C++ Primer 中文版（第 5 版）-第二单元

第二单元

练习 2.1

通过读下面程序，写出程序运行结果。

#include <iostream>int main()
{unsigned u = 10, u2 = 42;std::cout << u2 - u << std::endl;std::cout << u - u2 << std::endl;int i = 10, i2 = 42;std::cout << i2 - i << std::endl;std::cout << i - i2 << std::endl;std::cout << i - u << std::endl;std::cout << u - i << std::endl;return 0;
}

unsigned 表示无符号整数类型，当从无符号数中减去一个值时，不管这个值是不是无符号数，我们都必须确保结果不能是一个负值。
例如：unsigned u1 = 42，u2=10；
cout<< u1 - u2<< endl;//正确：输出32
cout<< u2 - u1<< endl;//正确：不过，结果是取模后的值
结果u2-u1 = （10-42）+2^32 = 4294967264

32
4294967264
32
-32
0
0
Program ended with exit code: 0

练习 2.2

请利用转义序列编写一段程序，要求先输出 2M，然后转到新一行。修改程序使其先输出 2，然后输出制表符，再输出 M，最后转到新一行。

#include <iostream>int main()
{std::cout << 2 << "\115\012";std::cout << 2 << "\t\115\012";return 0;
}

练习 2.3

const int buf;      // 不合法, const 对象必须初始化
int cnt = 0;        // 合法
const int sz = cnt; // 合法
++cnt; ++sz;        // 不合法, const 对象不能被改变int i = -1, &r = 0;         // 不合法, r 必须引用一个对象
int *const p2 = &i2;        // 合法，常量指针
const int i = -1, &r = 0;   // 合法
const int *const p3 = &i2;  // 合法
const int *p1 = &i2;        // 合法
const int &const r2;        // 不合法, r2 是引用, 引用自带顶层 const, 第二个const写法多余但合法, 但引用需要初始化.
const int i2 = i, &r = i;   // 合法int i, *const cp;       // 不合法, const 指针必须初始化
int *p1, *const p2;     // 不合法, const 指针必须初始化
const int ic, &r = ic;  // 不合法, const int 必须初始化
const int *const p3;    // 不合法, const 指针必须初始化
const int *p;           // 合法. 一个指针，指向 const int

练习 2.4

使用你自己的Sale_data类重写1.5.1节（第20页）、1.5.2节（第21页）和1.6节（第22页）的练习。眼下先把Sales_data类的定义和main函数放在一个文件里。

#include <iostream>
#include <string>struct Sale_data
{std::string bookNo;unsigned units_sold = 0;double revenue = 0.0;
};int main()
{Sale_data book;double price;std::cin >> book.bookNo >> book.units_sold >> price;book.revenue = book.units_sold * price;std::cout << book.bookNo << " " << book.units_sold << " " << book.revenue << " " << price<<std::endl;return 0;
}

// 1.5.2#include <iostream>
#include <string>struct Sale_data
{std::string bookNo;unsigned units_sold = 0;double revenue = 0.0;
};int main()
{Sale_data book1, book2;double price1, price2;std::cin >> book1.bookNo >> book1.units_sold >> price1;std::cin >> book2.bookNo >> book2.units_sold >> price2;book1.revenue = book1.units_sold * price1;book2.revenue = book2.units_sold * price2;if (book1.bookNo == book2.bookNo){unsigned totalCnt = book1.units_sold + book2.units_sold;double totalRevenue = book1.revenue + book2.revenue;std::cout << book1.bookNo << " " << totalCnt << " " << totalRevenue << " ";if (totalCnt != 0)std::cout << totalRevenue / totalCnt << std::endl;elsestd::cout << "(no sales)" << std::endl;return 0;}else{std::cerr << "Data must refer to same ISBN" << std::endl;return -1;  // indicate failure}
}

// 1.6#include <iostream>
#include <string>struct Sale_data
{std::string bookNo;unsigned units_sold = 0;double revenue = 0.0;
};int main()
{Sale_data total;double totalPrice;if (std::cin >> total.bookNo >> total.units_sold >> totalPrice){total.revenue = total.units_sold * totalPrice;Sale_data trans;double transPrice;while (std::cin >> trans.bookNo >> trans.units_sold >> transPrice){trans.revenue = trans.units_sold * transPrice;if (total.bookNo == trans.bookNo){total.units_sold += trans.units_sold;total.revenue += trans.revenue;}else{std::cout << total.bookNo << " " << total.units_sold << " " << total.revenue << " ";if (total.units_sold != 0)std::cout << total.revenue / total.units_sold << std::endl;elsestd::cout << "(no sales)" << std::endl;total.bookNo = trans.bookNo;total.units_sold = trans.units_sold;total.revenue = trans.revenue;}}std::cout << total.bookNo << " " << total.units_sold << " " << total.revenue << " ";if (total.units_sold != 0)std::cout << total.revenue / total.units_sold << std::endl;elsestd::cout << "(no sales)" << std::endl;return 0;}else{std::cerr << "No data?!" << std::endl;return -1;  // indicate failure}
}