SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Programming Language
every PERSON has a NAME, and an ADDRESS,
owns some CHILDREN
and may belong to the MASONS, a CHURCH, and a FAMILY
every X has a P, a Q, a R and an A
Set names should follow the same naming conventions as entities and attributes and must be unique.
Recall the guidelines on variable naming given in Chapter 1. It may now be apparent why care
should be taken in assigning names of the form letter-dot-name, as the declaration of set names implicitly
defines a number of attribute names, made up from the set name by just this form of prefixing.
An every statement defines a data structure. The next several paragraphs explain how these data
structures are created and used, and the items within them are given further definition.
4.4 Temporary Entities
An every statement defines the structure of a class of entities. Entity classes can be temporary or
permanent. This paragraph describes temporary entities. Paragraph 4.5 discusses permanent entities.
When the statement:
temporary entities
appears before a collection of every statements in a preamble, it declares that all following entities
are temporary. This means that storage is allocated to entities individually as they are created during
the course of program execution. Individual entity records are provided for each temporary entity
when a create statement is encountered. The form is:
create entity name called variable
A create statement allocates space in memory for the entity representation and assigns a pointer
to this space to the indicated variable. Each entity is a unique and distinct individual that is identified
by its pointer . As long as the variables into which these pointer values are placed are distinct,
the identity of individual entities is preserved. For example:
Entity definition in a preamble:
temporary entities
every SHIP has a NAME and a TONNAGE
Create statements in a program:
create SHIP called VESSEL
create SHIP called V(I)
These two create statements assign pointers to distinct copies of the SHIP entity structure to the
variables VESSEL and V(I). Figure 4-8 provides an illustration.
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