int MailBoxes::boxIndex(int box)
//postcondition: returns the index of the element in myBoxes
//               whose POBox field equals box if there is such an element;
//               otherwise, returns -1.
{
    int k = 0;
    while (k < myBoxCount && myBoxes[k].POBox != box)
    {
	k++;
    }
    if (k < myBoxCount) return k;    // found box
    else                return -1;   // box not found
}