The english number problem from Learn to Program by Chris Pine.

englishnum.rb

# This program should take a number, like 22, and return the English version of it 
#(in this case, the string 'twenty-two'). 
#For now, let’s have it work only on integers from 0 to 99999:

puts "Provide a number greater than or equal to 0 but less than 100,000"
num = gets.chomp.to_i
def englishnum number
  if number < 0 || number > 99999
    puts "Please enter a number greater than or equal to 0 but less than 100,000"
  end
  if number == 0
    puts "Zero"
  end
  
  number_string = ''
  ones_places = %w[one two three four five six seven eight nine]
  teens_places = %w[eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen nineteen]
  tens_places = %w[ten twenty thirty fourty fifty sixty seventy eighty ninety]
  
  #THOUSANDS (we'll find how many thousands are here)
  left = number
  write = left/1000
  left = left - write*1000
  
  if write > 0 #if there are thousands
    thousands = englishnum write #recursion is here
    
    if write >= 1 && write <= 9
      number_string = number_string + ones_places[write - 1] + " thousand"
    end
    if write >= 11 && write <= 19
      number_string = number_string + teens_places[write - 11] + " thousand"
    end
    if (write >= 20 && write <= 99) || write == 10
      digits = write.to_s.split('')
      first = digits[0].to_i
      second = digits[1].to_i
      if first == 0 && second > 0
        number_string = number_string + ones_places[second-1] + " thousand"
      end
      if first >= 1 && first <= 9 && second == 0
        number_string = number_string + tens_places[first-1] + " thousand"
      end
      if first >= 1 && first <= 9 && second > 0
        number_string = number_string + tens_places[first - 1] + ones_places[second-1] + " thousand"
      end
    end
    if left > 0
      number_string = number_string + " " #so we don't write two thousandfive hundred; leaving a space for the remaining left number
    end
  end

  #HUNDREDS
  write = left/100
  left = left - write*100
  if write > 0 #if there are hundreds
    number_string = number_string + ones_places[write-1] + " hundred"
  end
  if left > 0 
    number_string = number_string + " "
  end
  
  #TENS
  write = left/10
  left = left - write*10 #ones place
  
  if write > 0
    number_string = number_string + tens_places[write-1]
  end
  
  if left > 0 
    number_string = number_string + " " + ones_places[left-1]
  end
  
  return number_string

end
puts englishnum(num)