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.math.genetics;
018
019 import java.util.Collections;
020 import java.util.List;
021
022 import org.apache.commons.math.util.FastMath;
023
024 /**
025 * Population of chromosomes which uses elitism (certain percentace of the best
026 * chromosomes is directly copied to the next generation).
027 *
028 * @version $Revision: 990655 $ $Date: 2010-08-29 23:49:40 +0200 (dim. 29 ao??t 2010) $
029 * @since 2.0
030 */
031 public class ElitisticListPopulation extends ListPopulation {
032
033 /** percentage of chromosomes copied to the next generation */
034 private double elitismRate = 0.9;
035
036 /**
037 * Creates a new ElitisticListPopulation instance.
038 *
039 * @param chromosomes
040 * list of chromosomes in the population
041 * @param populationLimit
042 * maximal size of the population
043 * @param elitismRate
044 * how many best chromosomes will be directly transferred to the
045 * next generation [in %]
046 */
047 public ElitisticListPopulation(List<Chromosome> chromosomes, int populationLimit, double elitismRate) {
048 super(chromosomes, populationLimit);
049 this.elitismRate = elitismRate;
050 }
051
052 /**
053 * Creates a new ListPopulation instance and initializes its inner
054 * chromosome list.
055 *
056 * @param populationLimit maximal size of the population
057 * @param elitismRate
058 * how many best chromosomes will be directly transferred to the
059 * next generation [in %]
060 */
061 public ElitisticListPopulation(int populationLimit, double elitismRate) {
062 super(populationLimit);
063 this.elitismRate = elitismRate;
064 }
065
066 /**
067 * Start the population for the next generation. The
068 * <code>{@link #elitismRate}<code> percents of the best
069 * chromosomes are directly copied to the next generation.
070 *
071 * @return the beginnings of the next generation.
072 */
073 public Population nextGeneration() {
074 // initialize a new generation with the same parameters
075 ElitisticListPopulation nextGeneration = new ElitisticListPopulation(this.getPopulationLimit(), this.getElitismRate());
076
077 List<Chromosome> oldChromosomes = this.getChromosomes();
078 Collections.sort(oldChromosomes);
079
080 // index of the last "not good enough" chromosome
081 int boundIndex = (int) FastMath.ceil((1.0 - this.getElitismRate()) * oldChromosomes.size());
082 for (int i=boundIndex; i<oldChromosomes.size(); i++) {
083 nextGeneration.addChromosome(oldChromosomes.get(i));
084 }
085 return nextGeneration;
086 }
087
088 /**
089 * Sets the elitism rate, i.e. how many best chromosomes will be directly
090 * transferred to the next generation [in %].
091 *
092 * @param elitismRate
093 * how many best chromosomes will be directly transferred to the
094 * next generation [in %]
095 */
096 public void setElitismRate(double elitismRate) {
097 if (elitismRate < 0 || elitismRate > 1)
098 throw new IllegalArgumentException("Elitism rate has to be in [0,1]");
099 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 }