// Contract: grade_avg: int[] int -> double
// Purpose: print to the screen the average of the letter
//          grades within <lett_freqs>, which contains
//          <num_letts> elements, assuming that:
//          A = 4.0
//          B = 3.0
//          C = 2.0
//          D = 1.0
//          F = 0.0
//          ...and any other elements will be IGNORED for
//          averaging purposes.
//
// Examples: If int num_grades[6] = {3, 4, 1, 1, 1, 2},
//          then grade_avg(num_letts, 6) == 2.7
//
// by: Sharon M. Tuttle
// last modified: 12-02-05

double grade_avg(int lett_freqs[], int num_letts)
{
    const int NUM_GRADE_LETTERS = 5;
    double grade_sum = 0;
    int num_grades = 0;

    double grade_points[NUM_GRADE_LETTERS] = {4.0, 3.0, 2.0, 1.0, 0.0};

    // gather the sum of the grade points for the grades in lett_freqs

    for (int i=0; i < NUM_GRADE_LETTERS; i++)
    {
        num_grades += lett_freqs[i];
        grade_sum += lett_freqs[i] * grade_points[i];
    }

    // compute the average, avoiding dividing by zero!
    if (num_grades == 0)
    {
        return 0.0;
    }
    else
    {
        return grade_sum / num_grades;
    }
}