void GradeDistribution(istream & input, Grade & grades)
// precondition: input is open for reading
//               grades.freq.length() >= 4    
// postcondition: grades.freq contains the frequency count of
//                each grade in input,
//                grades.count contains the total number of grades
//                in input
{
    int k,index;
    char ch;
    
    for(k=0; k < 4; k++)
    {
        grades.freq[k] = 0;
    }


    while (input >> ch)
    {
        index = ch - 'A';         // map to 0, 1, 2, 3
        grades.freq[index]++;     // bump # occurrences
        grades.count++;           // bump total count
    }
}