A handy little Array class extension for calculating standard deviation of an array

stddev.rb

class Array
	def mean
		return ((self.map{|x| x.to_f}
			         .inject(:+)) /
		        (self.count))
	end

	def standard_deviation
		m = self.mean

		return (Math.sqrt((self.map{|x| (x - m) ** 2}
			                   .inject(:+)) /
		                  (self.count)))
	end
end

ary = [2, 4, 4, 4, 5, 5, 7, 9]

p ary
p ary.mean
p ary.standard_deviation