/* This program finds the average and standard deviation of numbers
 * in a file.  It reads the file first, finding the average, then
 * again, finding the sum of squares of differences.
 * (Note: you can do it all in one pass, but then what would be the
 * fun in that?)
 */

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

int main () {
	FILE	*f;		/* file pointer */
	int	n;		/* number of items read in */
	float	num, 		/* number to read in */
		sum, 		/* sum of all the numbers */
		average, 	/* average of the numbers */
		diff, 		/* difference of a number with average */
		sumdiffsq, 	/* sum of the squares of the differences */
		sd;		/* standard deviation */

	/* initialize sum accumulator to 0 */

	sum = 0.0;

	/* open the file called "numbers" for reading */

	f = fopen ("numbers", "r");

	/* get a number from the file */

	fscanf (f, "%f", &num);

	/* while we haven't reached the end of the file... */

	while (!feof (f)) {

		/* add the number to the total */

		sum += num;

		/* we have one more number, so add one to n */

		n++;

		/* get another number */

		fscanf (f, "%f", &num);
	}

	/* close the file */

	fclose (f);

	/* figure out the average from sum and n */

	average = sum / n;

	/* start the sum of squared differences accumulator as 0 */

	sumdiffsq = 0.0;

	/* open the file again for reading */

	f = fopen ("numbers", "r");

	/* get the first number from the file */

	fscanf (f, "%f", &num);

	/* while we're not at the end of the file... */

	while (!feof (f)) {

		/* find the difference of the number with the average */

		diff = num - average;

		/* add its square to the total */

		sumdiffsq += diff * diff;

		/* get another number from the file */

		fscanf (f, "%f", &num);
	}

	/* close the file */
	fclose (f);

	/* finally calculate the standard deviation */

	sd = sqrt (sumdiffsq / (n-1));

	/* print the average and sd */

	printf ("average = %f\tsd = %f\n", average, sd);
}