Package org.apache.commons.numbers.combinatorics

Class Combinations

java.lang.Object

org.apache.commons.numbers.combinatorics.Combinations

All Implemented Interfaces:

java.lang.Iterable<int[]>

public final class Combinations
extends java.lang.Object
implements java.lang.Iterable<int[]>

Utility to create combinations (n, k) of k elements in a set of n elements.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
private static class	Combinations.LexicographicComparator	Defines a lexicographic ordering of the combinations.
private static class	Combinations.LexicographicIterator	Lexicographic combinations iterator.
private static class	Combinations.SingletonIterator	Iterator with just one element to handle degenerate cases (full array, empty array) for combination iterator.

Field Summary

Fields

Modifier and Type	Field	Description
private int	k	Number of elements in each combination.
private int	n	Size of the set from which combinations are drawn.

Constructor Summary

Constructors

Modifier	Constructor	Description
private	Combinations(int n, int k)

Method Summary

Modifier and Type	Method	Description
java.util.Comparator<int[]>	comparator()	Creates a comparator.
int	getK()	Gets the number of elements in each combination.
int	getN()	Gets the size of the set from which combinations are drawn.
java.util.Iterator<int[]>	iterator()	Creates an iterator whose range is the k-element subsets of {0, ..., n - 1} represented as int[] arrays.
static Combinations	of(int n, int k)	Create an instance.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.lang.Iterable

forEach, spliterator

Field Detail

n

private final int n

Size of the set from which combinations are drawn.

k

private final int k

Number of elements in each combination.

Constructor Detail

Combinations

private Combinations(int n,
                     int k)

Parameters:

n - Size of the set from which subsets are selected.

k - Size of the subsets to be enumerated.

Throws:

java.lang.IllegalArgumentException - if n < 0.

java.lang.IllegalArgumentException - if k > n or k < 0.

Method Detail

of

public static Combinations of(int n,
                              int k)

Create an instance.

Parameters:

n - Size of the set from which subsets are selected.

k - Size of the subsets to be enumerated.

Returns:

a new instance.

Throws:

java.lang.IllegalArgumentException - if n < 0.

java.lang.IllegalArgumentException - if k > n or k < 0.

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 java.util.Iterator<int[]> iterator()

Creates an iterator 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.

Specified by:

iterator in interface java.lang.Iterable<int[]>

comparator

public java.util.Comparator<int[]> comparator()

Creates a comparator. When performing a comparison, if an element of the array is not within the interval [0, n), an IllegalArgumentException will be thrown.

Returns:

a comparator.