Our other task is to check that the transactions are for the same ISBN. If so, we’ll print their sum, otherwise, we’ll print an error message

if (data1.bookNo == data2.bookNo) {
unsigned totalCnt = data1.units_sold + data2.units_sold;
double totalRevenue = data1.revenue + data2.revenue;
// print: ISBN, total sold, total revenue, average price per book
std::cout << data1.bookNo << " " << totalCnt
<< " " << totalRevenue << " ";
if (totalCnt != 0)
std::cout << totalRevenue/totalCnt << std::endl;
else
std::cout << "(no sales)" << std::endl;
return 0; // indicate success
} else { // transactions weren't for the same ISBN
std::cerr << "Data must refer to the same ISBN"
<< std::endl;
return -1; // indicate failure
}