Quaestiones Informaticae Vol 5 No 3
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Item Generating relations using formal grammars(Computer Society of South Africa (on behalf of SAICSIT), 1987) von Solms, S. H.Grammars generating 2-dimensional arrays have been studied by many people [1, 2, 3, 4). One effort was Random Context Array Grammars [4], where different types of context conditions placed on the production, were used to control the generating process. Relations, as used in relational data bases, can be considered as rectangular arrays, and therefore it should be possible to generate and manipulate relations using 2-dimensional Grammars. Simple Relation Grammars generate relations, and can simulate some unary operations like select and project on these relations. Extended Relation Grammars also generate relations, but these relations can communicate with each other within a "Extended Relation Schema (ERS)". Within such an ERS binary operations like union and join can be simulated. This paper is a summary of a research project of which certain parts have already been submitted for publication.Item Pointers as a data type(Computer Society of South Africa (on behalf of SAICSIT), 1987) Philips, N.C.K.; Postma, Stef W.The distinction between an abstract data type and an implementation of a data type is not as clear as is commonly thought. A hash table can be used to implement the type bag but few of our colleagues think of a hash table as itself being a data type. Similarly, pointers can be used to give a linked implementation of the data type list, but few of us seem to think of pointers as themselves forming a data type. However we shall give an algebraic specification of a pointer data type which is an abstraction of the representation of pointers in Pascal. The usual linked structures can then be specified abstractly via mutually recursive type specifications. In addition we shall describe a uniform method of constructing implementations of data type specifications via operations on strings.Item Common approach to some informational systems(Computer Society of South Africa (on behalf of SAICSIT), 1987) Orlowska, M.E.In this paper we consider relationships between a complete system [6], an L-system [3], a stochastic system [13] and an N-system [8]. We introduce a notion of the induction of one system by another. We prove the important fact that an N-system is a common generalization of all the systems mentioned above. This fact is clearly demonstrated using geometric characterization of the set of subcontinua of the internal <0, l>.Item Three packaging rules for information system design(Computer Society of South Africa (on behalf of SAICSIT), 1987) Mende, J.After identifying the processing functions required in a computer based information system, the designer needs to combine them into an optimal set of load units. Some "packaging" arrangements yield a better system than others, depending upon characteristics of the data collected from external sources and the data extracted for external users. An effective and technically efficient system satisfies three rules. 1. If two user data types are needed at different times, the corresponding extract functions should be separated in different load units. 2. If source data predates the user data derived from it, the corresponding collect and extract functions should be separated in different load units. 3. If two source data types are available at different frequencies, one being less frequent than the user data derived from it, the corresponding collect functions should be separated in different load units.Item The ELSIM language: an FSM-based language for the ELSIM SEE(Computer Society of South Africa (on behalf of SAICSIT), 1987) Du Plessis, A.L.; Bornman, C.H.A fonnal requirements specification language, the ELSIM language, is presented. The language uses conceptual models for modelling data processing, and control characteristics and behavior of real-time systems, and is structured into two sections, an analysis section and a design section. An extended finite-state machine model is used for modelling the control features of a system. The ELSIM language incorporates the terminology, syntax and semantics of the real-time methodology (ELSIM). The full power of the language is realised when using it within the Software Environment developed under the SEM System. The fonnal nature of the language allows checking for completeness and consistency of the target system specification.