May 1978 - Vol. 21 No. 5
Features
A fast algorithm for copying list structures
An algorithm is presented for copying an arbitrarily linked list structure into a block of contiguous storage locations without destroying the original list. Apart from a fixed number of program variables, no auxillary storage, such as a stack, is used. The algorithm needs no mark bits and operates in linear time. It is shown to be significantly faster than Fisher's algorithm, the fastest previous linear-time algorithm for the same problem. Its speed comes mainly from its efficient list-traversal technique, which folds the processing stack into the structure being built, and from its classification of list cells into nine types, which enables processing operations to be optimized for each type.
A language extension for expressing constraints on data access
Controlled sharing of information is needed and desirable for many applications and is supported in operating systems by access control mechanisms. This paper shows how to extend programming languages to provide controlled sharing. The extension permits expression of access constraints on shared data. Access constraints can apply both to simple objects, and to objects that are components of larger objects, such as bank account records in a bank's data base. The constraints are stated declaratively, and can be enforced by static checking similar to type checking. The approach can be used to extend any strongly-typed language, but is particularly suitable for extending languages that support the notion of abstract data types.
Test data as an aid in proving program correctness
Proofs of program correctness tend to be long and tedious, whereas testing, though useful in detecting errors, usually does not guarantee correctness. This paper introduces a technique whereby test data can be used in proving program correctness. In addition to simplifying the process of providing correctness, this method simplifies the process of providing accurate specification for a program. The applicability of this technique to procedures and recursive programs is demonstrated.
Automatic data structure selection: an example and overview
The use of several levels of abstraction has proved to be very helpful in constructing and maintaining programs. When programs are designed with abstract data types such as sets and lists, programmer time can be saved by automating the process of filling in low-level implementation details. In the past, programming systems have provided only a single general purpose implementation for an abstract type. Thus the programs produced using abstract types were often inefficient in space or time. In this paper a system for automatically choosing efficient implementations for abstract types from a library of implementations is discussed. This process is discussed in detail for an example program. General issues in data structure selection are also reviewed.
Incorporation of units into programming languages
The issues of how a programming language might aid in keeping track of physical units (feet, sec, etc.) are discussed. A method is given for the introduction of relationships among units (a watt is volts*amps, a yard is three feet) and subsequent automatic conversion based upon these relationships. Various proposals for syntax are considered.
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.
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.
New sufficient optimality conditions for integer programming and their application
The purpose of this report is to present a new class of sufficient optimality conditions for pure and mixed integer programming problems. Some of the sets of sufficient conditions presented can be thought of as generalizations of optimality conditions based on primal-dual complementarity in linear programming. These sufficient conditions are particularly useful for the construction of difficult integer programming problems with known optimal solutions. These problems may then be used to test and/or “benchmark” integer programming codes.
Computer generation of gamma random variables
A new method for generating random variables from the gamma distribution with nonintegral shape parameter &agr; is proposed. This method is similar to two other methods recently given by Wallace and Fishman. It is compared with Fishman's and Ahrens and Dieter's methods. The core storage requirements and programming effort for this method are similar to those of Fishman's method. The proposed method is the same as Fishman's method for 1 ≤ &agr; < 2 and is faster than Fishman's method for 3 ≤ &agr; ≤ 19. Also, the proposed method is much simpler than Ahrens and Dieter's method and is faster for &agr; ≤ 8.
Optimal shift strategy for a block-transfer CCD memory
For the purposes of this paper, a block-transfer CCD memory is composed of serial shift registers whose shift rate can vary, but which have a definite minimum shift rate (the refresh rate) and a definite maximum shift rate. The bits in the shift registers are numbered 0 to N - 1, and blocks of N bits are always transferred, always starting at bit 0. What is the best shift strategy so that a block transfer request occurring at a random time will have to wait the minimal amount of time before bit 0 can be reached? The minimum shift rate requirement does not allow one to simply “park” at bit 0 and wait for a transfer request. The optimal strategy involves shifting as slowly as possible until bit 0 is passed, then shifting as quickly as possible until a critical boundary is reached, shortly before bit 0 comes around again. This is called the “hurry up and wait” strategy and is well known outside the computer field. The block-transfer CCD memory can also be viewed as a paging drum with a variable (bounded) rotation speed.
