Example of flattening an Array.

flatten.rb

module ArrayHelper
  
  # Returns a new Array which is the result 
  # flattening any nested arrays.
  #
  # @param [Array] the array to flatten
  # @param [Array] the array to append results to.
  #
  # @return [Array] the flattened array.
  #
  # @raise [ArgumentError] if the array to flatten is `nil`.
  #
  # @example
  #   flatten([[1,2,[3]],4]) #=> [1,2,3,4]
  #
  def flatten(a, result=[])
    raise ArgumentError, 'Array cannot be nil' if a.nil?

    a.each do |item|
      if item.is_a? Array
        flatten(item, result)
      else
        result << item
      end
    end
    result
  end
end

require 'minitest/autorun'

class FlattenTest < MiniTest::Unit::TestCase

  include ArrayHelper

  def test_nil
    assert_raises ArgumentError do
      flatten(nil)
    end
  end

  def test_empty_array
    assert_equal [], flatten([])
  end

  def test_already_flat_array
    assert_equal [1, 2, 3], flatten([1, 2, 3])
  end

  def test_single_nested_array
    assert_equal [1, 2, 3, 4], flatten([1, [2, 3], 4])
  end

  def test_multiple_single_nested_arrays
    assert_equal [1, 2, 3, 4], flatten([[1], [2, 3], 4])
  end

  def test_multi_level_nested_array
    assert_equal [1, 2, 3, 4], flatten([[1,2,[3]],4])
  end
end