/*
* Copyright 2003-2004 The Apache Software Foundation.
*
* Licensed 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.stat.descriptive.moment;
import org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
import org.apache.commons.math.stat.descriptive.summary.SumOfLogs;
/**
* Returns the <a href="http://www.xycoon.com/geometric_mean.htm">
* geometric mean </a> of the available values.
* <p>
* Uses a {@link SumOfLogs} instance to compute sum of logs and returns
* <code> exp( 1/n (sum of logs) ).</code> Therefore,
* <ul>
* <li>If any of values are < 0, the result is <code>NaN.</code></li>
* <li>If all values are non-negative and less than
* <code>Double.POSITIVE_INFINITY</code>, but at least one value is 0, the
* result is <code>0.</code></li>
* <li>If both <code>Double.POSITIVE_INFINITY</code> and
* <code>Double.NEGATIVE_INFINITY</code> are among the values, the result is
* <code>NaN.</code></li>
* </ul>
* <p>
* <strong>Note that this implementation is not synchronized.</strong> If
* multiple threads access an instance of this class concurrently, and at least
* one of the threads invokes the <code>increment()</code> or
* <code>clear()</code> method, it must be synchronized externally.
*
*
* @version $Revision: 348519 $ $Date: 2005-11-23 12:12:18 -0700 (Wed, 23 Nov 2005) $
*/
public class GeometricMean extends AbstractStorelessUnivariateStatistic {
/** Serializable version identifier */
private static final long serialVersionUID = -8178734905303459453L;
/** Wrapped SumOfLogs instance */
private SumOfLogs sumOfLogs;
/**
* Create a GeometricMean instance
*/
public GeometricMean() {
sumOfLogs = new SumOfLogs();
}
/**
* @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#increment(double)
*/
public void increment(final double d) {
sumOfLogs.increment(d);
}
/**
* @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getResult()
*/
public double getResult() {
if (sumOfLogs.getN() > 0) {
return Math.exp(sumOfLogs.getResult() / (double) sumOfLogs.getN());
} else {
return Double.NaN;
}
}
/**
* @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#clear()
*/
public void clear() {
sumOfLogs.clear();
}
/**
* Returns the geometric mean of the entries in the specified portion
* of the input array.
* <p>
* See {@link GeometricMean} for details on the computing algorithm.
* <p>
* Throws <code>IllegalArgumentException</code> if the array is null.
*
* @param values input array containing the values
* @param begin first array element to include
* @param length the number of elements to include
* @return the geometric mean or Double.NaN if length = 0 or
* any of the values are <= 0.
* @throws IllegalArgumentException if the input array is null or the array
* index parameters are not valid
*/
public double evaluate(
final double[] values, final int begin, final int length) {
return Math.exp(
sumOfLogs.evaluate(values, begin, length) / (double) length);
}
/**
* @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getN()
*/
public long getN() {
return sumOfLogs.getN();
}
}