How to calculate standard deviation with streaming inputs? Is there a formula that is capable of operating on streaming inputs and approximating standard  deviation of the set of numbers? 

You can use the formula $\sigma = \sqrt{\bar{x^2}-(\bar x)^2}=\sqrt{\frac {\sum x^2}N-\left(\frac {\sum x}N\right)^2}$  Each sum can be accumulated as the data comes in.  The disadvantage compared to averaging the data first and subtracting the average from each item is you are more prone to overflow and loss of significance, but mathematically it is equivalent.
