To convert number to words in ruby (for any positive integer)

numbersToWords.rb

def toWords(num)
  units = ['Zero', 'One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine']
  tens = ['', '', 'Twenty', 'Thirty', 'Fourty', 'Fifty', 'Sixty', 'Seventy', 'Eighty', 'Ninety']
  teens = ['Ten', 'Eleven', 'Tweleve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen',
                   'Seventeen', 'Eighteen', 'Nineteen']

  units_digit = num % 10
  tens_digit  = num / 10

  number_in_words = ""

  # Add more powers of ten as needed, otherwise results will come as seven million billion 
  # (which is technically correct, but most likely not desired)      
  powers_of_ten = { 10**9 => 'Billion',
                    10**6 => 'Million',
                    10**3 => 'Thousand',
                    10**2 => 'Hundred' }
  if num >= 100
    powers_of_ten.each { |power_of_ten, word|
      if (num/power_of_ten > 0)
         number_in_words += ' ' + toWords(num/power_of_ten) + ' ' + word
         if units_digit != 0
           number_in_words += ' ' + toWords(num % power_of_ten)
         end
         break
      end
    }
  elsif num >= 20
    number_in_words += tens[tens_digit]
    if units_digit != 0
        number_in_words += ' ' + units[units_digit]
    end
  elsif num >= 10
    number_in_words += ' ' + teens[units_digit]
  else
    number_in_words += ' ' + units[units_digit]
  end

  return number_in_words.strip
end

# Now let's test
examples = [43, 53, 23, 20, 60, 42, 18, 15, 10, 9, 0, 99, 153, 100, 1000, 1052, 1729, 1000432231, 45$
examples.each {|num| puts "#{num} => #{toWords(num)}"}


# Output
# 43 => Fourty Three
# 53 => Fifty Three
# 23 => Twenty Three
# 20 => Twenty
# 60 => Sixty
# 42 => Fourty Two
# 18 => Eighteen
# 15 => Fifteen
# 10 => Ten
# 9 => Nine
# 0 => Zero
# 99 => Ninety Nine
# 153 => One Hundred Fifty Three
# 100 => One Hundred
# 1000 => One Thousand
# 1052 => One Thousand Fifty Two
# 1729 => One Thousand Seven Hundred Twenty Nine
# 1000432231 => One Billion Four Hundred Thirty Two Thousand Two Hundred Thirty One
# 45345890345823345 => Fourty Five Million Billion Three Hundred Fourty Five Million Eight Hundred Twenty Three Thousand Three Hundred Fourty Five