/*

* grades_2.c

* This program reads a series of grades, terminated by (-1), and computes

* the average and variance.

* In case the user the code (-2) the program aborts and prints no statistics.

*/

#include <stdio.h>

int main ()

{

const int min_grade = 0;

const int max_grade = 100;

const int stop_code = -1;

const int abort_code = -2;

int curr_grade; /* The current grade. */

int sum_grades = 0; /* The sum of all grades so far. */

int sum_sqr_grades = 0; /* The sum of their squares. */

int n_grades = 0; /* The number of legal grades so far. */

float average;

float variance;

/* Notice that the loop is theoretically infinite. */

while (1)

{

/* Read the current grade. */

printf ("Please enter the next grade (or: -1 to stop, -2 to abort): ");

scanf ("%d", &curr_grade);

/* Stop if necessary. */

if (curr_grade == stop_code)

break;

/* Abort the program if necessary. */

if (curr_grade == abort_code)

goto abort;

/* Ignore illegal values. */

if (curr_grade < min_grade || curr_grade > max_grade)

{

printf ("Ignoring illegal grade (%d).\n", curr_grade);

continue;

}

/* Update our sums. */

sum_grades += curr_grade;

sum_sqr_grades += curr_grade * curr_grade;

n_grades++;

}

/* Compute the average and variance. */

if (n_grades > 0)

{

average = (float)sum_grades / (float)n_grades;

/* Using the equality: Var(X) = E(X^2) - E(X)*E(X) */

variance = (float)sum_sqr_grades / (float)n_grades - average * average;

printf ("Average grade is %g (variance = %g).\n", average, variance);

}

return (0);

/* Code executed in case the user requested to abort the program: */

abort:

printf ("Program aborted!\n");

return (1);

}