Putting numbers into words

I have some embarrassingly long code which puts into words any number up into the trillions. As a newbie, and understanding that shorter, non-repetitive code is best, I am looking for suggestions on how to reduce this code to a respectable quantity. I realize that there is a lot of repetition in it. However, depending on the number being evaluated, the math looks a little different with each rep, so I am not sure if I can reduce that. I have tried rewriting it solely as an if/else (without the recursion) but it quickly becomes just as bad if not worse.

```
class :: Fixnum

def in_words(number)

if number 999999999999

if write 9) && (write 19) && (write 19,000,000 - 999,000,000
millions = in_words(write)
numString = numString + millions + ' trillion'
end

if left > 0
numString = numString + ' '
end

#-----------------------------------------billions
#left = number
write = left/1000000000
left = left - (write*1000000000)

if number > 999999999

if write 9) && (write 19) && (write 19,000,000 - 999,000,000
millions = in_words(write)
numString = numString + millions + ' billion'
end

if left > 0
numString = numString + ' '
end

# ----------------------------------------millions
write = left/1000000
left = left - (write*1000000)

if number > 999999

if write 9) && (write 19) && (write 19,000,000 - 999,000,000
millions = in_words(write)
numString = numString + millions + ' million'
end

if left > 0
numString = numString + ' '
end

#-----------------------------------------thousands
write = left/1000
left = left - (write*1000)

if number > 999

if write 9) && (write 19) && (write 19,000 - 999,000
thousands = in_words(write)
numString = numString + thousands + ' thousand'
end

if left > 0
numString = numString + ' '
end
end

Rather than rewrite your whole solution, I'll point out more idiomatic ways to do this stuff in Ruby.

Recursion

You don't need an argument to do recursion, self is enough.

class ::Fixnum
  def in_words
    # ...
    millions = write.in_words
    # ...
  end
end

String Arrays

%w{} is handy for making arrays containing one-word strings

ones_place = %{one two three four five six seven eight nine}

Long Numbers

_ can be used like a comma (or period, if you're European) in long numbers.

billion = 1_000_000_000

For powers of 10, you can also use exponentiation

trillion = 10 ** 12

Modulus

% gets the remainder of a number after dividing.

write = left / 10 ** 9
left = left % 10 ** 9

Assignment Shortcut

x = x y can be written as x = y where ` is any operator.

numString += ' '
left %= 10 ** 9

Loops

Once you incorporate all of those, you should be able to see how that huge nested if` can be turned into a tiny

[12, 9, 6, 3].each do |power|
  break if self < 10 ** power
  # ...
end

StackExchange Code Review Q#85010, answer score: 4