1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 package org.apache.commons.math.genetics; 18 19 import java.util.Collections; 20 import java.util.List; 21 22 import org.apache.commons.math.util.FastMath; 23 24 /** 25 * Population of chromosomes which uses elitism (certain percentace of the best 26 * chromosomes is directly copied to the next generation). 27 * 28 * @version $Revision: 990655 $ $Date: 2010-08-29 23:49:40 +0200 (dim. 29 aot 2010) $ 29 * @since 2.0 30 */ 31 public class ElitisticListPopulation extends ListPopulation { 32 33 /** percentage of chromosomes copied to the next generation */ 34 private double elitismRate = 0.9; 35 36 /** 37 * Creates a new ElitisticListPopulation instance. 38 * 39 * @param chromosomes 40 * list of chromosomes in the population 41 * @param populationLimit 42 * maximal size of the population 43 * @param elitismRate 44 * how many best chromosomes will be directly transferred to the 45 * next generation [in %] 46 */ 47 public ElitisticListPopulation(List<Chromosome> chromosomes, int populationLimit, double elitismRate) { 48 super(chromosomes, populationLimit); 49 this.elitismRate = elitismRate; 50 } 51 52 /** 53 * Creates a new ListPopulation instance and initializes its inner 54 * chromosome list. 55 * 56 * @param populationLimit maximal size of the population 57 * @param elitismRate 58 * how many best chromosomes will be directly transferred to the 59 * next generation [in %] 60 */ 61 public ElitisticListPopulation(int populationLimit, double elitismRate) { 62 super(populationLimit); 63 this.elitismRate = elitismRate; 64 } 65 66 /** 67 * Start the population for the next generation. The 68 * <code>{@link #elitismRate}<code> percents of the best 69 * chromosomes are directly copied to the next generation. 70 * 71 * @return the beginnings of the next generation. 72 */ 73 public Population nextGeneration() { 74 // initialize a new generation with the same parameters 75 ElitisticListPopulation nextGeneration = new ElitisticListPopulation(this.getPopulationLimit(), this.getElitismRate()); 76 77 List<Chromosome> oldChromosomes = this.getChromosomes(); 78 Collections.sort(oldChromosomes); 79 80 // index of the last "not good enough" chromosome 81 int boundIndex = (int) FastMath.ceil((1.0 - this.getElitismRate()) * oldChromosomes.size()); 82 for (int i=boundIndex; i<oldChromosomes.size(); i++) { 83 nextGeneration.addChromosome(oldChromosomes.get(i)); 84 } 85 return nextGeneration; 86 } 87 88 /** 89 * Sets the elitism rate, i.e. how many best chromosomes will be directly 90 * transferred to the next generation [in %]. 91 * 92 * @param elitismRate 93 * how many best chromosomes will be directly transferred to the 94 * next generation [in %] 95 */ 96 public void setElitismRate(double elitismRate) { 97 if (elitismRate < 0 || elitismRate > 1) 98 throw new IllegalArgumentException("Elitism rate has to be in [0,1]"); 99 this.elitismRate = elitismRate; 100 } 101 102 /** 103 * Access the elitism rate. 104 * @return the elitism rate 105 */ 106 public double getElitismRate() { 107 return this.elitismRate; 108 } 109 110 } 111
