Statistical Calculations

Question

Is there a built-in library that I can use to calculate the median in java ??

I worked with apache.commons.math for other statistical functions, but no median was found anywhere.

Thanks,

Answer 1

What version of Apache Commmons Math are you using? There is a middle class, at least starting with version 2.1 (an older version, I'm not sure). You can use it like:

Median median = new Median(); median.evaluate(values); 

Answer 2

Put all the numbers in the list, sort the list and take the average (or the average of two average values ​​for an even size). No calculations required

Answer 3

Try the Median object.

Answer 4

Get values ​​in a List object. Suppose the values ​​are integers and the list is called "values". Then

 List<Integer> values; ... populate values ... Collections.sort(values); int median; int midpoint=values.size()/2; if (values.size()%2==1) median=values.get(midpoint+1).intValue(); else median=(values.get(midpoint).intValue()+values.get(midpoint+1).intValue())/2; 

If the number of values ​​is large, like hundreds or more, messing around with mod-2 may be technically correct, but redundant.

Maybe there is a more efficient way to do this - sorting is pretty slow on a large list, but it will work.

Oh, and you really need to check the list with null entries. Perhaps I am missing other boundary conditions.

Answer 5

From the “too much time on my hands” department: here is a little MedianGenerator class:

 /** * Methods to calculate the median value of a supplied {@link List}. */ public final class MedianGenerator{ private MedianGenerator(){ } /** * Calculate the median of a supplied list. * <ol> * <li>A copy will be generated</li> * <li>this copy will be sorted with the supplied comparator</li> * <li>the median will be calculated, using the supplied averageCalculator * for collections with an even number of items</li> * </ol> * * @param data * @param comparator * @param averageCalculator * @return the median */ public static <T> T calculateMedian(final List<T> data, final Comparator<? super T> comparator, final AverageCalculator<T> averageCalculator){ final List<T> copy = new ArrayList<T>(data); Collections.sort(copy, comparator); return doCalculateMedian(data, averageCalculator); } /** * Calculate the median of a supplied list. * <ol> * <li>A copy will be generated</li> * <li>this copy will be sorted with the supplied comparator</li> * <li>the median will be calculated, using the {@link #ALWAYS_FIRST} {@link AverageCalculator} * for collections with an even number of items</li> * </ol> * * @param data * @param comparator * @return the median */ @SuppressWarnings("unchecked") public static <T> T calculateMedian(final List<T> data, final Comparator<? super T> comparator){ return calculateMedian(data, comparator, (AverageCalculator<T>) ALWAYS_FIRST); } /** * Calculate the median of a supplied list. * <ol> * <li>A copy will be generated</li> * <li>this copy will be sorted using natural ordering</li> * <li>the median will be calculated, using the {@link #ALWAYS_FIRST} {@link AverageCalculator} * for collections with an even number of items</li> * </ol> * * @param data * @return the median */ @SuppressWarnings("unchecked") public static <T extends Comparable<? super T>> T calculateMedian(final List<T> data){ return calculateMedian(data, (AverageCalculator<T>) ALWAYS_FIRST); } /** * Calculate the median of a supplied list. * <ol> * <li>A copy will be generated</li> * <li>this copy will be sorted using natural ordering</li> * <li>the median will be calculated, using the supplied averageCalculator * for collections with an even number of items</li> * </ol> * * @param data * @param averageCalculator * @return the median */ public static <T extends Comparable<? super T>> T calculateMedian(final List<T> data, final AverageCalculator<T> averageCalculator){ final List<T> copy = new ArrayList<T>(data); Collections.sort(copy); return doCalculateMedian(copy, averageCalculator); } private static <T> T doCalculateMedian(final List<T> sortedData, final AverageCalculator<T> averageCalculator){ T result; if(sortedData.isEmpty()){ result = null; } else{ final int size = sortedData.size(); if(size % 2 == 0){ result = averageCalculator.getAverage(sortedData.get(size / 2 - 1), sortedData.get(size / 2)); } else{ result = sortedData.get(size / 2 - 1); } } return result; } /** * Generic accessor method for {@link #ALWAYS_FIRST}. */ @SuppressWarnings("unchecked") public static <T> AverageCalculator<T> alwaysFirst(){ return ALWAYS_FIRST; } /** * {@link AverageCalculator} implementation that always returns the lower * bound unchanged. */ @SuppressWarnings("rawtypes") public static final AverageCalculator ALWAYS_FIRST = new AverageCalculator(){ @Override public Object getAverage(final Object first, final Object second){ return first; } }; /** * When there is an even number of items, this interface is used to generate * the average between the two middle items. */ public static interface AverageCalculator<E> { E getAverage(E first, E second); } }