SIMSCRIPT II.5 Programming Language

Modelling Concepts
WORKER
value of AGE
value of DEPENDENTS
value of SOCIAL.SECURITY.NUMBER
Figure 4-2. Order of Storage of the Attributes of an Entity
4.3 Sets
Entities and their attributes allow some structuring of related data. Sets, in turn, provide a higher
level of structuring of these data. Sets are organized collections of entities. Sets are like arrays in
that each of the entity elements of which they are composed may be identified and manipulated, but
in contrast with the static structuring imposed on array elements, the organization of entities in sets
may be dynamic and changeable. The set concept and the underlying mechanism are introduced
here by way of an example.
Consider the following situation: Over the years, residents of a community join various clubs and
societies. As residents are born, grow up, remain in or move out of the community, and die, the
club memberships change. To model the relationships that exist between the members of the community,
both over time and at particular instants of time, requires some way of grouping the individual
society and club members together. Such groupings might be defined by the statements:
every COMMUNITY owns a MASONS,
and a BOY.SCOUTS
every MAN may belong to the MASONS,
and the BOY.SCOUTS
The first statement declares that each entity of the class COMMUNITY owns a set called MASONS and
a set called BOY.SCOUTS. Each of these sets corresponds to a logical grouping of residents in the
community. This statement does not specify which residents belong to the particular sets; rather, it
establishes a system of set pointers and set attributes for the owner entities that enable set memberships
to be constructed. For each COMMUNITY entity, the attributes shown in figure 4-3 are automatically
defined to exist.
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