// AP Computer Science, A Exam, 1996, Question 1, part A in C++

double LetterAverage(const apvector<int>  & scores, 
                     const apvector<char> & letters, char grade)
// postcondition: returns 0.0 if no element of letters == grade
//                otherwise, returns average of all scores[n],
//                for all 0 <= n < NUM_GRADES, such that letters[n] == grade
{
    double sum = 0.0;       // use double for sum, avoid cast at end
    int count = 0;          // # of grades equal to parameter grade
    int k;

    for(k=0; k < NUM_GRADES; k++)
    {
	if (letters[k] == grade)
	{
	    sum += scores[k];
	    count++;
	}
    }
    if (sum == 0.0) return 0.0;
    else            return sum/count;
}