README.md

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A combinatorics library for Java.

3.4.0 - the latest stable release version

The latest release of the library is v3.4.0. It is available through The Maven Central Repository here. Add the following section into your pom.xml file.

<dependency>
    <groupId>com.github.dpaukov</groupId>
    <artifactId>combinatoricslib3</artifactId>
    <version>3.4.0</version>
</dependency>

Examples

You can check out an example project to see how to use the library combinatoricslib3-example

3.4.1 - current version under development

1. Simple combinations
2. Combinations with repetitions
3. Simple permutations
4. Permutations with repetitions
5. k-Permutations
6. Subsets
7. Integer partitions
8. Cartesian product

Description	Is Order Important?	Is Repetition Allowed?	Stream
Simple combinations	No	No	Generator.combination(...).simple(n).stream()
Combinations with repetitions	No	Yes	Generator.combination(...).multi(n).stream()
Simple permutations	Yes	No	Generator.permutation(...).simple().stream()
Permutations with repetitions	Yes	Yes	Generator.permutation(...).withRepetitions(n).stream()

1. Simple combinations

A simple k-combination of a finite set S is a subset of k distinct elements of S. Specifying a subset does not arrange them in a particular order. As an example, a poker hand can be described as a 5-combination of cards from a 52-card deck: the 5 cards of the hand are all distinct, and the order of the cards in the hand does not matter.

Let's generate all 3-combination of the set of 5 colors (red, black, white, green, blue).

   Generator.combination("red", "black", "white", "green", "blue")
       .simple(3)
       .stream()
       .forEach(System.out::println);

And the result of 10 combinations

   [red, black, white]
   [red, black, green]
   [red, black, blue]
   [red, white, green]
   [red, white, blue]
   [red, green, blue]
   [black, white, green]
   [black, white, blue]
   [black, green, blue]
   [white, green, blue]

2. Combinations with repetitions

A k-multicombination or k-combination with repetition of a finite set S is given by a sequence of k not necessarily distinct elements of S, where order is not taken into account.

As an example. Suppose there are 2 types of fruits (apple and orange) at a grocery store, and you want to buy 3 pieces of fruit. You could select

• (apple, apple, apple)
• (apple, apple, orange)
• (apple, orange, orange)
• (orange, orange, orange)

   Generator.combination("apple", "orange")
       .multi(3)
       .stream()
       .forEach(System.out::println);

And the result will be:

   [apple, apple, apple]
   [apple, apple, orange]
   [apple, orange, orange]
   [orange, orange, orange]

3. Simple permutations

A permutation is an ordering of a set in the context of all possible orderings. For example, the set containing the first three digits, 123, has six permutations: 123, 132, 213, 231, 312, and 321.

This is an example of the permutations of the 3 string items (apple, orange, cherry):

   Generator.permutation("apple", "orange", "cherry")
       .simple()
       .stream()
       .forEach(System.out::println);

   [apple, orange, cherry]
   [apple, cherry, orange]
   [cherry, apple, orange]
   [cherry, orange, apple]
   [orange, cherry, apple]
   [orange, apple, cherry]

This generator can produce the permutations even if an initial vector has duplicates. For example, all permutations of (1, 1, 2, 2):

   Generator.permutation(1, 1, 2, 2)
       .simple()
       .stream()
       .forEach(System.out::println);

The result does not have duplicates. All permutations are distinct by default.

   [1, 1, 2, 2]
   [1, 2, 1, 2]
   [1, 2, 2, 1]
   [2, 1, 1, 2]
   [2, 1, 2, 1]
   [2, 2, 1, 1]

Notice that we have 6 permutations here instead of 24. If you still need all permutations, you should call method simple(PermutationGenerator.TreatDuplicatesAs.IDENTICAL).

4. Permutations with repetitions

Permutation may have more elements than slots. For example, all possible permutation of 12 in three slots are: 111, 211, 121, 221, 112, 212, 122, and 222.

Let's generate all possible permutations with repetitions of 3 elements from the set of apple and orange.

   Generator.permutation("apple", "orange")
        .withRepetitions(3)
        .stream()
        .forEach(System.out::println);

And the list of all 8 permutations

   [apple, apple, apple]
   [orange, apple, apple]
   [apple, orange, apple]
   [orange, orange, apple]
   [apple, apple, orange]
   [orange, apple, orange]
   [apple, orange, orange]
   [orange, orange, orange]

5. k-Permutations

You can generate k-permutations with and without repetitions using the combination and permutation generators together. For example, 2-permutations without repetitions of the list (1, 2, 3):

    Generator.combination(1, 2, 3)
       .simple(2)
       .stream()
       .forEach(combination -> Generator.permutation(combination)
          .simple()
          .forEach(System.out::println));

This will print six 2-permutations of (1, 2, 3):

   [1, 2]
   [2, 1]
   [1, 3]
   [3, 1]
   [2, 3]
   [3, 2]

Similarly, you can get 2-Permutations with repetitions of the list (1, 2, 3):

    Generator.combination(1, 2, 3)
       .multi(2)
       .stream()
       .forEach(combination -> Generator.permutation(combination)
           .simple()
           .forEach(System.out::println));

This will print all nine 2-permutations of (1, 2, 3):

   [1, 1]
   [1, 2]
   [2, 1]
   [1, 3]
   [3, 1]
   [2, 2]
   [2, 3]
   [3, 2]
   [3, 3]

6. Subsets

A set A is a subset of a set B if A is "contained" inside B. A and B may coincide. The relationship of one set being a subset of another is called inclusion or sometimes containment.

Examples:

The set (1, 2) is a proper subset of (1, 2, 3). Any set is a subset of itself, but not a proper subset. The empty set, denoted by ∅, is also a subset of any given set X. All subsets of (1, 2, 3) are:

• ()
• (1)
• (2)
• (1, 2)
• (3)
• (1, 3)
• (2, 3)
• (1, 2, 3)

Here is a piece of code that generates all possible subsets of (one, two, three)

   Generator.subset("one", "two", "three")
        .simple()
        .stream()
        .forEach(System.out::println);

And the list of all 8 subsets

   []
   [one]
   [two]
   [one, two]
   [three]
   [one, three]
   [two, three]
   [one, two, three]

7. Integer Partitions

In number theory, a partition of a positive integer n is a way of writing n as a sum of positive integers. Two sums that differ only in the order of their summands are considered to be the same partition; if order matters then the sum becomes a composition. A summand in a partition is also called a part.

The partitions of 5 are listed below:

• 1 + 1 + 1 + 1 + 1
• 2 + 1 + 1 + 1
• 2 + 2 + 1
• 3 + 1 + 1
• 3 + 2
• 4 + 1
• 5

Let's generate all possible partitions of 5:

   Generator.partition(5)
       .stream()
       .forEach(System.out::println);

And the result of all 7 integer possible partitions:

   [1, 1, 1, 1, 1]
   [2, 1, 1, 1]
   [2, 2, 1]
   [3, 1, 1]
   [3, 2]
   [4, 1]
   [5]

8. Cartesian Product

In set theory, a Cartesian Product A × B is the set of all ordered pairs (a, b) where a ∈ A and b ∈ B.

As an example, suppose there are 2 sets of number, (1, 2, 3) and (4, 5, 6), and you want to get the Cartesian product of the two sets.

Source: Cartesian Product

       Generator.cartesianProduct(Arrays.asList(1, 2, 3), Arrays.asList(4, 5, 6))
           .stream()
           .forEach(System.out::println);

And the result will be:

   [1, 4]
   [1, 5]
   [1, 6]
   [2, 4]
   [2, 5]
   [2, 6]
   [3, 4]
   [3, 5]
   [3, 6]