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Java > Open Source Codes > org > apache > commons > math > stat > descriptive > moment > GeometricMean


1 /*
2  * Copyright 2003-2004 The Apache Software Foundation.
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  * http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 package org.apache.commons.math.stat.descriptive.moment;
17
18 import org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
19 import org.apache.commons.math.stat.descriptive.summary.SumOfLogs;
20
21 /**
22  * Returns the <a HREF="http://www.xycoon.com/geometric_mean.htm">
23  * geometric mean </a> of the available values.
24  * <p>
25  * Uses a {@link SumOfLogs} instance to compute sum of logs and returns
26  * <code> exp( 1/n (sum of logs) ).</code> Therefore,
27  * <ul>
28  * <li>If any of values are < 0, the result is <code>NaN.</code></li>
29  * <li>If all values are non-negative and less than
30  * <code>Double.POSITIVE_INFINITY</code>, but at least one value is 0, the
31  * result is <code>0.</code></li>
32  * <li>If both <code>Double.POSITIVE_INFINITY</code> and
33  * <code>Double.NEGATIVE_INFINITY</code> are among the values, the result is
34  * <code>NaN.</code></li>
35  * </ul>
36  * <p>
37  * <strong>Note that this implementation is not synchronized.</strong> If
38  * multiple threads access an instance of this class concurrently, and at least
39  * one of the threads invokes the <code>increment()</code> or
40  * <code>clear()</code> method, it must be synchronized externally.
41  *
42  *
43  * @version $Revision$ $Date: 2005-02-26 05:11:52 -0800 (Sat, 26 Feb 2005) $
44  */
45 public class GeometricMean extends AbstractStorelessUnivariateStatistic {
46
47     /** Serializable version identifier */
48     static final long serialVersionUID = -8178734905303459453L;
49     
50     /** Wrapped SumOfLogs instance */
51     private SumOfLogs sumOfLogs;
52
53     /**
54      * Create a GeometricMean instance
55      */
56     public GeometricMean() {
57         sumOfLogs = new SumOfLogs();
58     }
59     
60     /**
61      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#increment(double)
62      */
63     public void increment(final double d) {
64         sumOfLogs.increment(d);
65     }
66
67     /**
68      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getResult()
69      */
70     public double getResult() {
71         if (sumOfLogs.getN() > 0) {
72             return Math.exp(sumOfLogs.getResult() / (double) sumOfLogs.getN());
73         } else {
74             return Double.NaN;
75         }
76     }
77
78     /**
79      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#clear()
80      */
81     public void clear() {
82         sumOfLogs.clear();
83     }
84
85     /**
86      * Returns the geometric mean of the entries in the specified portion
87      * of the input array.
88      * <p>
89      * See {@link GeometricMean} for details on the computing algorithm.
90      * <p>
91      * Throws <code>IllegalArgumentException</code> if the array is null.
92      *
93      * @param values input array containing the values
94      * @param begin first array element to include
95      * @param length the number of elements to include
96      * @return the geometric mean or Double.NaN if length = 0 or
97      * any of the values are &lt;= 0.
98      * @throws IllegalArgumentException if the input array is null or the array
99      * index parameters are not valid
100      */
101     public double evaluate(
102         final double[] values, final int begin, final int length) {
103         return Math.exp(
104             sumOfLogs.evaluate(values, begin, length) / (double) length);
105     }
106     
107     /**
108      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getN()
109      */
110     public long getN() {
111         return sumOfLogs.getN();
112     }
113
114 }
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