--Global--
double total = 0, totalSq = 0;
double stdev = 0, var;

--Loop--
total += sample;
totalSq += pow(sample, 2);
double mean = total/nSmpl;
if (nSmpl > 1)
  var = (totalSq - pow(total, 2)/nSmpl)/(nSmpl - 1);
stdev = sqrt(var);