org.apache.commons.math3.util

## Class Combinations

• All Implemented Interfaces:
Iterable<int[]>

public class Combinations
extends Object
implements Iterable<int[]>
Utility to create combinations (n, k) of k elements in a set of n elements.
Since:
3.3
• ### Constructor Summary

Constructors
Constructor and Description
Combinations(int n, int k)
Creates an instance whose range is the k-element subsets of {0, ..., n - 1} represented as int[] arrays.
• ### Method Summary

Methods
Modifier and Type Method and Description
Comparator<int[]> comparator()
Defines a lexicographic ordering of combinations.
int getK()
Gets the number of elements in each combination.
int getN()
Gets the size of the set from which combinations are drawn.
Iterator<int[]> iterator()
• ### Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
• ### Constructor Detail

• #### Combinations

public Combinations(int n,
int k)
Creates an instance whose range is the k-element subsets of {0, ..., n - 1} represented as int[] arrays.

The iteration order is lexicographic: the arrays returned by the iterator are sorted in descending order and they are visited in lexicographic order with significance from right to left. For example, new Combinations(4, 2).iterator() returns an iterator that will generate the following sequence of arrays on successive calls to next():
[0, 1], [0, 2], [1, 2], [0, 3], [1, 3], [2, 3]

If k == 0 an iterator containing an empty array is returned; if k == n an iterator containing [0, ..., n - 1] is returned.
Parameters:
n - Size of the set from which subsets are selected.
k - Size of the subsets to be enumerated.
Throws:
NotPositiveException - if n < 0.
NumberIsTooLargeException - if k > n.
• ### Method Detail

• #### getN

public int getN()
Gets the size of the set from which combinations are drawn.
Returns:
the size of the universe.
• #### getK

public int getK()
Gets the number of elements in each combination.
Returns:
the size of the subsets to be enumerated.
• #### iterator

public Iterator<int[]> iterator()
Specified by:
iterator in interface Iterable<int[]>
• #### comparator

public Comparator<int[]> comparator()
Defines a lexicographic ordering of combinations. The returned comparator allows to compare any two combinations that can be produced by this instance's iterator. Its compare(int[],int[]) method will throw exceptions if passed combinations that are inconsistent with this instance:
• DimensionMismatchException if the array lengths are not equal to k,
• OutOfRangeException if an element of the array is not within the interval [0, n).
Returns:
a lexicographic comparator.