StdDeviator = {}
StdDeviator.count = 0
StdDeviator.sum = 0
StdDeviator.sumOfSquares = 0

StdDeviator.add = function(x)
    self.count = self.count + 1
    self.sum = self.sum + x
    self.sumOfSquares = self.sumOfSquares + x*x
end function

StdDeviator.stddev = function()
    m = self.sum / self.count
    return sqrt(self.sumOfSquares / self.count - m*m)
end function

sd = new StdDeviator
for x in [2, 4, 4, 4, 5, 5, 7, 9]
    sd.add x
end for
print sd.stddev