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 /**
023 * Population of chromosomes which uses elitism (certain percentace of the best
024 * chromosomes is directly copied to the next generation).
025 *
026 * @version $Revision: 811685 $ $Date: 2009-09-05 13:36:48 -0400 (Sat, 05 Sep 2009) $
027 * @since 2.0
028 */
029 public class ElitisticListPopulation extends ListPopulation {
030
031 /** percentage of chromosomes copied to the next generation */
032 private double elitismRate = 0.9;
033
034 /**
035 * Creates a new ElitisticListPopulation instance.
036 *
037 * @param chromosomes
038 * list of chromosomes in the population
039 * @param populationLimit
040 * maximal size of the population
041 * @param elitismRate
042 * how many best chromosomes will be directly transferred to the
043 * next generation [in %]
044 */
045 public ElitisticListPopulation(List<Chromosome> chromosomes, int populationLimit, double elitismRate) {
046 super(chromosomes, populationLimit);
047 this.elitismRate = elitismRate;
048 }
049
050 /**
051 * Creates a new ListPopulation instance and initializes its inner
052 * chromosome list.
053 *
054 * @param populationLimit maximal size of the population
055 * @param elitismRate
056 * how many best chromosomes will be directly transferred to the
057 * next generation [in %]
058 */
059 public ElitisticListPopulation(int populationLimit, double elitismRate) {
060 super(populationLimit);
061 this.elitismRate = elitismRate;
062 }
063
064 /**
065 * Start the population for the next generation. The
066 * <code>{@link #elitismRate}<code> percents of the best
067 * chromosomes are directly copied to the next generation.
068 *
069 * @return the beginnings of the next generation.
070 */
071 public Population nextGeneration() {
072 // initialize a new generation with the same parameters
073 ElitisticListPopulation nextGeneration = new ElitisticListPopulation(this.getPopulationLimit(), this.getElitismRate());
074
075 List<Chromosome> oldChromosomes = this.getChromosomes();
076 Collections.sort(oldChromosomes);
077
078 // index of the last "not good enough" chromosome
079 int boundIndex = (int) Math.ceil((1.0 - this.getElitismRate()) * oldChromosomes.size());
080 for (int i=boundIndex; i<oldChromosomes.size(); i++) {
081 nextGeneration.addChromosome(oldChromosomes.get(i));
082 }
083 return nextGeneration;
084 }
085
086 /**
087 * Sets the elitism rate, i.e. how many best chromosomes will be directly
088 * transferred to the next generation [in %].
089 *
090 * @param elitismRate
091 * how many best chromosomes will be directly transferred to the
092 * next generation [in %]
093 */
094 public void setElitismRate(double elitismRate) {
095 if (elitismRate < 0 || elitismRate > 1)
096 throw new IllegalArgumentException("Elitism rate has to be in [0,1]");
097 this.elitismRate = elitismRate;
098 }
099
100 /**
101 * Access the elitism rate.
102 * @return the elitism rate
103 */
104 public double getElitismRate() {
105 return this.elitismRate;
106 }
107
108 }