Accumulator.java

package edu.northwestern.at.utils.math.statistics;

/*	Please see the license information at the end of this file. */

import java.util.*;

/**	Accumulate basic statistical information for a set of doubles.
 *
 *	<p>
 *	Accumulates the following quantities.
 *	</p>
 *
 *	<ul>
 *	<li>Count</li>
 *	<li>Maximum</li>
 *	<li>Minimum</li>
 *	<li>Mean</li>
 *	<li>Sum of squares</li>
 *	</ul>
 *
 *	<p>
 *	Returns these values as well as the variance and standard deviation.
 *	</p>
 */

public class Accumulator
{
	/**	Count of elements. */

	protected long count		= 0;

	/**	Mean of elements. */

	protected double mean		= 0.0D;

    /**	Sum of squares of elements. */

	protected double sumOfSquares	= 0.0D;

	/**	Minimum of elements. */

	protected double minimum	= Double.POSITIVE_INFINITY;

    /**	Maximum of elements. */

	protected double maximum	= Double.NEGATIVE_INFINITY;

	/**	Create an empty accumulator.
	 */

	public Accumulator()
	{
	}

	/**	Add a value to the accumulation.
	 *
	 *	@param	value	The value to add.
	 */

	public void addValue( double value )
	{
								//	Increment count of values.
		count++;

		double prevMean = mean;

								//	Incrementally update
								//	mean and sum of squares.

		mean 			+= ( value - mean ) / (double)count;
		sumOfSquares	+=
			( value - mean ) * ( value - prevMean );

								//	Update minimum and maximum.

		if ( minimum > value ) minimum = value;
		if ( maximum < value ) maximum = value;
	}

	/**	Add collection of values to the accumulation.
	 *
	 *	@param	values	The collection of values to add.
	 */

	public void addValues( Collection<Double> values )
	{
		if ( values != null )
		{
			Iterator<Double> iterator	= values.iterator();

			while ( iterator.hasNext() )
			{
				double value	= iterator.next();
				addValue( value );
			}
		}
	}

	/**	Add array of values to the accumulation.
	 *
	 *	@param	values	The collection of values to add.
	 */

	public void addValues( double[] values )
	{
		if ( values != null )
		{
			for ( int i = 0 ; i < values.length ; i++ )
			{
				addValue( values[ i ] );
			}
		}
	}

	/**	Return count.
	 *
	 *	@return		Count of elements as a long.
	 */

	public long getCount()
	{
		return count;
	}

	/**	Return maximum.
	 *
	 *	@return		Maximum of elements.
	 */

	public double getMaximum()
	{
		return maximum;
	}

	/**	Return minimum.
	 *
	 *	@return		Minimum of elements.
	 */

	public double getMinimum()
	{
		return minimum;
	}

	/**	Return mean.
	 *
	 *	@return		Mean of elements.
	 */

	public double getMean()
	{
		return mean;
	}

	/**	Return variance.
	 *
	 *	@return		Variance of elements.
	 *
	 *	<p>
	 *	The variance is the sum of squares divided by the
	 *	number of values.
	 *	</p>
	 */

	public double getVariance()
	{
		return sumOfSquares / (double)count;
	}

	/**	Return standard deviation.
	 *
	 *	@return		Standard deviation of elements.
	 *
	 *	<p>
	 *	The standard deviation is the square root of the variance.
	 *	</p>
	 */

	public double getStandardDeviation()
	{
		return Math.sqrt( getVariance() );
	}

	/**	Return sum of squares.
	 *
	 *	@return		Sum of squares of elements.
	 */

	public double getSumOfSquares()
	{
		return sumOfSquares;
	}
}

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