SIMSCRIPT II.5 Programming Language

Modelling Concepts
preamble
normally dimension is 2
the system has an EXCESS
define EXCESS as an integer array
normally dimension is 0, mode is real
the system has a VALUE and owns a COLLECTION
permanent entities
every SAMPLE belongs to the COLLECTION
and has a PRICE and a NAME
temporary entities
every POINT has an IDENTITY and a
COLLECT.TIME
define NAME and IDENTITY as text variables
This preamble defines five system attributes, one of which is real-valued (VALUE), one of which
is a base pointer for a two-dimensional array (EXCESS) whose elements are integer-valued, one
of which is an integer counter for set members (N.COLLECTION), and two of which are set
pointers (F.COLLECTION and L.COLLECTION). The preamble also defines a class of permanent
entities (SAMPLE) and a class of temporary entities (POINT). Each entity of type SAMPLE has two
set pointer attributes (P.COLLECTION and S.COLLECTION), an integer set membership flag
(M.COLLECTION), a real attribute (PRICE), and a text attribute (NAME). These attributes are stored
as one-dimensional arrays, of dimension N.SAMPLE. Each entity of type POINT has a text attribute
(IDENTITY) and a real attribute (COLLECT.TIME). These attributes are stored in individual
locations within the temporary entity structures.
Figure 4-10 illustrates the storage of the attributes of the N.SAMPLE permanent entities of the class
SAMPLE. Figure 4-11 illustrates the layout of an entity record for a temporary entity of the class
POINT. Figure 4-12 shows the arrangement in memory of the system attributes VALUE, EXCESS,
F.COLLECTION, L.COLLECTION, and N.COLLECTION.
4.8 Sets: Their Declaration and Use
Sets are declared in every statements when their owner and member entities are defined. Every
set must have an owner, either an entity or the system, and can have either permanent or temporary
entities as members, but not both.
Sets named in every statements have the following properties:
1. Owner entities have first and last-in-set pointers named F.set and L.set.
2. Member entities have predecessor and successor pointers named P.set and S.set.
3. Set members are ranked on a first-in, first-out basis when they are put in a set.
4. Each member entity has a membership named M.set that is a non-zero value if an entity is
in the set, and zero if it is not. Note that M.set is non-zero if an entity is in any set with
the given name. A non-zero value does not guarantee the entity is in one specific set.
151