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Research and Advances Sep 1 1978

Simulations of dynamic sequential search algorithms

In [3], R.L. Rivest presents a set of methods for dynamically reordering a sequential list containing N records in order to increase search efficiency. The method Ai (for i between 1 and N) performs the following operation each time that a record R has been successfully retrieved: Move R forward i positions in the list, or to the front of the list if it was in a position less than i. The method A1 is called the transposition method, and the method AN-1 is called the move-to-front method.
Research and Advances Aug 1 1978

Fast parallel sorting algorithms

A parallel bucket-sort algorithm is presented that requires time O(log n) and the use of n processors. The algorithm makes use of a technique that requires more space than the product of processors and time. A realistic model is used in which no memory contention is permitted. A procedure is also presented to sort n numbers in time O(k log n) using n1+1/k processors, for k an arbitrary integer. The model of computation for this procedure permits simultaneous fetches from the same memory location.
Research and Advances Aug 1 1978

Communicating sequential processes

This paper suggests that input and output are basic primitives of programming and that parallel composition of communicating sequential processes is a fundamental program structuring method. When combined with a development of Dijkstra's guarded command, these concepts are surprisingly versatile. Their use is illustrated by sample solutions of a variety of a familiar programming exercises.
Research and Advances Jul 1 1978

Analysis of the availability of computer systems using computer-aided algebra

Analytical results, related to the availability of a computer system constructed of unreliable processors, are presented in this paper. These results are obtained by using various computer-aided algebraic manipulation techniques. A major purpose of this paper is to demonstrate that the difficulties of obtaining analytical solutions to Markov processes can be considerably reduced by the application of symbol manipulation programs. Since many physical systems can be modeled by Markov and semi-Markov processes, the potential range of application of these techniques is much wider than the problem of availability analyzed here.
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Research and Advances Jul 1 1978

An O(n) algorithm for determining a near-optimal computation order of matrix chain products

This paper discusses the computation of matrix chain products of the form M1 × M22 × ··· × Mn where Mi's are matrices. The order in which the matrices are computed affects the number of operations. A sufficient condition about the association of the matrices in the optimal order is presented. An O(n) algorithm to find an order of computation which takes less than 25 percent longer than the optimal time Topt is also presented. In most cases, the algorithm yields the optimal order or an order which takes only a few percent longer than Topt (less than 1 percent on the average).
Research and Advances Jul 1 1978

Time, clocks, and the ordering of events in a distributed system

The concept of one event happening before another in a distributed system is examined, and is shown to define a partial ordering of the events. A distributed algorithm is given for synchronizing a system of logical clocks which can be used to totally order the events. The use of the total ordering is illustrated with a method for solving synchronization problems. The algorithm is then specialized for synchronizing physical clocks, and a bound is derived on how far out of synchrony the clocks can become.
Research and Advances Jul 1 1978

Proving the correctness of heuristically optimized code

A system for proving that programs written in a high level language are correctly translated to a low level language is described. A primary use of the system is as a postoptimization step in code generation. The low level language programs need not be generated by a compiler and in fact could be hand coded. Examples of the usefulness of such a system are given. Some interesting results are the ability to handle programs that implement recursion by bypassing the start of the program, and the detection and pinpointing of a wide class of errors in the low level language programs. The examples demonstrate that optimization of the genre of this paper can result in substantially faster operation and the saving of memory in terms of program and stack sizes.
Research and Advances Jul 1 1978

An English language question answering system for a large relational database

By typing requests in English, casual users will be able to obtain explicit answers from a large relational database of aircraft flight and maintenance data using a system called PLANES. The design and implementation of this system is described and illustrated with detailed examples of the operation of system components and examples of overall system operation. The language processing portion of the system uses a number of augmented transition networks, each of which matches phrases with a specific meaning, along with context registers (history keepers) and concept case frames; these are used for judging meaningfulness of questions, generating dialogue for clarifying partially understood questions, and resolving ellipsis and pronoun reference problems. Other system components construct a formal query for the relational database, and optimize the order of searching relations. Methods are discussed for handling vague or complex questions and for providing browsing ability. Also included are discussions of important issues in programming natural language systems for limited domains, and the relationship of this system to others.
Research and Advances Jun 1 1978

A selective traversal algorithm for binary search trees

The problem of selecting data items from a binary search tree according to a list of range conditions is considered. The process of visiting a minimal number of nodes to retrieve data satisfying the range conditions is called selective traversal. Presented in this paper is an algorithm for selective traversal which uses a tag field for each node in the tree. The algorithm is particularly useful and efficient when examination of data is more time consuming than examination of a tag field.
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Research and Advances Jun 1 1978

Automatic error recovery for LR parsers

In this paper we present a scheme for detecting and recovering from syntax errors in programs. The scheme, which is based on LR parsing, is driven by information which is directly and automatically obtainable from the information that is already present in an LR parser. The approach, which is patterned after that of Levy and Graham and Rhodes, appears to provide error recovery which is both simple and powerful.
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Research and Advances Jun 1 1978

An optimal method for deletion in one-sided height-balanced trees

A one-sided height-balanced tree is a binary tree in which every node's right subtree has a height which is equal to or exactly one greater than the height of its left subtree. It has an advantage over the more general AVL tree in that only one bit of balancing information is required (two bits are required for the AVL tree). It is shown that deletion of an arbitrary node of such a tree can be accomplished in O(log n) operations, where n is the number of nodes in the tree. Moreover the method is optimal in the sense that its complexity cannot be reduced in order of magnitude. This result, coupled with earlier results by Hirschberg, indicates that, of the three basic problems of insertion, deletion, and retrieval, only insertion is adversely affected by this modification of an AVL tree.
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Research and Advances Jun 1 1978

Automated welfare client-tracking and service integration: the political economy of computing

The impacts of an automated client-tracking system on the clients, caseworkers, administrators, and operations of the welfare agencies that use it are reported. The major impact of this system was to enhance the administrative attractiveness of the using agencies in the eyes of funders rather than to increase their internal administrative efficiency. This impact is a joint product of both the technical features of the computer-based system and of the organizational demands placed upon different agencies, administrators, and caseworkers. It illustrates the way “successful” automated information systems fit the political economies of the groups that use them.
Research and Advances May 1 1978

An interference matching technique for inducing abstractions

A method for inducing knowledge by abstraction from a sequence of training examples is described. The proposed method, interference matching, induces abstractions by finding relational properties common to two or more exemplars. Three tasks solved by a program that uses an interference-matching algorithm are presented. Several problems concerning the description of the training examples and the adequacy of interference matching are discussed, and directions for future research are considered.
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Research and Advances May 1 1978

The SL5 procedure mechanism

This paper describes an integrated procedure mechanism that permits procedures to be used as recursive functions or as coroutines. This integration is accomplished by treating procedures and their activation records (called environments) as data objects and by decomposing procedure invocation into three separate components at the source-language level. In addition, argument binding is under the control of the programmer, permitting the definition of various methods of argument transmission in the source language itself. The resulting procedure mechanism, which is part of the SL5 programming language, is well suited to goal-oriented problems and to other problems that are more readily programmed by using coroutines. Several examples are given.
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Research and Advances Apr 1 1978

Optimal conversion of extended-entry decision tables with general cost criteria

A general dynamic programming algorithm for converting limited, extended, or mixed entry decision tables to optimal decision trees is presented which can take into account rule frequencies or probabilities, minimum time and/or space cost criteria, common action sets, compressed rules and ELSE rules, sequencing constraints on condition tests, excludable combinations of conditions, certain ambiguities, and interrupted rule masking.
Research and Advances Apr 1 1978

List processing in real time on a serial computer

A real-time list processing system is one in which the time required by the elementary list operations (e.g. CONS, CAR, CDR, RPLACA, RPLACD, EQ, and ATOM in LISP) is bounded by a (small) constant. Classical implementations of list processing systems lack this property because allocating a list cell from the heap may cause a garbage collection, which process requires time proportional to the heap size to finish. A real-time list processing system is presented which continuously reclaims garbage, including directed cycles, while linearizing and compacting the accessible cells into contiguous locations to avoid fragmenting the free storage pool. The program is small and requires no time-sharing interrupts, making it suitable for microcode. Finally, the system requires the same average time, and not more than twice the space, of a classical implementation, and those space requirements can be reduced to approximately classical proportions by compact list representation. Arrays of different sizes, a program stack, and hash linking are simple extensions to our system, and reference counting is found to be inferior for many applications.
Research and Advances Apr 1 1978

A technique for isolating differences between files

A simple algorithm is described for isolating the differences between two files. One application is the comparing of two versions of a source program or other file in order to display all differences. The algorithm isolates differences in a way that corresponds closely to our intuitive notion of difference, is easy to implement, and is computationally efficient, with time linear in the file length. For most applications the algorithm isolates differences similar to those isolated by the longest common subsequence. Another application of this algorithm merges files containing independently generated changes into a single file. The algorithm can also be used to generate efficient encodings of a file in the form of the differences between itself and a given “datum” file, permitting reconstruction of the original file from the diference and datum files.
Research and Advances Mar 1 1978

Implications of structured programming for machine architecture

Based on an empirical study of more than 10,000 lines of program text written in a GOTO-less language, a machine architecture specifically designed for structured programs is proposed. Since assignment, CALL, RETURN, and IF statements together account for 93 percent of all executable statements, special care is given to ensure that these statements can be implemented efficiently. A highly compact instruction encoding scheme is presented, which can reduce program size by a factor of 3. Unlike a Huffman code, which utilizes variable length fields, this method uses only fixed length (1-byte) opcode and address fields. The most frequent instructions consist of a single 1-byte field. As a consequence, instruction decoding time is minimized, and the machine is efficient with respect to both space and time.
Research and Advances Mar 1 1978

The use of an interactive information storage and retrieval system in medical research

This paper presents the results of a study of the use of an interactive computerized storage and retrieval system. A monitor built into the computer system provided usage data for the study. Additional data on user reactions were gathered from a questionnaire. The results show the important role played by frequently chosen laboratory reference leaders in influencing the use of this system. The implications of the study for the design of similar systems are discussed.
Research and Advances Feb 1 1978

Is “sometime” sometimes better than “always”?: intermittent assertions in proving program correctness

This paper explores a technique for proving the correctness and termination of programs simultaneously. This approach, the intermittent-assertion method, involves documenting the program with assertions that must be true at some time when control passes through the corresponding point, but that need not be true every time. The method, introduced by Burstall, promises to provide a valuable complement to the more conventional methods. The intermittent-assertion method is presented with a number of examples of correctness and termination proofs. Some of these proofs are markedly simpler than their conventional counterparts. On the other hand, it is shown that a proof of correctness or termination by any of the conventional techniques can be rephrased directly as a proof using intermittent assertions. Finally, it is shown how the intermittent-assertion method can be applied to prove the validity of program transformations and the correctness of continuously operating programs.
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Research and Advances Feb 1 1978

Some new methods of detecting step edges in digital pictures

This note describes two operators that respond to step edges, but not to ramps. The first is similar to the digital Laplacian, but uses the max, rather than the sum, of the x and y second differences. The second uses the difference between the mean and median gray levels in a neighborhood. The outputs obtained from these operators applied to a set of test pictures are compared with each other and with the standard digital Laplacian and gradient. A third operator, which uses the distance between the center and centroid of a neighborhood as an edge value, is also briefly considered; it turns out to be equivalent to one of the standard digital approximations to the gradient.
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Research and Advances Feb 1 1978

Covering edges by cliques with regard to keyword conflicts and intersection graphs

Kellerman has presented a method for determining keyword conflicts and described a heuristic algorithm which solves a certain combinatorial optimization problem in connection with this method. This optimization problem is here shown to be equivalent to the problem of covering the edges of a graph by complete subgraphs with the objective of minimizing the number of complete subgraphs. A relationship between this edge-clique-cover problem and the graph coloring problem is established which allows algorithms for either one of these problems to be constructed from algorithms for the other. As consequences of this relationship, the keyword conflict problem and the edge-clique-cover problem are shown to be NP-complete, and if P ≠ NP then they do not admit polynomial-time approximation algorithms which always produce solutions within a factor less than 2 from the optimum.
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