>_<.

class Integer
    def to_comma_str
        original_string = self.to_s
        
        result_string = ""
        3.step(original_string.size, 3) do |i|
            result_string = "," + original_string[-i, 3] + result_string
        end
        reminder = original_string[0, original_string.size % 3]

        if reminder == ""
            result_string[1..result_string.size-1]
        else
            reminder + result_string
        end
    end
end

puts 1.to_comma_str
puts 12.to_comma_str
puts 123.to_comma_str
puts 1234.to_comma_str
puts 12345.to_comma_str
puts 123456.to_comma_str
puts 1234567890.to_comma_str
puts 123456789012345678901234567890.to_comma_str

1
12
123
1,234
12,345
123,456
1,234,567,890
123,456,789,012,345,678,901,234,567,890