10.3.2 AssociationMap Container

Contents

• Introduction
• Different Types of AssociationMap
• Data Storage in an AssociationMap
• AssociationMap Interface
• AssociationMap Interactive Access
• Generating AssociationMap Dictionaries
• Information Sources
• Review Status

Introduction

In many cases it is convenient to associate quantities to objects existing in a collection. This is true for different reasons, some of which could be:

• to add extra information to an object without modifying its structure
• to reprocess only the associated quantities without needing to reprocess the main object
• to make it possible to drop extra information when writing to disk in order to save disk space

We provide a generic association map implementation that uses EDM persistent references (edm::Ref<..>) in the template:

AssociationMap maps object already existing in a collection to other objects that could either be stored in a different collection (association by reference) or stored inside the map itself (association by value).

• AssociationMap<T>

where T is one of several possible helper classes described below.

Different Types of AssociationMap

Different types of AssociationMap are supported and can be used for specifying different types of the template argument T in AssociationMap<T>.

The supported types for the parameter T are:

• edm::OneToValue <CKey, Val, index>: associates an object of type Val to an object in a collection of type CKey. The associated object is contained by value in the map. The reference to the object in the collection of type CKey is stored as an index of type index, which, by default, is the type unsigned int. Shorter indices can be used for collections with a small number of objects in order to save disk space.
• edm::OneToOne <CKey, CVal, index>: associates an object in a collection of type CVal to an object in a collection of type CKey. The association is stored by reference in the map. As before, the reference to the object in the collection of type CKey is stored as an index of type index.
• edm::OneToMany <CKey, CVal, index>: associates many objects in a collection of type CVal to an object in a collection of type CKey.
• edm::OneToManyWithQuality <CKey, CVal, Q, index>: associates many objects in a collection of type CVal to an object in a collection of type CKey. The association is stored by reference in the map in conjunction with an object of type Q that is intended to measure the quality of the match. The class Q should support the operator " < ".

Data Storage in an AssociationMap

With the exception of data stored by value in AssociationMap<OneToValue> type, references to objects are stored as indices that are of type unsigned int by default. But these indices can be of different types if they refer to collections with a small number of objects, and this may help save disk space. For instance, if the collection has less than 65536 objects, you can use an unsigned short, if the collection has less than 256 objects, you can use unsigned char.

A reference to the collection as a whole (edm::RefProd<...>) contains the product identifier of the collection (again an unsigned integer index). This is stored only once in the AssociationMap.

So, the total size of a map containing N associations is determined by:

• one product identifier for the main collection of type CKey
• one product identifier for the associated collection of type CVal (not stored for OneToValue map)
• N indices for the associated objects in the collection of type CKey
• either:
  • N objects of type Val, for OneToValue map
  • N indices for the associated objects from the collection of type CVal for OneToOne map
  • N vectors of indices for the type OneToMany map
  • N vectors of indices plus quality Q pair for the type OneToManyWithQuality map

AssociationMap Interface

The AssociationMap interface is designed in a way similar to std::map, from the Standard C++ Library.

Whenever the associated object is accessed by reference, a persistent edm::Ref<..> is used in place of a C++ reference.

An example of how to fill and retrieve objects from an AssociationMap is the following, that associates multiple references to tracks to jets:

  typedef 
    edm::AssociationMap<
      edm::OneToMany<CaloJetCollection, TrackCollection> 
    > JetTracksMap;

  JetTracksMap map;

  CaloJetRef rj = ...; 
  TrackRef rt = ...;
  map->insert( rj, rt );
  TrackRefVector tracks = map[ rj ];

  JetTracksMap::const_iterator i = map.find( rj );
  assert( i != map.end() );
  const CaloJetRef & jet1 = i->key;
  const TrackRefVector & tracks1 = i->val;

AssociationMap Interactive Access

AssociationMas provides two method called keys() and values() that returns purely transient vectors filled with object pointers (or values, for OneToValue maps). For instance, if you have a branch called "assoc" containing an association of type:

  AssociationMap<OneToOne<TrackCollection, SuperClusterCollection> >

you could plot the track momentum vs super-cluster energy with:

   Events.Draw("assoc.keys().pt():assoc.values().energy()");

In order to use the above interactive ROOT command, you need to create, in addition to the dictionary of the AssociationMap, also a dictionary for the types:

    std::vector<const reco::Track *>
    std::vector<const reco::SuperCluster *>

The following transient vector types are returned by the methods keys() and values() respectively, and require a dictionary if you want to allow interactive access:

• for AssociationMap<OneToOne<K, V> > :

       std::vector<const K::value_type *>
       std::vector<const K::value_type *>

• for AssociationMap<OneToValue<K, V> > :

       std::vector<const K::value_type *>
       std::vector<V> 

• for AssociationMap<OneToMany<K, V> > :

       std::vector<const K::value_type *>
       std::vector<std::vector<const V::value_type *> >

• for AssociationMap<OneToManyWithQuality<K, V, Q> > :

       std::vector<const K::value_type>
       std::vector<std::vector<std::pair<const V::value_type *, Q> > >

Generating AssociationMap Dictionaries

If you wish to store an AssociationMap in the event, it is mandatory to define a dictionary of the map type you are using.

In order to create a dictionary of an AssociationMap type, the following guidelines should be followed:

• references to products (collections) are stored using the template type edm::helpers::KeyVal<CKey, CVal> for all maps except for OneToValue, which needs one one reference, and uses the template edm::helpers::Key&CKey>. Those template specializations should be added to the dictionary
• the internally stored map type should be added added to the dictionary if not already defined in DataFormats/Common library. In particular:
  • AssociationMap<OneToValue<CKey, Val, index> > requires std::map<index, Val> that is already defined in DataFormats/Common for some trivial cases of the type Val
  • AssociationMap<OneToOne<CKey, CVal, index> > requires std::map<index, index> that in most of the cases is already defined in DataFormats/Common library
  • AssociationMap<OneToMany<CKey, CVal, index> > requires std::map<index, std::vector<index> > that in most of the cases is already defined in DataFormats/Common library
  • AssociationMap<OneToManyWithQuality<CKey, CVal, Q, index> > requires std::map<index, std::vector<std::pair<index, Q> > >
• the type edm::AssociationMap<...> should be added declaring the field transientMap_ as transient data member
• the wrapper edm::Wrapper<edm::AssociationMap<...> > should be added, as for any EDM type

The following dictionary is also required for OneToValue map:

• edm::helpers::KeyVal<edm::Ref<CKey>, Val>

The table below summarises the internally stored map type for the different association map types.

An example of dictionary generation is the following. It associates many tracks to a jet, and is inspired by DataFormats/BTauReco:

<lcgdict>
  <class name="edm::helpers::KeyVal<edm::RefProd<std::vector<reco::CaloJetCollection> >,
                       edm::RefProd<std::vector<reco::TrackCollection> > >" />
  <class name="edm::AssociationMap<edm::OneToMany<std::vector<reco::CaloJet>, 
                       std::vector<reco::Track>, unsigned int > >">
    <field name="transientMap_" transient="true" />
  </class>
  <!-- the dictionary for std::map<unsigned int, std::vector<unsigned int> > is not needed
         because it is defined in DataFormats/Common library -->
</lcgdict>

Review status

Responsible: LucaLista
Last reviewed by: