calculate standard deviation single pass

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var=sum((X(i)-mean)^2)/(N-1)

sum((X(i)-mean)^2)
=sum(X(i)^2 - 2*X(i)*mean + mean^2)
=sum(X(i)^2) - 2*sum(X(i))*mean + N*mean^2

because sum(X(i)) = N*mean, so we get

=sum(X(i)^2) - 2*N*mean*mean + N*mean^2
=sum(X(i)^2) - N*mean^2

then std = sqrt((sum(X(i)^2) - N*mean^2)/(N-1))