Athena Developer Guide

Athena Chapter 12 N-tuple and Event Collection facilities Version/Issue: 2.0.0
the series (and the event not selected for the collection) if one of the algorithms in the sequence fails to
pass a filter.
12.3.1.1 Defining the Address Tag
The event data is accessed using a special n-tuple tag of the type
NTuple::Item m_evtAddress
It is defined in the algorithm’s header file in addition to any other ordinary n-tuple tags, as described in
section 12.2.2.1. When booking the n-tuple, the address tag must be declared like any other tag, as
shown in Listing 12.1. It is recommended to use the name "Address" for this tag.
Listing 12.1 Connecting an address tag to an n-tuple.
1: NTuplePtr nt(ntupleSvc(), "/NTUPLES/EvtColl/Collection");
1: ... Book N-tuple
2: // Add an event address column
3: StatusCode status = nt->addItem ("Address", m_evtAddress);
The usage of this tag is identical to any other tag except that it only accepts variables of type
IOpaqueAddress - the information necessary to retrieve the event data.
12.3.1.2 Filling the Event Collection
At fill time the address of the event must be supplied to the Address item. Otherwise the n-tuple may
be written, but the information to retrieve the corresponding event data later will be lost. Listing 12.2
also demonstrates the setting of a filter to steer whether the event is written out to the event collection.
Listing 12.2 Fill the address tag of an n-tuple at execution time:
1: SmartDataPtr evt(eventSvc(),"/Event");
2: if ( evt ) {
3: ... Some data analysis deciding whether to keep the event or not
4: // keep_event=true if event should be written to event collection
5: setFilterPassed( keep_event );
6: m_evtAddrColl = evt->address();
7: }
12.3.1.3 Writing out the Event Collection
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The event collection is written out by an EvtCollectionStream, which is the last member of the
event collection Sequencer. Listing 12.3 (which is taken from the job options of EvtCollection
example), shows how to set up such a sequence consisting of a user written Selector algorithm
(which could for example contain the code in Listing 12.2), and of the EvtCollectionStream.