calculate mean and variance in a single iteration

gistfile1.js

// http://en.wikipedia.org/wiki/Variance
// http://upload.wikimedia.org/math/a/3/3/a336557f138eb90bafc9e6ebc00e926d.png

var data = [{x: 1, p: 0.1}, {x: 2, p: 0.3}, {x: 8, p: 0.6}]; //{x:probability}

var mu = 0, sigma = 0;
for (i in data) {
  mu += i.p * i.x;
  sigma += i.p * i.x * i.x;
}

console.log("mu = " + mu + ", sigma = " + sigma);