1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.math.stat.descriptive.summary;
18
19 import java.io.Serializable;
20
21 import org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
22
23 /**
24 * Returns the sum of the natural logs for this collection of values.
25 * <p>
26 * Uses {@link java.lang.Math#log(double)} to compute the logs. 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>Double.NEGATIVE_INFINITY.</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></p>
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.</p>
41 *
42 * @version $Revision: 617953 $ $Date: 2008-02-02 22:54:00 -0700 (Sat, 02 Feb 2008) $
43 */
44 public class SumOfLogs extends AbstractStorelessUnivariateStatistic implements Serializable {
45
46 /** Serializable version identifier */
47 private static final long serialVersionUID = -370076995648386763L;
48
49 /**Number of values that have been added */
50 private int n;
51
52 /**
53 * The currently running value
54 */
55 private double value;
56
57 /**
58 * Create a SumOfLogs instance
59 */
60 public SumOfLogs() {
61 value = 0d;
62 n = 0;
63 }
64
65 /**
66 * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#increment(double)
67 */
68 public void increment(final double d) {
69 value += Math.log(d);
70 n++;
71 }
72
73 /**
74 * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getResult()
75 */
76 public double getResult() {
77 if (n > 0) {
78 return value;
79 } else {
80 return Double.NaN;
81 }
82 }
83
84 /**
85 * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getN()
86 */
87 public long getN() {
88 return n;
89 }
90
91 /**
92 * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#clear()
93 */
94 public void clear() {
95 value = 0d;
96 n = 0;
97 }
98
99 /**
100 * Returns the sum of the natural logs of the entries in the specified portion of
101 * the input array, or <code>Double.NaN</code> if the designated subarray
102 * is empty.
103 * <p>
104 * Throws <code>IllegalArgumentException</code> if the array is null.</p>
105 * <p>
106 * See {@link SumOfLogs}.</p>
107 *
108 * @param values the input array
109 * @param begin index of the first array element to include
110 * @param length the number of elements to include
111 * @return the sum of the natural logs of the values or Double.NaN if
112 * length = 0
113 * @throws IllegalArgumentException if the array is null or the array index
114 * parameters are not valid
115 */
116 public double evaluate(final double[] values, final int begin, final int length) {
117 double sumLog = Double.NaN;
118 if (test(values, begin, length)) {
119 sumLog = 0.0;
120 for (int i = begin; i < begin + length; i++) {
121 sumLog += Math.log(values[i]);
122 }
123 }
124 return sumLog;
125 }
126 }