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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.math.stat.descriptive.moment;
import java.io.Serializable;
/**
* Computes a statistic related to the Second Central Moment. Specifically,
* what is computed is the sum of squared deviations from the sample mean.
* <p>
* The following recursive updating formula is used:</p>
* <p>
* Let <ul>
* <li> dev = (current obs - previous mean) </li>
* <li> n = number of observations (including current obs) </li>
* </ul>
* Then</p>
* <p>
* new value = old value + dev^2 * (n -1) / n.</p>
* <p>
* Returns <code>Double.NaN</code> if no data values have been added and
* returns <code>0</code> if there is just one value in the data set.</p>
* <p>
* <strong>Note that this implementation is not synchronized.</strong> If
* multiple threads access an instance of this class concurrently, and at least
* one of the threads invokes the <code>increment()</code> or
* <code>clear()</code> method, it must be synchronized externally.</p>
*
* @version $Revision: 811685 $ $Date: 2009-09-05 19:36:48 +0200 (sam. 05 sept. 2009) $
*/
public class SecondMoment extends FirstMoment implements Serializable {
/** Serializable version identifier */
private static final long serialVersionUID = 3942403127395076445L;
/** second moment of values that have been added */
protected double m2;
/**
* Create a SecondMoment instance
*/
public SecondMoment() {
super();
m2 = Double.NaN;
}
/**
* Copy constructor, creates a new {@code SecondMoment} identical
* to the {@code original}
*
* @param original the {@code SecondMoment} instance to copy
*/
public SecondMoment(SecondMoment original) {
super(original);
this.m2 = original.m2;
}
/**
* {@inheritDoc}
*/
@Override
public void increment(final double d) {
if (n < 1) {
m1 = m2 = 0.0;
}
super.increment(d);
m2 += ((double) n - 1) * dev * nDev;
}
/**
* {@inheritDoc}
*/
@Override
public void clear() {
super.clear();
m2 = Double.NaN;
}
/**
* {@inheritDoc}
*/
@Override
public double getResult() {
return m2;
}
/**
* {@inheritDoc}
*/
@Override
public SecondMoment copy() {
SecondMoment result = new SecondMoment();
copy(this, result);
return result;
}
/**
* Copies source to dest.
* <p>Neither source nor dest can be null.</p>
*
* @param source SecondMoment to copy
* @param dest SecondMoment to copy to
* @throws NullPointerException if either source or dest is null
*/
public static void copy(SecondMoment source, SecondMoment dest) {
FirstMoment.copy(source, dest);
dest.m2 = source.m2;
}
}
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