Advanced Genetic Algorithm Toolkit
Project description
This package implements some of the advanced algorithms on top of my python wrapper pgapy of the Parallel Genetic Algorithm Package PGAPack. In the following we’re using GA to mean genetic algorithm.
Currently we cover two variants of probabilistic model building GAs, also called Estimation of Distribution Algorithm (EDA) by other authors. The variants implemented here all operate on binary genes.
You can implement your own solver for an optimization problem on top of this implementation by studying the documentation of pgapy and the user guide of PGAPack (which comes bundled with an installation of pgapy). In addition looking into the implementation of the deceptive function it becomes clear how to code your own problem.
The package is meant as a toolkit where you can mix and match various advanced GA algorithms.
Deceptive Functions
For testing these algorithms, deceptive functions have been proposed [DG92], these work on k bit strings and all, except one value lead the genetic algorithm in the wrong direction. A concrete example are k bit trap functions. The function evaluates to the number of 1-bits minus 1, except for the single case with all zeros for which the evaluation is k. So for all values the algorithm gains when adding more 1-bits but the case with all 0-bits is the optimum. Several of these trap functions can be concatenated into a more complex problem, the evaluation being the sum of all individual trap functions.
An implementation of order k trap functions is given in deceptive.py, the number of bits and the number of concatenated trap functions can be specified on the command-line. By default, all trap functions are concatenated in order, making the problem solveable by a simple GA. With the --shuffle option, the bits belonging to individual trap functions are shuffled across the whole gene. Due to the disruptive effect of crossover in the simple GA, with shuffled genes, the simple GA can no longer solve these problems.
The newer algorithms build a probabilistic model during optimization. This model can detect which genes have a high correlation and should be treated as a unit. This makes even shuffled deceptive problems solveable by these algorithms.
Probabilistic Model Building Genetic Algorithms (PMBGA)
This implements some of the probabilistic model building genetic algorithms from the literature. We also implement the typical test functions the so-called deceptive functions. As a basis it needs PGApy, my wrapper around the Parallel Genetic Algorithm Library, PGApack. For the latter I’ve recently implemented restricted tournament replacement [Har95] as one of the replacement strategies which is better at preserving genetic variability for longer that the standard replacements schemes.
PMBGAs typically replace the mutation and crossover operators of traditional genetic algorithms [Hol75], [Gol89] with a statistical model built during the search. Instead of crossover and mutation the model is sampled to create the next generation.
Extended Compact Genetic Algorithm (ECGA)
The ECGA, originally proposed by Harik [Har99] replaces the mutation and crossover of a genetic algorithm by building and later sampling a statistical model of the population in each generation. The model uses a minimum description length (MDL) model. A better description with an example is found in [HLS06]. Note that we’re using the restricted tournament replacement, originally called restricted tournament selection by Harik [Har94], [Har95] and later named restricted tournament replacement by Pelikan [Pel05] for population replacement which has recently been implemented in the genetic algorithm library PGApack. Note that currently we support only bitstring (bool) genes. Since we’re using model building not the standard GA operations, we disable mutation by setting the mutation probability to 0.
Hierarchical Bayesian Optimization Algorithm (hBOA)
Bayesian optimization, first introduced as the Bayesian Optimization Algorithm (BOA) [PGC99] uses a Bayesian network to model the dependencies among the individual genes. It was later extended to include local structures in the Bayesian network which allows more granular model building and named hBOA [Pel05]. The hBOA, originally proposed by Pelikan and Goldberg [PG00], [PG01] and described in Pelikan’s dissertation [Pel02] which was developed in a book on Bayesian optimization algorithms [Pel05] builds a Bayesian network to model the dependencies of the variables in the genetic algorithm. Like other PMBGAs it samples the resulting model to generate individuals as candidates for the next generation. It uses restricted tournament replacement (RTR) [Har95] as the replacement strategy. For implementing it using PGApy, we use the RTR strategy from PGApack which was recently introduced. When building the model it uses a Bayesian dirichlet metric and a term that penalizes models which are too complex ([Pel05] p.113). In the code, this term is named cutoff because it stops the addition of further edges to the Bayesian network. It was later discovered that when using noisy selection strategies like tournament selection, the algorithm would add spurious dependencies [LLPG10], [LLPG11], especially during the final phase of each network-building stage. This could be countered by adding a penalty factor to the cutoff parameter. The best value according to the tests was the tournament size, e.g. 2 for binary tournament, this is the reason the factor was called s-penalty. To further limit the spurious dependencies generated the code has an additional parameter min_split that will not split the graph further when the number of samples affected is below the threshold given by the min_split parameter.
Kalyanmoy Deb and David E. Goldberg. Analyzing deception in trap functions. In L. Darrell Whitley, editor, Foundation of Genetic Algorithms (FOGA) 2, pages 93–108. Elsevier, 1992.
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Martin Pelikan, David E. Goldberg, and Erick Cantú-Paz. BOA: The Bayesian optimization algorithm. In Wolfgang Banzhaf, Jason M. Daida, A. E. Eiben, Max H. Garzon, Vasant G. Honavar, Mark J. Jakiela, and Robert E. Smith, editors, Genetic and Evolutionary Computation GECCO 1999, page 525–532, Orlando, Florida, July 1999. Morgan Kaufmann.
Version 0.2: Updated initial release
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