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Java > Open Source Codes > org > apache > commons > math > stat > descriptive > summary > SumOfLogs


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.summary;
17
18 import java.io.Serializable JavaDoc;
19
20 import org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
21
22 /**
23  * Returns the sum of the natural logs for this collection of values.
24  * <p>
25  * Uses {@link java.lang.Math#log(double)} to compute the logs. Therefore,
26  * <ul>
27  * <li>If any of values are < 0, the result is <code>NaN.</code></li>
28  * <li>If all values are non-negative and less than
29  * <code>Double.POSITIVE_INFINITY</code>, but at least one value is 0, the
30  * result is <code>Double.NEGATIVE_INFINITY.</code></li>
31  * <li>If both <code>Double.POSITIVE_INFINITY</code> and
32  * <code>Double.NEGATIVE_INFINITY</code> are among the values, the result is
33  * <code>NaN.</code></li>
34  * </ul>
35  * <p>
36  * <strong>Note that this implementation is not synchronized.</strong> If
37  * multiple threads access an instance of this class concurrently, and at least
38  * one of the threads invokes the <code>increment()</code> or
39  * <code>clear()</code> method, it must be synchronized externally.
40  *
41  * @version $Revision$ $Date: 2005-02-26 05:11:52 -0800 (Sat, 26 Feb 2005) $
42  */
43 public class SumOfLogs extends AbstractStorelessUnivariateStatistic implements Serializable JavaDoc {
44
45     /** Serializable version identifier */
46     static final long serialVersionUID = -370076995648386763L;
47
48     /**Number of values that have been added */
49     private int n;
50     
51     /**
52      * The currently running value
53      */
54     private double value;
55     
56     /**
57      * Create a SumOfLogs instance
58      */
59     public SumOfLogs() {
60        value = 0d;
61        n = 0;
62     }
63
64     /**
65      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#increment(double)
66      */
67     public void increment(final double d) {
68         value += Math.log(d);
69         n++;
70     }
71
72     /**
73      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getResult()
74      */
75     public double getResult() {
76         if (n > 0) {
77             return value;
78         } else {
79             return Double.NaN;
80         }
81     }
82
83     /**
84      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getN()
85      */
86     public long getN() {
87         return n;
88     }
89     
90     /**
91      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#clear()
92      */
93     public void clear() {
94         value = 0d;
95         n = 0;
96     }
97
98     /**
99      * Returns the sum of the natural logs of the entries in the specified portion of
100      * the input array, or <code>Double.NaN</code> if the designated subarray
101      * is empty.
102      * <p>
103      * Throws <code>IllegalArgumentException</code> if the array is null.
104      * <p>
105      * See {@link SumOfLogs}.
106      *
107      * @param values the input array
108      * @param begin index of the first array element to include
109      * @param length the number of elements to include
110      * @return the sum of the natural logs of the values or Double.NaN if
111      * length = 0
112      * @throws IllegalArgumentException if the array is null or the array index
113      * parameters are not valid
114      */
115     public double evaluate(final double[] values, final int begin, final int length) {
116         double sumLog = Double.NaN;
117         if (test(values, begin, length)) {
118             sumLog = 0.0;
119             for (int i = begin; i < begin + length; i++) {
120                 sumLog += Math.log(values[i]);
121             }
122         }
123         return sumLog;
124     }
125 }
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