void GradeInfo::Read(istream & input)
// precondition: input is open for reading
// postcondition: myGrades contains the frequency count of
//                each grade in input,
//                myCount contains the total number of grades
//                in input
{
    int k,index;
    char ch;

    // initialize all counters to 0
    
    for(k=0; k < NUM_GRADES; k++)   
    {
        myGrades[k] = 0;
    }

    // read file

    while (input >> ch)
    {
        index = CharToIndex(ch);  // map char to an index
        myGrades[index]++;        // bump # occurrences
        myCount++;                // bump total count
    }   
}