void MailBoxes::processMail(istream & input)
// precondition:  input is formatted correctly, is open for reading,
//                 and is positioned at the beginning of the file;
//                 the file contains no more than MAX_BOXES different
//                 P.O. box numbers    
{
    int box;
    while (input >> box)
    {
	int index = boxIndex(box);
	if (index != -1)
	{
	    myBoxes[index].mailCount++;
	}
	else
	{
	    myBoxes[myBoxCount].POBox = box;
	    myBoxes[myBoxCount].mailCount = 1;
	    myBoxCount++;
	}	
    }
}