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class Array; def sum; inject( nil ) { |sum,x| sum ? sum+x : x }; end; end
class Array; def mean; sum / size; end; end

def sd(population)
  m=population.mean
  Math.sqrt(population.collect{|e|(m-e)*(m-e)}.mean)
end

set=[2,4,4,4,5,5,7,9]
puts sd(set)

class Array; def sum; inject( nil ) { |sum,x| sum ? sum+x : x }; end; end
class Array; def mean; sum / size; end; end
class Array; def sd; m=mean; Math.sqrt(collect{|e|(m-e)**2}.mean); end; end