Stats.java

package bsearch.util;

import java.util.Collections;
import java.util.List;

public class Stats {

	/** calculates sum of a list of numbers */
	public static double sum( List<Double> dlist )
	{
		double sum = 0;
		for (double d: dlist)
		{
			sum += d;
		}
		return sum ;
	}

	/** calculates arithmetic mean of a list of numbers */
	public static double mean( List<Double> dlist )
	{
		double sum = 0;
		for (double d: dlist)
		{
			sum += d;
		}
		return sum / dlist.size();
	}

	/** calculates arithmetic mean of a list of (not necessarily sorted) numbers*/
	public static double median(List<Double> dlist)
	{
		List<Double> sortedlist = new java.util.ArrayList<Double>(dlist);
		Collections.sort(sortedlist);
		int len = sortedlist.size(); 
		if (len % 2 == 0) 
		{
			return (sortedlist.get(len / 2 - 1) + sortedlist.get(len / 2)) / 2;
		}
		else
		{
			return sortedlist.get(len / 2);
		}		
	}

	/** calculates sample variance (as opposed to population variances) */ 
	public static double variance( List<Double> dlist )
	{
		double n = dlist.size();
		if (n <= 1)
		{
			return 0;
		}
		double mean = mean(dlist);
		
		double sum = 0;
		double sumsq = 0;
		for (double d: dlist)
		{
			sum += (d - mean);
			sumsq += (d - mean) * (d - mean);
		}
		return (sumsq - sum * sum / n) / (n - 1);
	}
	
	public static double stdev(List<Double> dlist)
	{
		return StrictMath.sqrt(variance(dlist));
	}	
}