练习1.20

#include <iostream>
#include "Sales_item.h"
using namespace std;int main() {Sales_item book;cout << "请输入销售记录:" << endl;while (cin >> book) {cout << "ISBN,售出本数，销售额和平均售价为" << book << endl;}return 0;
}

练习1.21

#include <iostream>
#include "Sales_item.h"
using namespace std;int main() {Sales_item trans1, trans2;cout << "请输入两条ISBN相同的销售记录:" << endl;cin >> trans1 >> trans2;if (trans1.isbn == trans2.isbn) {cout << "汇总信息:ISBN，销出本数，销售额和平均售价为" << trans1 + trans2 << endl;} else {cout << "两条销售记录的ISBN不同" << endl;}return 0;
}

练习1.22

#include <iostream>
#include "Sales_item.h"using namespace std;int main() {Sales_item total, trans;cout << "请输入几条ISBN相同的销售记录:" << endl;if (cin >> total) {while (cin >> trans) {if (trans.isbn == total.isbn)total = total + trans;else {cout << "ISBN不同" << endl;return -1;}}cout << "汇总信息:ISBN,销售本数，销售额和平均售价为" << total << endl;} else {cout << "没有数据" << endl;return -1;}return 0;
}