I used my prime generator from Problem 27 for this one. It would have been faster to build the rotation into my generator, but it ran fine without it.

Problem 35

How many circular primes are there below one million?

require 'prime_generator'

primer = Prime_Generator.new 1_000_000

def is_rot_prime? primer, chars
	chars.size.times do |i|
		chars = Array.new(chars.size) { |i| chars[i - 1] }
		return false if !primer.is_prime?(chars.join("").to_i)
	end
	true
end

count = 0
primer.stack.each do |n|
	count += 1 if is_rot_prime? primer, n.to_s.split("")
end

# subtract 1 because "1" doesn't count
puts count - 1

Speaking of the rotation, the ruby array initialize methods and negative indexers make it a cinch to rotate. How cool is this?

chars = Array.new(chars.size) { |i| chars[i - 1] }