snapmobile.rb

array_of_numbers = [1, 2, 3, 4, 8, 9, 10, 12, 14, 18]

def rewrite_array(array)
  answer_string  = ""
  first_of_range = nil

  array.each_with_index do |value, index|
    previous_value = array[index-1]

    # outputs last of range with dash
    if previous_value != array[index]-1 && previous_value != first_of_range
      answer_string += "-#{previous_value}" if index > 0
    end

    # outputs first of range
    if previous_value != value-1
      answer_string += ", " if index > 0
      answer_string += "#{value}" 
      first_of_range = value
    end
  end

  answer_string
end

puts rewrite_array(array_of_numbers) == "1-4, 8-10, 12, 14, 18" # => true