
• Inside initialize( )
• If (service(StoreGateSvc, m_sgSvc).isFailure(
  ))
• log ltlt MSGALWAYS ltlt No StoreGate!!!!!!! ltlt
  endreq
• Access Atlfast collections inside execute( )
• const DataHandleltReconstructedParticleCollectiongt
  particles
• const DataHandleltCellCollectiongt cells
• const DataHandleltClusterCollectiongt clusters
• m_sgSvc-gtretrieve(particles, m_particleLocation)
• m_sgSvc-gtretrieve(cells, m_cellLocation)
• m_sgSvc-gtretrieve(clusters, m_clusterLocation)

11
Hands On Access Object Collections

• Algorithms can declare additional member
  functions
• Add typedef definition of the collection
• include AtlfastCode/ReconstructedParticleCollect
  ion.h
• Declare and define private member function
• void dumpParticles(MsgStream log,
  ReconstructedParticleCollection particles)
• Inside execute( )
• // Work on collections
• MsgStream log (msgSvc(), name())
• if (particles-gtsize( )) dumpParticles(log,
  particles)
• Inside dumpParticles( ), iterate in the
  collection
• ReconstructedParticleCollectionconst_iterator
  theParticle
• for (theParticle particles-gtbegin()
• theParticle lt particles-gtend()
• theParticle)

12
Hands On Access Object Collections

• For each selected particle, printout values
• // dump out particles
• log ltlt MSGINFO ltlt theParticle ltlt endreq
• // print pT
• double pT (theParticle)-gtpT( )
• log ltlt MSGINFO ltlt Particle pT ltlt pT ltlt
  endreq
• Additional access methods for particles
• eta( )
• phi( )
• eT( )
• mT( )
• momentum( ) // HepLorentzVector

Set ApplicationMgr.EvtMax 100