001/*-------------------------------------------------------------------------+
002|                                                                          |
003| Copyright 2005-2011 The ConQAT Project                                   |
004|                                                                          |
005| Licensed under the Apache License, Version 2.0 (the "License");          |
006| you may not use this file except in compliance with the License.         |
007| You may obtain a copy of the License at                                  |
008|                                                                          |
009|    http://www.apache.org/licenses/LICENSE-2.0                            |
010|                                                                          |
011| Unless required by applicable law or agreed to in writing, software      |
012| distributed under the License is distributed on an "AS IS" BASIS,        |
013| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
014| See the License for the specific language governing permissions and      |
015| limitations under the License.                                           |
016+-------------------------------------------------------------------------*/
017package org.conqat.lib.commons.math;
018
019import java.util.Collection;
020
021/**
022 * Calculates the standard deviation. This calculation is based on
023 * {@link VarianceAggregator}.
024 */
025public class StdDevAggregator implements IAggregator {
026
027        /** The variance aggregator used to calculate the standard deviation. */
028        private final VarianceAggregator varianceAggregator;
029
030        /**
031         * Constructor.
032         * 
033         * @param sample
034         *            if true sample standard deviation is calculated, population
035         *            standard deviation otherwise.
036         */
037        public StdDevAggregator(boolean sample) {
038                varianceAggregator = new VarianceAggregator(sample);
039        }
040
041        /**
042         * Aggregates by finding the average value
043         * 
044         * @return {@link Double#NaN} for empty input collection
045         */
046        @Override
047        public double aggregate(Collection<? extends Number> values) {
048                if (values.isEmpty()) {
049                        return Double.NaN;
050                }
051                double variance = varianceAggregator.aggregate(values);
052                return Math.sqrt(variance);
053        }
054
055        /** Returns {@link Double#NaN}. */
056        @Override
057        public double getNeutralElement() {
058                return Double.NaN;
059        }
060}