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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.math.genetics;
import java.util.Collections;
import java.util.List;
import org.apache.commons.math.util.FastMath;
/**
* Population of chromosomes which uses elitism (certain percentace of the best
* chromosomes is directly copied to the next generation).
*
* @version $Revision: 990655 $ $Date: 2010-08-29 23:49:40 +0200 (dim. 29 août 2010) $
* @since 2.0
*/
public class ElitisticListPopulation extends ListPopulation {
/** percentage of chromosomes copied to the next generation */
private double elitismRate = 0.9;
/**
* Creates a new ElitisticListPopulation instance.
*
* @param chromosomes
* list of chromosomes in the population
* @param populationLimit
* maximal size of the population
* @param elitismRate
* how many best chromosomes will be directly transferred to the
* next generation [in %]
*/
public ElitisticListPopulation(List<Chromosome> chromosomes, int populationLimit, double elitismRate) {
super(chromosomes, populationLimit);
this.elitismRate = elitismRate;
}
/**
* Creates a new ListPopulation instance and initializes its inner
* chromosome list.
*
* @param populationLimit maximal size of the population
* @param elitismRate
* how many best chromosomes will be directly transferred to the
* next generation [in %]
*/
public ElitisticListPopulation(int populationLimit, double elitismRate) {
super(populationLimit);
this.elitismRate = elitismRate;
}
/**
* Start the population for the next generation. The
* <code>{@link #elitismRate}<code> percents of the best
* chromosomes are directly copied to the next generation.
*
* @return the beginnings of the next generation.
*/
public Population nextGeneration() {
// initialize a new generation with the same parameters
ElitisticListPopulation nextGeneration = new ElitisticListPopulation(this.getPopulationLimit(), this.getElitismRate());
List<Chromosome> oldChromosomes = this.getChromosomes();
Collections.sort(oldChromosomes);
// index of the last "not good enough" chromosome
int boundIndex = (int) FastMath.ceil((1.0 - this.getElitismRate()) * oldChromosomes.size());
for (int i=boundIndex; i<oldChromosomes.size(); i++) {
nextGeneration.addChromosome(oldChromosomes.get(i));
}
return nextGeneration;
}
/**
* Sets the elitism rate, i.e. how many best chromosomes will be directly
* transferred to the next generation [in %].
*
* @param elitismRate
* how many best chromosomes will be directly transferred to the
* next generation [in %]
*/
public void setElitismRate(double elitismRate) {
if (elitismRate < 0 || elitismRate > 1)
throw new IllegalArgumentException("Elitism rate has to be in [0,1]");
this.elitismRate = elitismRate;
}
/**
* Access the elitism rate.
* @return the elitism rate
*/
public double getElitismRate() {
return this.elitismRate;
}
}
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