[エラトステネスのふるい](http://e-words.jp/w/E382A8E383A9E38388E382B9E38386E3838DE382B9E381AEE7AFA9.html) について

pm.rb

require 'pp'
max = 100
first_prime = 2
prime = [2]
list = (2..max).to_a


i = 2
while list.size > 1 do
  list = list.select{|x| x % i != 0}
  prime << list.first
  i = list.first
end
pp prime

=begin
[2,
 3,
 5,
 7,
 11,
 13,
 17,
 19,
 23,
 29,
 31,
 37,
 41,
 43,
 47,
 53,
 59,
 61,
 67,
 71,
 73,
 79,
 83,
 89,
 97]
=end