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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 java.io.Serializable;
19  
20  import org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
21  import org.apache.commons.math.stat.descriptive.summary.Sum;
22  
23  /**
24   * Returns the arithmetic mean of the available values. Uses the definitional 
25   * formula:
26   * <p>
27   * mean = sum(x_i) / n
28   * <p>
29   * where <code>n</code> is the number of observations.
30   * <p>
31   * The value of the statistic is computed using the following recursive
32   * updating algorithm:
33   * <p>
34   * <ol>
35   * <li>Initialize <code>m = </code> the first value</li>
36   * <li>For each additional value, update using <br>
37   *   <code>m = m + (new value - m) / (number of observations)</code></li>
38   * </ol>
39   * <p>
40   *  Returns <code>Double.NaN</code> if the dataset is empty.
41   * <p>
42   * <strong>Note that this implementation is not synchronized.</strong> If 
43   * multiple threads access an instance of this class concurrently, and at least
44   * one of the threads invokes the <code>increment()</code> or 
45   * <code>clear()</code> method, it must be synchronized externally.
46   * 
47   * @version $Revision: 348519 $ $Date: 2005-11-23 12:12:18 -0700 (Wed, 23 Nov 2005) $
48   */
49  public class Mean extends AbstractStorelessUnivariateStatistic 
50      implements Serializable {
51  
52      /** Serializable version identifier */
53      private static final long serialVersionUID = -1296043746617791564L;    
54      
55      /** First moment on which this statistic is based. */
56      protected FirstMoment moment;
57  
58      /** 
59       * Determines whether or not this statistic can be incremented or cleared.
60       * <p>
61       * Statistics based on (constructed from) external moments cannot
62       * be incremented or cleared.
63       */
64      protected boolean incMoment;
65  
66      /** Constructs a Mean. */
67      public Mean() {
68          incMoment = true;
69          moment = new FirstMoment();
70      }
71  
72      /**
73       * Constructs a Mean with an External Moment.
74       * 
75       * @param m1 the moment
76       */
77      public Mean(final FirstMoment m1) {
78          this.moment = m1;
79          incMoment = false;
80      }
81  
82      /**
83       * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#increment(double)
84       */
85      public void increment(final double d) {
86          if (incMoment) {
87              moment.increment(d);
88          }
89      }
90  
91      /**
92       * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#clear()
93       */
94      public void clear() {
95          if (incMoment) {
96              moment.clear();
97          }
98      }
99  
100     /**
101      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getResult()
102      */
103     public double getResult() {
104         return moment.m1;
105     }
106 
107     /**
108      * @see org.apache.commons.math.stat.descriptive.StorelessUnivariateStatistic#getN()
109      */
110     public long getN() {
111         return moment.getN();
112     }
113 
114     /**
115      * Returns the arithmetic mean of the entries in the specified portion of
116      * the input array, or <code>Double.NaN</code> if the designated subarray
117      * is empty.
118      * <p>
119      * Throws <code>IllegalArgumentException</code> if the array is null.
120      * <p>
121      * See {@link Mean} for details on the computing algorithm.
122      * 
123      * @param values the input array
124      * @param begin index of the first array element to include
125      * @param length the number of elements to include
126      * @return the mean of the values or Double.NaN if length = 0
127      * @throws IllegalArgumentException if the array is null or the array index
128      *  parameters are not valid
129      */
130     public double evaluate(final double[] values,final int begin, final int length) {
131         if (test(values, begin, length)) {
132             Sum sum = new Sum();
133             return sum.evaluate(values, begin, length) / ((double) length);
134         }
135         return Double.NaN;
136     }
137 }