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Task/Standard-deviation/Python/standard-deviation-2.py

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+>>> class SD(object): # Plain () for python 3.x
+	def __init__(self):
+		self.sum, self.sum2, self.n = (0,0,0)
+	def sd(self, x):
+		self.sum  += x
+		self.sum2 += x*x
+		self.n    += 1.0
+		sum, sum2, n = self.sum, self.sum2, self.n
+		return sqrt(sum2/n - sum*sum/n/n)
+
+>>> sd_inst = SD()
+>>> for value in (2,4,4,4,5,5,7,9):
+	print (value, sd_inst.sd(value))