SIMSCRIPT II.5 Programming Language

4. Modelling Concepts
4.1 Introduction
In Chapter 1, a programming language was presented as a means of describing both instructions and
the data on which they operate. The description of data items has been limited to naming the data
items, specifying modes, and, in the case of arrays, describing some simple data structuring. This
chapter describes the additional data-structuring facilities and commands provided by SIMSCRIPT
II.5 and illustrates their potential use. These data structures are designed to aid in problem definition,
particularly in the areas of simulation and modelling.
The provision of enhanced data-structuring facilities is necessary for two reasons: (1) the need for
more organizational structure than simple arrays afford, and (2) the lack of clarity of programs
written within the descriptive limits of variable name and subscript expression conventions.
SIMSCRIPT II.5 provides needed structure and narrative clarity through statements that define and
manipulate entities, attributes, and sets.
This chapter is organized into three parts: definition, organization, and manipulation. First, definitions
are provided for the three constituents of the SIMSCRIPT II.5 world view: entities, attributes,
and sets. Next the relationships between these constituents are discussed, with special
attention to how they are organized. Finally, statements that use these constituents to perform useful
functions are presented.
4.2 Entities and Attributes
An entity is a structured data item that represents some element of a modelled system. Similar to a
subscripted variable. It may have more than a single value to define a particular configuration or
state of the entity. Unlike subscripted variables, the attributes of entities are referenced by name
rather than by a subscript number, enhancing readability and model description. Using subscripted
variables, a collection of ten workers having the attributes of age, number of dependents, and social
security number might be represented in SIMSCRIPT II.5 by a two-dimensional array reserved as
follows:
reserve WORKER as 3 by 10
with the understanding that WORKER(1,4) represents the age of the fourth worker, WORKER(3,6)
the social security number of the sixth worker, etc., according to the layout shown in figure 4-1.
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