Standard Deviation One-liner

[Alain Ketterlin]

Billy Mays <noway at nohow.com> writes:

> I'm trying to shorten a one-liner I have for calculating the standard
> deviation of a list of numbers.  I have something so far, but I was
> wondering if it could be made any shorter (without imports).

> a=lambda d:(sum((x-1.*sum(d)/len(d))**2 for x in d)/(1.*(len(d)-1)))**.5

You should make it two half-liners, because this one repeatedly computes
sum(d). I would suggest:

aux = lambda s1,s2,n: (s2 - s1*s1/n)/(n-1)
sv = lambda d: aux(sum(d),sum(x*x for x in d),len(d))

(after some algebra). Completely untested, assumes data come in as
floats. You get the idea.

-- Alain.