Ruby Language Arrays Get all combinations / permutations of an array
Example
The permutation method, when called with a block yields a two dimensional array consisting of all ordered sequences of a collection of numbers.
If this method is called without a block, it will return an enumerator. To convert to an array, call the to_a method.
| Example | Result |
|---|---|
[1,2,3].permutation | #<Enumerator: [1,2,3]:permutation |
[1,2,3].permutation.to_a | [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]] |
[1,2,3].permutation(2).to_a | [[1,2],[1,3],[2,1],[2,3],[3,1],[3,2]] |
[1,2,3].permutation(4).to_a | [] -> No permutations of length 4 |
The combination method on the other hand, when called with a block yields a two-dimensional array consisting of all sequences of a collection of numbers. Unlike permutation, order is disregarded in combinations. For example, [1,2,3] is the same as [3,2,1]
| Example | Result |
|---|---|
[1,2,3].combination(1) | #<Enumerator: [1,2,3]:combination |
[1,2,3].combination(1).to_a | [[1],[2],[3]] |
[1,2,3].combination(3).to_a | [[1,2,3]] |
[1,2,3].combination(4).to_a | [] -> No combinations of length 4 |
Calling the combination method by itself will result in an enumerator. To get an array, call the to_a method.
The repeated_combination and repeated_permutation methods are similar, except the same element can be repeated multiple times.
For example the sequences [1,1], [1,3,3,1], [3,3,3] would not be valid in regular combinations and permutations.
| Example | # Combos |
|---|---|
[1,2,3].combination(3).to_a.length | 1 |
[1,2,3].repeated_combination(3).to_a.length | 6 |
[1,2,3,4,5].combination(5).to_a.length | 1 |
[1,2,3].repeated_combination(5).to_a.length | 126 |
