scholarly journals Conflict-Directed Backjumping Revisited

2001 ◽  
Vol 14 ◽  
pp. 53-81 ◽  
Author(s):  
X. Chen ◽  
P. Van Beek
Keyword(s):  
Constraint Satisfaction Problems ◽  
Local Consistency ◽  
Backtracking Search ◽  
Arc Consistency ◽  
Look Ahead ◽  
Variable Ordering ◽  
Look Back ◽  
The Look ◽  
The Relationship ◽  
Theoretical Results

In recent years, many improvements to backtracking algorithms for solving constraint satisfaction problems have been proposed. The techniques for improving backtracking algorithms can be conveniently classified as look-ahead schemes and look-back schemes. Unfortunately, look-ahead and look-back schemes are not entirely orthogonal as it has been observed empirically that the enhancement of look-ahead techniques is sometimes counterproductive to the effects of look-back techniques. In this paper, we focus on the relationship between the two most important look-ahead techniques---using a variable ordering heuristic and maintaining a level of local consistency during the backtracking search---and the look-back technique of conflict-directed backjumping (CBJ). We show that there exists a ``perfect'' dynamic variable ordering such that CBJ becomes redundant. We also show theoretically that as the level of local consistency that is maintained in the backtracking search is increased, the less that backjumping will be an improvement. Our theoretical results partially explain why a backtracking algorithm doing more in the look-ahead phase cannot benefit more from the backjumping look-back scheme. Finally, we show empirically that adding CBJ to a backtracking algorithm that maintains generalized arc consistency (GAC), an algorithm that we refer to as GAC-CBJ, can still provide orders of magnitude speedups. Our empirical results contrast with Bessiere and Regin's conclusion (1996) that CBJ is useless to an algorithm that maintains arc consistency.

scholarly journals Constraint programming viewed as rule-based programming

2001 ◽  
Vol 1 (6) ◽  
pp. 713-750 ◽  
Author(s):  
KRZYSZTOF R. APT ◽  
ERIC MONFROY
Keyword(s):  
Constraint Satisfaction ◽  
Constraint Programming ◽  
Constraint Satisfaction Problems ◽  
Rule Based ◽  
Local Consistency ◽  
Arc Consistency ◽  
Computation Process ◽  
Rule Based Approach ◽  
Rule Consistency

We study here a natural situation when constraint programming can be entirely reduced to rule-based programming. To this end we explain first how one can compute on constraint satisfaction problems using rules represented by simple first-order formulas. Then we consider constraint satisfaction problems that are based on predefined, explicitly given constraints. To solve them we first derive rules from these explicitly given constraints and limit the computation process to a repeated application of these rules, combined with labeling. We consider two types of rule here. The first type, that we call equality rules, leads to a new notion of local consistency, called rule consistency that turns out to be weaker than arc consistency for constraints of arbitrary arity (called hyper-arc consistency in Marriott & Stuckey (1998)). For Boolean constraints rule consistency coincides with the closure under the well-known propagation rules for Boolean constraints. The second type of rules, that we call membership rules, yields a rule-based characterization of arc consistency. To show feasibility of this rule-based approach to constraint programming, we show how both types of rules can be automatically generated, as CHR rules of Frühwirth (1995). This yields an implementation of this approach to programming by means of constraint logic programming. We illustrate the usefulness of this approach to constraint programming by discussing various examples, including Boolean constraints, two typical examples of many valued logics, constraints dealing with Waltz's language for describing polyhedral scenes, and Allen's qualitative approach to temporal logic.

The value of online surveys: a look back and a look ahead

2018 ◽  
Vol 28 (4) ◽  
pp. 854-887 ◽  
Author(s):  
Joel R. Evans ◽  
Anil Mathur
Keyword(s):  
Survey Research ◽  
Online Survey ◽  
New Technologies ◽  
Actual Behavior ◽  
Online Surveys ◽  
Content Type ◽  
Look Ahead ◽  
Look Back ◽  
The Future ◽  
The Look

Purpose The purpose of this paper is to present a detailed and critical look at the evolution of online survey research since Evans and Mathur’s (2005) article on the value of online surveys. At that time, online survey research was in its early stages. Also covered are the present and future states of online research. Many conclusions and recommendations are presented. Design/methodology/approach The look back focuses on online surveys, strengths and weaknesses of online surveys, the literature on several aspects of online surveys and online survey best practices. The look ahead focuses on emerging survey technologies and methodologies, and new non-survey technologies and methodologies. Conclusions and recommendations are provided. Findings Online survey research is used more frequently and better accepted by researchers than in 2005. Yet, survey techniques are still regularly transformed by new technologies. Non-survey digital research is also more prominent than in 2005 and can better track actual behavior than surveys can. Hybrid surveys will be widespread in the future. Practical implications The paper aims to provide insights for researchers with different levels of online survey experience. And both academics and practitioners should gain insights. Social implications Adhering to a strong ethics code is vital to gain respondents’ trust and to produce valid results. Originality/value Conclusions and recommendations are offered in these specific areas: defining concepts, understanding the future role of surveys, developing and implementing surveys and a survey code of ethics. The literature review cites more than 200 sources.

Efficient Singleton Consistency by Combining Forward Checking and Bound Consistency

2014 ◽  
Vol 23 (04) ◽  
pp. 1460017
Author(s):  
Jinsong Guo ◽  
Hongbo Li ◽  
Zhanshan Li ◽  
Yonggang Zhang ◽  
Xianghua Jia
Keyword(s):  
Constraint Satisfaction ◽  
Search Algorithm ◽  
Computational Cost ◽  
Search Space ◽  
Search Algorithms ◽  
Constraint Satisfaction Problems ◽  
Local Consistency ◽  
Arc Consistency ◽  
Strong Bound

Maintaining local consistencies can improve the efficiencies of the search algorithms solving constraint satisfaction problems (CSPs). Comparing with arc consistency which is the most widely used local consistency, stronger local consistencies can make the search space smaller while they require higher computational cost. In this paper, we make an attempt on the compromise between the pruning ability and the computational cost. A new local consistency called singleton strong bound consistency (SSBC) and its light version, light SSBC, are proposed. The search algorithm maintaining light SSBC can outperform MAC on a considerable number of problems.

Analysis of chlamydia contacts and the look-back interval at a UK sexual health clinic

2021 ◽  
Vol 32 (6) ◽  
pp. 533-537
Author(s):  
Catherine Turner ◽  
Sally E Howlett ◽  
Hannah Loftus
Keyword(s):  
Sexual Health ◽  
Sexual Intercourse ◽  
Index Case ◽  
Contact Tracing ◽  
Health Clinic ◽  
Significant Difference ◽  
Look Back ◽  
The Look ◽  
The Relationship ◽  
Current Guidelines

Guidance on contact tracing in Chlamydia trachomatis (CT) is limited. CT contacts data over 12 months (1 December 2018–29 November 2019) at a UK sexual health clinic were analysed to determine the appropriateness of the currently recommended six-month ‘look-back’ interval. Age and sex of CT contacts were associated with clinical outcomes. Subgroups of 100 CT positive/negative contacts (each N = 100) were randomly selected. The relationship between time since sexual intercourse with the index case (Last Sexual Intercourse; LSI) and CT positivity was examined; suitability of varying look-back intervals was explored. Of 891 CT contacts (mean age = 25.0 years, 66.2% men), 66.9% tested positive for CT. Positive CT contacts were significantly younger (23.8 ± 6.8 years vs. 27.4 ± 9.1, p < 0.001) and more often women (36.4% vs. 28.5%, p = 0.018) than negative contacts. In the subgroups, the Mann–Whitney U test revealed no significant difference between the LSI of positive and negative contacts ( p = 0.081). 95% of positive CT contacts ( N = 82) were captured within a hypothetical three-month look-back interval. While most CT positive contacts were captured within three months, they appeared to remain proportionately represented beyond this point. Although this supports current guidelines, further research should investigate whether CT contacts involved in longer look-back intervals may require disproportionately greater resources to trace.

scholarly journals Path-Wise Test Data Generation Based on Heuristic Look-Ahead Methods

2014 ◽  
Vol 2014 ◽  
pp. 1-19 ◽  
Author(s):  
Ying Xing ◽  
Yun-Zhan Gong ◽  
Ya-Wen Wang ◽  
Xu-Zhou Zhang
Keyword(s):  
Test Data ◽  
Search Space ◽  
Optimization Method ◽  
Search Tree ◽  
Test Data Generation ◽  
Data Generation ◽  
Look Ahead ◽  
Variable Ordering ◽  
The Look ◽  
Monotonicity Analysis

Path-wise test data generation is generally considered an important problem in the automation of software testing. In essence, it is a constraint optimization problem, which is often solved by search methods such as backtracking algorithms. In this paper, the backtracking algorithm branch and bound and state space search in artificial intelligence are introduced to tackle the problem of path-wise test data generation. The former is utilized to explore the space of potential solutions and the latter is adopted to construct the search tree dynamically. Heuristics are employed in the look-ahead stage of the search. Dynamic variable ordering is presented with a heuristic rule to break ties, values of a variable are determined by the monotonicity analysis on branching conditions, and maintaining path consistency is achieved through analysis on the result of interval arithmetic. An optimization method is also proposed to reduce the search space. The results of empirical experiments show that the search is conducted in a basically backtrack-free manner, which ensures both test data generation with promising performance and its excellence over some currently existing static and dynamic methods in terms of coverage. The results also demonstrate that the proposed method is applicable in engineering.

170-OR: Association of Timing of Physical Activity Bouts with Cardiorespiratory Fitness in Adults with Type 2 Diabetes in the Look AHEAD Study

Diabetes ◽  
2020 ◽  
Vol 69 (Supplement 1) ◽  
pp. 170-OR
Author(s):  
JINGYI QIAN ◽  
MICHAEL P. WALKUP ◽  
SHYH-HUEI CHEN ◽  
PETER H. BRUBAKER ◽  
DALE BOND ◽  
...  
Keyword(s):  
Physical Activity ◽  
Type 2 Diabetes ◽  
Cardiorespiratory Fitness ◽  
Look Ahead ◽  
The Look

scholarly journals The look ahead trace back optimizer for genomic selection under transparent and opaque simulators

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Fatemeh Amini ◽  
Felipe Restrepo Franco ◽  
Guiping Hu ◽  
Lizhi Wang
Keyword(s):  
Genomic Selection ◽  
Genetic Gain ◽  
Genomic Prediction ◽  
In Planta ◽  
Data Set ◽  
Look Ahead ◽  
Trace Back ◽  
New Variant ◽  
The Look ◽  
Partially Observable

AbstractRecent advances in genomic selection (GS) have demonstrated the importance of not only the accuracy of genomic prediction but also the intelligence of selection strategies. The look ahead selection algorithm, for example, has been found to significantly outperform the widely used truncation selection approach in terms of genetic gain, thanks to its strategy of selecting breeding parents that may not necessarily be elite themselves but have the best chance of producing elite progeny in the future. This paper presents the look ahead trace back algorithm as a new variant of the look ahead approach, which introduces several improvements to further accelerate genetic gain especially under imperfect genomic prediction. Perhaps an even more significant contribution of this paper is the design of opaque simulators for evaluating the performance of GS algorithms. These simulators are partially observable, explicitly capture both additive and non-additive genetic effects, and simulate uncertain recombination events more realistically. In contrast, most existing GS simulation settings are transparent, either explicitly or implicitly allowing the GS algorithm to exploit certain critical information that may not be possible in actual breeding programs. Comprehensive computational experiments were carried out using a maize data set to compare a variety of GS algorithms under four simulators with different levels of opacity. These results reveal how differently a same GS algorithm would interact with different simulators, suggesting the need for continued research in the design of more realistic simulators. As long as GS algorithms continue to be trained in silico rather than in planta, the best way to avoid disappointing discrepancy between their simulated and actual performances may be to make the simulator as akin to the complex and opaque nature as possible.

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