February 1970 - Vol. 13 No. 2

February 1970 issue cover image

Features

Research and Advances

A formal system for information retrieval from files

A generalized file structure is provided by which the concepts of keyword, index, record, file, directory, file structure, directory decoding, and record retrieval are defined and from which some of the frequently used file structures such as inverted files, index-sequential files, and multilist files are derived. Two algorithms which retrieve records from the generalized file structure are presented.
Research and Advances

The multistore parser for hierarchical syntactic structures

A syntactic parser is described for hierarchical concatenation patterns that are presented to the analyzer in the form of linear strings. Particular emphasis is given to the system of “significant addresses” by means of which processing times for large-scale matching procedures can be substantially reduced. The description makes frequent use of examples taken from the fully operational implementation of the parser in an experimental English sentence analyzer. By structuring an area of the computer's central core storage in such a way that the individual locations of bytes and bits come to represent the data involved in the matching procedure, the shifting of information is reduced to a minimum, and the searching of lists is eliminated altogether. The matches are traced by means of binary masks and the state of single bits determines the operational flow of the procedure. The method could be implemented with any interpretive grammar, provided it can be expressed by the functional classification of the items composing the input hierarchical structures.
Research and Advances

Translation equations

Input limited transduction expressions, or translation equations, are used to describe the syntax and left-context sensitive semantics for context-free languages. A formal procedure is given for deriving from a set of translation equations the specifications for a pushdown translator. The translator consists of Mealy form finite-state automata interacting by means of a pushdown stack. Within the framework described, string recognition and parsing may be treated as special cases of the translation problem.
Research and Advances

Spelling correction in systems programs

Several specialized techniques are shown for efficiently incorporating spelling correction algorithms into compilers and operating systems. These include the use of syntax and semantics information, the organization of restricted keyword and symbol tables, and the consideration of a limited class of spelling errors. Sample 360 coding for performing spelling correction is presented. By using systems which perform spelling correction, the number of debugging runs per program has been decreased, saving both programmer and machine time.
Research and Advances

An efficient context-free parsing algorithm

A parsing algorithm which seems to be the most efficient general context-free algorithm known is described. It is similar to both Knuth's LR(k) algorithm and the familiar top-down algorithm. It has a time bound proportional to n3 (where n is the length of the string being parsed) in general; it has an n2 bound for unambiguous grammars; and it runs in linear time on a large class of grammars, which seems to include most practical context-free programming language grammars. In an empirical comparison it appears to be superior to the top-down and bottom-up algorithms studied by Griffiths and Petrick.
Research and Advances

The use of quadratic residue research

Some of the problems of simulating discrete event systems, particularly computer systems, on a conventional digital computer are dealt with. The systems are assumed to be described as a network of interconnected sequential processes. Briefly reviewed are the common techniques used to handle such simulations when simultaneous events do not occur, can be ignored, or can be handled by simple priority rules. Following this, the problem of dealing with simultaneous events in separate processes is introduced. An abstraction of this problem is developed which admits solution for a majority of commonly encountered problems. The technique will either find a method of simulating the parallel events or report that none can be found. In some of the latter cases it is shown to be possible to find a solution by extending the information available to the solution technique, but in many cases the technique becomes computationally unfeasible when the additional information is provided.
Research and Advances

Computer education in a graduate school of management

Several years of experience have led to the belief that the creative design and evaluation of management information systems requires a thorough understanding of the related computer technology. Concepts such as paging and priority interrupt systems can best be explained at the machine language level. Any machine used for exposition should fulfill several criteria. It should: (1) raise as few spurious issues as possible; (2) allow, without undue effort, the solution of interesting problems; (3) be capable of exposing all outstanding issues of significance, within the chosen machine; (4) be useful for pursuing issues in great depth when appropriate; (5) not be committed to the equipment provided by any manufacturer; (6) be able to provide the student with diagnostic aids to a great depth; (7) allow the student ready access to the machine; (8) be capable of extension to expose new issues as they come along. We have constructed a simulated machine and its associated software which meets these criteria. This system, called the PRISM system, is documented by a primer and a reference manual.
Research and Advances

An interactive computer system using graphical flowchart input

An interactive computer system operational on a graphical computer terminal is described. This system was designed to demonstrate a method of programming by computer interpretation of a flowchart. The user draws a description of a sampled-data system and specifies an original input signal on a display oscilloscope. This graphical description is transmitted to a large scale computer. The design is simulated, and a graphic representation of the processed signal is returned to the scope. A successful design may require numerous modifications of the original design. A graphical interactive system provides an environment to perform this iterative process efficiently and effectively.

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