// Arup Guha
// 4/17/2012
// Illustrating standard deviation calculation using constant extra storage.
// With NUMTRIALS = 50, NUMVALUES = 20000000, this took 18 sec. on my desktop.

#include <stdio.h>
#include <time.h>
#include <math.h>

#define NUMTRIALS 50
#define NUMVALUES 20000000

int main() {

    srand(time(0));
    int start = time(0);

    int loop;
    for (loop = 0; loop<NUMTRIALS; loop++) {

        double sum = 0, sumsq = 0;

        int i;

        // Generate each number and add into both our
        // sum and sum of squares.
        for (i=0; i<NUMVALUES; i++) {
            int num = rand();
            sum += num;
            sumsq += (num*num);
        }

        // Calculate results.
        double ave = sum/NUMVALUES;
        //printf("Ave = %lf\n", ave);
        //printf("Std Dev = %lf\n", sqrt(sumsq/NUMVALUES - ave*ave));
    }

    int end = time(0);
    printf("One loop: Took %d seconds.\n", end-start);

    return 0;
}