Array#permutation

arr.permutation(n) { |p| block }

Array#permutation generates all possible ordered arrangements of array elements of a given size. This method is essential for solving combinatorial problems, generating test cases, and exploring mathematical relationships in your data.

Syntax

array.permutation(n) { |p| block }

Returns an Enumerator when called without a block, allowing you to chain other Enumerable methods or convert to an array.

Parameters

Parameter	Type	Default	Description
n	Integer	Required	The size of each permutation to generate

Examples

Basic usage

# Generate all permutations of 2 elements
[1, 2, 3].permutation(2).to_a
# => [[1, 2], [1, 3], [2, 1], [2, 3], [3, 1], [3, 2]]

With a block

# Using a block with permutation
[1, 2].permutation(2) do |p|
  puts "Permutation: \#{p.join(, )}"
end
# Permutation: 1, 2
# Permutation: 2, 1

Practical examples

# Find all possible orders for scheduling 3 tasks
tasks = [:design, :code, :test]
schedules = tasks.permutation(3).to_a
# => [[:design, :code, :test], [:design, :test, :code],
#     [:code, :design, :test], [:code, :test, :design],
#     [:test, :design, :code], [:test, :code, :design]]

Common Patterns

Generating all possible orderings

letters = [a, b, c]
letters.permutation(2).to_a
# => [["a", "b"], ["a", "c"], ["b", "a"], ["b", "c"], ["c", "a"], ["c", "b"]]

Finding unique arrangements

# Get all 3-letter arrangements from a set of letters
[A, B, C, D].permutation(3).to_a
# Returns 24 possible ordered arrangements

Error Cases

# Negative size raises ArgumentError
[1, 2, 3].permutation(-1)
# => ArgumentError: negative permutation size (-1)

# Size greater than array length returns empty enumerator
[1, 2].permutation(5).to_a
# => []

# Size equal to zero returns single empty array
[1, 2, 3].permutation(0).to_a
# => [[]]

See Also

• Array#combination
• Array#each_slice
• Array#pop