gistfile1.rb

#!/usr/bin/env ruby
class Integer
  def prime?
    return false if self < 2
    return true if self == 2
    return false if self % 2 == 0
    max = Math::sqrt(self).ceil
    (3..max).step(2) {|i|
      return false if (self % i == 0)
    }
    true
  end
end

def primeList num
  return [] if num < 2
  arr = []
  prime = [2]
  (2..num).each{|i| arr << i }
  while (prime.last < Math::sqrt(arr.last).ceil) do
    arr.reject!{|i|
      i % prime.last == 0
    }
    prime << arr.shift
  end
  prime + arr
end

# prime? entry
(1..100).each {|i|
  puts i if i.prime?
}

# primeList entry
puts primeList(100)