# Checking whether a number is prime can be
# much simpler than this. But this example demonstrates
# loop construct and duck-typing.
 
# A simple duck-typing example.
# Add a new method/functionality to an existing class
class Fixnum
    def is_prime?
        (2..Math.sqrt(self).floor).each {|x| return false if self % x == 0 }
        return true
    end
end
  
 
loop do
   print "Enter a number (0 to exit): "
   num = gets.to_i
   break if num == 0
   puts "#{num} is" + (num.is_prime? ? " " : " NOT ") + "prime."
end
- See more at: http://www.chandrashekar.info/vault/ruby-scripts/loops.html#sthash.GbDNNtmh.dpuf