class StdDevAccumulator(n=0, sum=0, sumofsquares=0) {
  method <<(num) {
    n += 1
    sum += num
    sumofsquares += num**2
    self
  }
 
  method stddev {
    sqrt(sumofsquares/n - pow(sum/n, 2))
  }
 
  method to_s {
    self.stddev.to_s
  }
}
 
var i = 0
var sd = StdDevAccumulator()
[2,4,4,4,5,5,7,9].each {|n|
    say "adding #{n}: stddev of #{i+=1} samples is #{sd << n}"
}