standard deviation

double sum = 0.0, sumsq = 0.0;foreach (double i in values) {  sum += i;  sumsq += i*i;}double avg = sum / (double)n.Length;double sqavg = sumsq / (double)n.Length;return Math.Sqrt(sqavg - avg * avg);

#include <iostream>#include <cstdlib>#include <ctime>#include <vector>#include <cmath>#include <sys/time.h>double standard_deviation(double const *data, unsigned n){  double sum=0.0;  for(unsigned i=0;i<n;++i){    double x=data;    sum+=x;  }  double avg = sum/n;    double sum2=0.0;  for(unsigned i=0;i<n;++i){    double x=data-avg;    sum2+=x*x;  }  double avg2 = sum2/n;    return std::sqrt(avg2);}double standard_deviation_fast(double const *data, unsigned n){  double sum=0.0;  double sum2=0.0;  for(unsigned i=0;i<n;++i){    double x=data;    sum+=x;    sum2+=x*x;  }  double avg = sum/n;  double avg2 = sum2/n;    return std::sqrt(avg2 - avg*avg);}double now(){  struct timeval tp;  gettimeofday(&tp,0);  return tp.tv_sec+.000001*tp.tv_usec;}int main(){  std::srand(std::time(0));  std::vector<double> v;    v.reserve(100000000);  for(unsigned i=0;i<100000000;++i)    v.push_back(double(std::rand())/RAND_MAX);  double start = now();  double stddev = standard_deviation(&v[0], v.size());  double elapsed = now() - start;  double start_fast = now();  double stddev_fast = standard_deviation_fast(&v[0], v.size());  double elapsed_fast = now() - start_fast;  std::cout << "2 pass: " << elapsed << " (" << stddev << ")\n";  std::cout << "1 pass: " << elapsed_fast << " (" << stddev_fast << ")\n";}