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.moment;
18  
19  import junit.framework.Test;
20  import junit.framework.TestSuite;
21  
22  import org.apache.commons.math.stat.descriptive.StorelessUnivariateStatisticAbstractTest;
23  import org.apache.commons.math.stat.descriptive.UnivariateStatistic;
24  
25  /**
26   * Test cases for the {@link UnivariateStatistic} class.
27   * 
28   * @version $Revision: 762118 $ $Date: 2009-04-05 12:55:59 -0400 (Sun, 05 Apr 2009) $
29   */
30  public class StandardDeviationTest extends StorelessUnivariateStatisticAbstractTest{
31  
32      protected StandardDeviation stat;
33      
34      /**
35       * @param name
36       */
37      public StandardDeviationTest(String name) {
38          super(name);
39      }
40  
41      /**
42       * {@inheritDoc}
43       */
44      @Override
45      public UnivariateStatistic getUnivariateStatistic() {
46          return new StandardDeviation();
47      }
48  
49      public static Test suite() {
50          TestSuite suite = new TestSuite(StandardDeviationTest.class);
51          suite.setName("StandardDeviation Tests");
52          return suite;
53      }
54      
55      /**
56       * {@inheritDoc}
57       */
58      @Override
59      public double expectedValue() {
60          return this.std;
61      }
62      
63      /**
64       * Make sure Double.NaN is returned iff n = 0
65       *
66       */
67      public void testNaN() {
68          StandardDeviation std = new StandardDeviation();
69          assertTrue(Double.isNaN(std.getResult()));
70          std.increment(1d);
71          assertEquals(0d, std.getResult(), 0);
72      }
73      
74      /**
75       * Test population version of variance
76       */ 
77      public void testPopulation() {
78          double[] values = {-1.0d, 3.1d, 4.0d, -2.1d, 22d, 11.7d, 3d, 14d};
79          double sigma = populationStandardDeviation(values);
80          SecondMoment m = new SecondMoment();
81          m.evaluate(values);  // side effect is to add values
82          StandardDeviation s1 = new StandardDeviation();
83          s1.setBiasCorrected(false);
84          assertEquals(sigma, s1.evaluate(values), 1E-14);
85          s1.incrementAll(values);
86          assertEquals(sigma, s1.getResult(), 1E-14);
87          s1 = new StandardDeviation(false, m);
88          assertEquals(sigma, s1.getResult(), 1E-14);     
89          s1 = new StandardDeviation(false);
90          assertEquals(sigma, s1.evaluate(values), 1E-14);
91          s1.incrementAll(values);
92          assertEquals(sigma, s1.getResult(), 1E-14);     
93      }
94      
95      /**
96       * Definitional formula for population standard deviation
97       */
98      protected double populationStandardDeviation(double[] v) {
99          double mean = new Mean().evaluate(v);
100         double sum = 0;
101         for (int i = 0; i < v.length; i++) {
102             sum += (v[i] - mean) * (v[i] - mean); 
103         }
104         return Math.sqrt(sum / v.length);
105     }
106 
107 }