isPrime.rb

class Integer #Override
  def prime? #boolean: itself is prime or not.
    return true if self == 2
    return false if self < 2 || self % 2 == 0
    limit = Math::sqrt(self).ceil
    (3..limit).step(2) { |i|
      return false if self % i == 0
    }
    return true
  end #prime?
end #Integer

#test
(1..100).map{ |i|
  if i.prime?
    puts i
  end #if i.prime?
}