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


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 JavaDoc;
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 $Revision$ $Date: 2005-02-26 05:11:52 -0800 (Sat, 26 Feb 2005) $
48  */
49 public class Mean extends AbstractStorelessUnivariateStatistic
50     implements Serializable JavaDoc {
51
52     /** Serializable version identifier */
53     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 }
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