Genotype representations in grammatical evolution
Introduction
Grammatical evolution (GE) [20], [19], [25] is a form of grammar-based genetic programming. A special feature of GE is that, unlike genetic programming, it has a clear distinction between the genotype and phenotype. The mapping of the genotype and phenotype is governed by a grammar and this grammar can contain domain knowledge to bias the form a phenotypic solution can take. By separating the search and solution spaces, GE allows the implementation of generic search algorithms without a requirement to tailor the diversity-generating operators to the nature of the phenotype. A substantial literature has emerged on GE and its applications [20], [2], [24], [22], [4]. Some of the more recent developments of GE are focused on the various components of the GE approach including the use of alternative search engines [15], [14], [18], the use of alternative grammar constructs [16], [5], [12], [21], and the examination of different mapping processes [17]. One aspect of GE which has seen less research is the examination of the impact of the choice of genotypic representation, and associated diversity-generation operators, on GE’s efficiency and effectiveness. A recent paper by Oetzel and Rothlauf [24] examined the locality properties of a binary representation in GE and found that a genotypic bit-mutation operator produced non-local changes in the phenotype. The authors of this study proposed that further research be undertaken in order to find other representations and associated mutation operators which would produce higher locality, suggesting that this would increase the performance and effectiveness of GE. This study addresses this research issue by investigating the impact of four mutation operators using two different representations, binary and gray code representation, on the performance of GE. In addition, for the first time, a direct comparison is made between the two forms of genotypic representation adopted in the GE literature, namely integer versus binary codons. The combinations are tested using three standard benchmark problems, symbolic regression, the Santa Fe ant trail and the even-5-parity problem. In addition, a novel wrapping operator is proposed for the binary and gray representations and its performance compared to the standard wrapping operator.
The remainder of the paper is structured as follows. Section 2 describes GE and provides background on earlier work on representations. Section 3 details the experimental approach adopted and results, and finally Section 5 details conclusions and future work.
Section snippets
Background
This section provides an introduction to GE and to some prior work on the importance of representation in evolutionary algorithms. GE is a grammar-based form of genetic programming (GP) [9]. Rather than representing the programs as parse trees, as in GP, a linear genome representation is used. A genotype–phenotype mapping is employed such that each individual’s variable length binary string, contains in its codons (groups of 8 bits) the information to select production rules from a Backus Naur
Experimental setup and results
This section outlines the experimental setup used in this study, details the results of these experiments, and provides a discussion of the key findings.
Discussion
A number of experiments on different genotypic representations and operators have been presented, and are conducted on three diverse and well understood standard benchmark problems from the Genetic Programming literature. One of the primary motivations for this study was the fact that at least two variants of Grammatical Evolution exist within the literature, which adopt different genotypic representations (i.e. binary and integer). For the first time, in this study we compare the performance
Conclusions and future work
The object of this study was to examine the impact of different genotypic representations in GE in order to determine whether certain representations, and associated diversity-generation operators, improve GE’s efficiency and effectiveness. Two main variants of genotype representation exist in the literature to date, namely, binary and integer encodings. This is the first time these two representations have been formally compared and analysed.
Four mutation operators using two different
Acknowledgement
This publication has emanated from research conducted with the financial support of Science Foundation Ireland.
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