commaifier.rb

def divisible_by_3?(number)
  number%3 == 0
end

def commaifier(number)
  reversed = number.to_s.reverse

  result = ""

  number.to_s.size.times do |i|
    result << reversed[i]
    result << "," if divisible_by_3?(i+1)
  end

  result.reverse
end

# > commaifier(25164150)
# => "25,164,150"
# > commaifier(1234567890)
# => "1,234,567,890"

yours is shorter than mine...