org.apache.commons.math3.util

## Class Combinations

- java.lang.Object
- - org.apache.commons.math3.util.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.

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