Recursively flatten an array without using built-in methods.

flatten_array.rb

#!/bin/ruby

require 'test/unit'
require 'test/unit/ui/console/testrunner'

class Array

  # Flatten the Array of nested arrays recursively
  # without using any pre-written methods.
  def flatten_array(result = [])
    self.each do |element|
      if element.instance_of?( Array )
        element.flatten_array(result)
      else
        result.push(element)
      end
    end
    result
  end
  
end

class ArrayTest < Test::Unit::TestCase

  # Main test case
  def test_array
    array = [[1,2,[3]],4]
    expected_result = [1,2,3,4]
    actual_result = array.flatten_array

    puts "Result: #{actual_result}"
    
    assert_equal expected_result, actual_result, 'Arrays are not equal!'
  end

  def test_array_individual_nesting
    array = [[[1],[2],[3]],[4]]
    expected_result = [1,2,3,4]
    actual_result = array.flatten_array

    puts "Result: #{actual_result}"
    
    assert_equal expected_result, actual_result, 'Arrays are not equal!'
  end

  def test_array_multiple_nesting
    array = [[[1],[[[2]]],[3]],[4]]
    expected_result = [1,2,3,4]
    actual_result = array.flatten_array

    puts "Result: #{actual_result}"
    
    assert_equal expected_result, actual_result, 'Arrays are not equal!'
  end

end

# Run it...
my_tests = Test::Unit::TestSuite.new('Array tests')
my_tests << ArrayTest.new('test_array')
my_tests << ArrayTest.new('test_array_individual_nesting')
my_tests << ArrayTest.new('test_array_multiple_nesting')

Test::Unit::UI::Console::TestRunner.run(my_tests)