Edward Brown

Approximating minimum-length-sequence metrics: a cautionary note☆

Abstract In numerical taxonomy one may wish to measure the dissimilarity of classifications S and T by computing the distance between them with an appropriate metric. A minimum-length-sequence (MLS) metric requires that the user identify a set X of meaningful transformations of classifications; the MLS metric μ x is then defined by requiring that μ x ( S,T ) be the length of a shortest sequence of transformations from X that carries S into T . For a given application it may be relatively easy to identify an appropriate set X of transformations, but it may be difficult or impossible to design an efficient algorithm to compute μ x . In this case it is natural to restrict the definition to obtain an approximation ϱ to the original metric μ x such that ϱ has an efficient algorithm for its computation. This restriction process must be performed carefully lest the approximation fail to satisfy the metric properties. We present a general result about this problem and apply it in two ways. First we prove that a published ‘metric’ on partitions of a set in fact violates the triangle inequality and so is merely a semimetric. Then we clarify the relationship between the nearest neighbor interchange metric on labeled binary trees and the closest partition distance measure proposed by Waterman and Smith (1978).

A computationally efficient approximation to the nearest neighbor interchange metric

The nearest neighbor interchange (nni) metric is a distance measure providing a quantitative measure of dissimilarity between two unrooted binary trees with labeled leaves. The metric has a transparent definition in terms of a simple transformation of binary trees, but its use in nontrivial problems is usually prevented by the absence of a computationally efficient algorithm. Since recent attempts to discover such an algorithm continue to be unsuccessful, we address the complementary problem of designing an approximation to the nni metric. Such an approximation should be well-defined, efficient to compute, comprehensible to users, relevant to applications, and a close fit to the nni metric; the challenge, of course, is to compromise these objectives in such a way that the final design is acceptable to users with practical and theoretical orientations. We describe an approximation algorithm that appears to satisfy adequately these objectives. The algorithm requires O(n) space to compute dissimilarity between binary trees withn labeled leaves; it requires O(n logn) time for rooted trees and O(n2 logn) time for unrooted trees. To help the user interpret the dissimilarity measures based on this algorithm, we describe empirical distributions of dissimilarities between pairs of randomly selected trees for both rooted and unrooted cases.

Learning by linking: Pedagogical environments for hypermedia authoring

APPROACHES TO HYPERTEXT have often emphasized the additional flexibility associative links provide the author and reader in the organization of text. Our interest is in the creative challenge hypertext links present to the author. The problems of organizing ideas and creating associative links can be framed in a pedagogical context. Our objective is to develop appropriate software environments to exploit the learning potential of hypertext authoring. In this paper we outline a pedagogical model of hypertext, illustrated through examples fromAnchors Aweigh, an implementation of this model. We also report on the impact that the effort of selecting anchor icons and assigning meaning to them has on thinking, as revealed in verbal protocols collected during hypertext authoring usingAnchors Aweigh.

Dynamic Routing Using Health Information Policy with Apache Camel

Apache Camel is an implementation of a messaging framework using the enterprise integration patterns advanced by Hohpe and Woolf. This framework allows the messaging architecture to be configured at run-time, by changing routing rules which determine the disposition of messages transmitted by the system. Our work illustrates an original use of the dynamic routing capability to implement health information policy, by putting the routing rules in the hands of policy administrators rather than network managers. This paper is an extended version of reference [1].

A Metaphor for Technology Integration

This article examines the use of an organizing metaphor, the walking tour, as a means of integrating learning activities with appropriate media support. The article pursues two themes; first, the effectiveness of the metaphor for knowledge acquisition activities, and second, the student-directed nature of the activities this approach fosters (the selective use of technology where it can be most effective). Of importance to both these themes is the notion that multimedia technology can enhance learning opportunities, but need not control the instructional agenda. It is not multimedia resources that make a difference in teaching, it is how they are used … if you do not have the appropriate conditions for learning, then the students will not learn [1].

A Framework for Student Projects in Computer Networks

This paper describes the delivery of a computer networks course with accompanying materials and software. The course is offered at the fourth-year undergraduate level, and has few formal pre-requisites, other than general programming skills. There is a substantial architectural and coding component to any practical course on networks, and it is important to challenge the experienced and knowledgeable students without making the material too difficult for the inexperienced students who have minimal experience in programming practical applications. Some students have an industrial background and are expecting to build practical working applications. At the other extreme are students with no exposure to operating system principles, and therefore lacking pertinent knowledge, such as concurrency concepts. Our programming framework provides an event-polling approach to avoid the complexity of more general concurrency techniques. Protocol layers are implemented with data types and use a finite state machine model to control their behaviors. The resulting course offers code structure and support to students with minimal background, without sacrificing pertinent concepts, and provides the ability to construct elaborate network applications using the coding structures introduced in the course.