SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Programming Language
TANKER
TUG
word 1 SPEED SPEED
word 2 P.HARBOR.SET P.HARBOR.SET
word 3 S.HARBOR.SET S.HARBOR.SET
word 4 M.HARBOR.SET M.HARBOR.SET
word 5 CARGO
Figure 4-21. Entity Structures for TANKER and TUG
Word numbers are described in detail in Chapter 6.
Care should be taken not to reference CARGO with a pointer which could be identifying an instance
of a TUG entity. Thus:
for each SHIP in HARBOR.SET
with CARGO(SHIP) greater than 100
.
.
would be incorrect if there were, in fact, any TUG entities in the HARBOR.SET. Clearly TUG entities
do not have any CARGO.
In general, where common sets are used, it is good practice to declare a common attribute which
can serve to discriminate between the different entity types sharing membership of a common set.
Recall that in the context of permanent entities, attribute references are actually references to attribute
arrays. Multiple definition of such an array is not permitted. Hence, permanent attributes
cannot be declared as common.
4.11 Compound Entities
At times it is convenient for several entities jointly to have attributes and own sets. Such entities
are called compound entities. Statements such as:
permanent entities
every MAN and WOMAN owns a FAMILY and has
a BANK.ACCOUNT
every CITY, COUNTY, STATE has a CENSUS
every MODEL, COLOR, YEAR, MFG has a SALES.VOLUME
define compound entities composed of 2, 3, and 4 permanent entities, respectively. The first defines
four two-dimensional arrays: F.FAMILY, L.FAMILY, N.FAMILY, and BANK.ACCOUNT, each dimensioned
as N.MAN BY N.WOMAN. The second defines a three-dimensional array CENSUS dimen-
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