SIMSCRIPT II.5 Programming Language

Modelling Concepts
destroy entity name called entity name
4.5 Permanent Entities
Permanent entities are defined in much the same way as temporary entities, but these declarations
must be preceded by the statement:
permanent entities
Entities declared as permanent are stored collectively rather than in individually identifiable
records. The entire group of permanent entities of a given class is created by a single statement.
The attributes of the entities in the group are stored as indexed arrays. The attributes for a particular
entity are accessed by effectively selecting a common index for all the associated attribute arrays.
The number of entities in a particular entity group is maintained in an implicitly declared global
variable called N.entity. The attribute arrays are allocated by a create statement of a different
form than that used for temporary entities. Given the preamble declaration:
permanent entities
every HOME has an ADDRESS and an AREA
the number of HOME entities to exist may be set by assigning a value to N.HOME by a read or let
statement. The statement:
create each HOME
allocates arrays for all the attributes of the N.HOME entities of the class by executing the statement
reserve ADDRESS and AREA as N.HOME. The words every and all can be used in place of
each. Several permanent entities can be created together by naming a list of entity names, as in:
create every HOME, HOTEL and RESTAURANT
which is of the general form:
create permanent entity name list
As an alternative to assigning a value to N.entity before permanent entity creation, an arithmetic
expression can be used in the create statement to indicate the size of the attribute arrays. For
example, the following statements are equivalent:
let N.HOME = 5
create every HOME
create every HOME(5)
When the second form is used, N.entity is thereafter set to the value of the parenthesized expression.
Permanent entities are not referred to by a pointer value, but by an index value that varies between
1 and N.entity. Thus we speak of the attributes of each HOME as ADDRESS(1), ADDRESS(2),
..., ADDRESS(5), AREA(1), AREA(2), ..., AREA(5). The layout of these attributes is
shown in figure 4-9.
147