001 /* 002 * Licensed to the Apache Software Foundation (ASF) under one or more 003 * contributor license agreements. See the NOTICE file distributed with 004 * this work for additional information regarding copyright ownership. 005 * The ASF licenses this file to You under the Apache License, Version 2.0 006 * (the "License"); you may not use this file except in compliance with 007 * the License. You may obtain a copy of the License at 008 * 009 * http://www.apache.org/licenses/LICENSE-2.0 010 * 011 * Unless required by applicable law or agreed to in writing, software 012 * distributed under the License is distributed on an "AS IS" BASIS, 013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 014 * See the License for the specific language governing permissions and 015 * limitations under the License. 016 */ 017 package org.apache.commons.math3.genetics; 018 019 import java.util.ArrayList; 020 import java.util.List; 021 022 import org.apache.commons.math3.exception.MathIllegalArgumentException; 023 import org.apache.commons.math3.exception.util.LocalizedFormats; 024 025 /** 026 * Tournament selection scheme. Each of the two selected chromosomes is selected 027 * based on n-ary tournament -- this is done by drawing {@link #arity} random 028 * chromosomes without replacement from the population, and then selecting the 029 * fittest chromosome among them. 030 * 031 * @since 2.0 032 * @version $Id: TournamentSelection.java 1416643 2012-12-03 19:37:14Z tn $ 033 */ 034 public class TournamentSelection implements SelectionPolicy { 035 036 /** number of chromosomes included in the tournament selections */ 037 private int arity; 038 039 /** 040 * Creates a new TournamentSelection instance. 041 * 042 * @param arity how many chromosomes will be drawn to the tournament 043 */ 044 public TournamentSelection(final int arity) { 045 this.arity = arity; 046 } 047 048 /** 049 * Select two chromosomes from the population. Each of the two selected 050 * chromosomes is selected based on n-ary tournament -- this is done by 051 * drawing {@link #arity} random chromosomes without replacement from the 052 * population, and then selecting the fittest chromosome among them. 053 * 054 * @param population the population from which the chromosomes are chosen. 055 * @return the selected chromosomes. 056 * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size 057 */ 058 public ChromosomePair select(final Population population) throws MathIllegalArgumentException { 059 return new ChromosomePair(tournament((ListPopulation) population), 060 tournament((ListPopulation) population)); 061 } 062 063 /** 064 * Helper for {@link #select(Population)}. Draw {@link #arity} random chromosomes without replacement from the 065 * population, and then select the fittest chromosome among them. 066 * 067 * @param population the population from which the chromosomes are choosen. 068 * @return the selected chromosome. 069 * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size 070 */ 071 private Chromosome tournament(final ListPopulation population) throws MathIllegalArgumentException { 072 if (population.getPopulationSize() < this.arity) { 073 throw new MathIllegalArgumentException(LocalizedFormats.TOO_LARGE_TOURNAMENT_ARITY, 074 arity, population.getPopulationSize()); 075 } 076 // auxiliary population 077 ListPopulation tournamentPopulation = new ListPopulation(this.arity) { 078 public Population nextGeneration() { 079 // not useful here 080 return null; 081 } 082 }; 083 084 // create a copy of the chromosome list 085 List<Chromosome> chromosomes = new ArrayList<Chromosome> (population.getChromosomes()); 086 for (int i=0; i<this.arity; i++) { 087 // select a random individual and add it to the tournament 088 int rind = GeneticAlgorithm.getRandomGenerator().nextInt(chromosomes.size()); 089 tournamentPopulation.addChromosome(chromosomes.get(rind)); 090 // do not select it again 091 chromosomes.remove(rind); 092 } 093 // the winner takes it all 094 return tournamentPopulation.getFittestChromosome(); 095 } 096 097 /** 098 * Gets the arity (number of chromosomes drawn to the tournament). 099 * 100 * @return arity of the tournament 101 */ 102 public int getArity() { 103 return arity; 104 } 105 106 /** 107 * Sets the arity (number of chromosomes drawn to the tournament). 108 * 109 * @param arity arity of the tournament 110 */ 111 public void setArity(final int arity) { 112 this.arity = arity; 113 } 114 115 }