C++primer（第五版）习题答案

前两章习题较简单，这里就不作整理了，直接从第三章开始(持续更新)：

Chapter 3. Strings, Vectors and Arrays

Exercise 3.1

part 1

#include <iostream>using std::cin;
using std::cout;
using std::endl;int main()
{int sum = 0, val = 1;while (val <= 10) {sum += val;++val;}cout << "Sum of 1 to 10 inclusive is " << sum << endl;return 0;
}

part 2

#include <iostream>using std::cin;
using std::cout;
using std::endl;
using std::cerr;struct Sales_data   {std::string bookNo;unsigned unit_sold = 0;double revenue = 0.0;
};int main()
{Sales_data data1, data2;double price = 0;cin >> data1.bookNo >> data1.unit_sold >> price;data1.revenue = data1.unit_sold * price;cin >> data2.bookNo >> data2.unit_sold >> price;data2.revenue = data2.unit_sold * price;if (data1.bookNo == data2.bookNo)   {unsigned totalCnt = data1.unit_sold + data2.unit_sold;double totalRevenue = data1.revenue + data2. revenue;cout << data1.bookNo << " " << totalCnt << " " << totalRevenue << " ";if (totalCnt != 0)cout << totalRevenue/totalCnt << endl;elsecout << "(no sale)" << endl;return 0;}else{cerr << "Data must refer to the same ISBN" << endl;return -1;}
}

Exercise 3.2

part 1

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string line;while (getline(cin, line))  {cout << line << endl;}return 0;
}

part 2

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string word;while (cin >> word)cout << word << endl;return 0;
}

Exercise 3.3

string类输入运算符(>>)：忽视一切空格，从第一个非空格字符开始，到第一个空格结束；

getline()：包含换行符前的所有空格；

Exercise 3.4

part 1

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string st1, st2;cin >> st1 >> st2;if (st1 == st2)cout << "The two strings are equal" << endl;else if (st1 > st2)cout << "The maximum string is " << st1 << endl;else cout << "The maximum string is " << st2 << endl;   return 0;
}

part 2

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string st1, st2;cin >> st1 >> st2;if (st1.size() == st2.size())cout << "The two strings have the same length " << endl;else if (st1.size() > st2.size())cout << "The longest string is " << st1 << endl;else cout << "The longest string is " << st2 << endl;   return 0;
}

Exercise 3.5

part 1

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string total_str, str;while (cin >> str)total_str += str;cout << "The concatenated string is " << total_str << endl;return 0;
}

part 2

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string total_str, str;while (cin >> str)if (total_str.empty())total_str += str;elsetotal_str += " " +str;cout << "The concatenated string is " << total_str << endl;return 0;
}

Exercise 3.6

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string str("I wanna go home!");for (auto &c : str)  {c = 'X';}cout << str << endl;return 0;
}

Exercise 3.7

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string str("I wanna go home!");for (char c : str)  {c = 'X';}   cout << str << endl;return 0;
}
//输出仍为I wanna go home!

本质上是把string中的每一个字符拷贝给循环控制变量c，此时对c更改只是改变了c字符，原string并没有任何变化，想要通过c改变string的值，就必须把c定义为对字符的引用。

Exercise 3.8

part 1

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string str("I wanna go home!");decltype(str.size()) n = 0; //这里注意必须初始化while ( n < str.size()) {str[n] = 'X';++n;}cout << str << endl;cout << n << endl;return 0;
}

part 2

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{string str("I wanna go home!");for ( decltype(str.size()) index = 0; index < str.size(); ++index)  {str[index] = 'X';}cout << str << endl;return 0;
}

使用范围for语句更简洁，逻辑也更清晰。

Exercise 3.9

不合法，不能用下标访问空字符串。

Exercise 3.10

#include <iostream>
#include <string>using std::string;
using std::cin;
using std::cout;
using std::endl;int main()
{cout << "Please input a string with punctuation: " << endl;string str, result;cin >> str;if (!str.empty())   {for (auto &c : str)  {if ( !ispunct(c))result += c;}cout << result << endl;return 0;}cout << "Your string is fucking empty!" << endl;return -1;
}

注：这里使用ispunct()但不Include <cctype>是因为iostream中间接包含了cctype。

Exercise 3.11

c为常量字符串的引用（底层const）,只读取数据——合法，通过其修改数据——不合法。

Exercise 3.12

（a）正确，元素为向量，元素的元素为int；

（b）错误，拷贝初始化类型必须统一；

（c）正确，向量拥有10个空字符串元素；

Exercise 3.13

（a）无元素；

（b）10个元素，值均为0；

（c）10个元素，值均为42；

（d）一个元素，值为10；

（e）两个元素，值分别为10、42；

（f）10个元素，均为空字符串；

（g）10个元素，均为"hi"；

Exercise 3.14

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{vector<int> ivec;int i;while (cin >> i) {ivec.push_back(i);return 0;}
}

Exercise 3.15

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{vector<string> svec;string str;while (cin >> str) {svec.push_back(str);return 0;}
}

Exercise 3.16:

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{//v1vector<int> v1;cout << "The size is: "<< v1.size() << endl;cout <<  "The value is: ";for (auto i : v1)cout << i << " ";     cout << endl;//v2vector<int> v2(10);cout << "The size is: "<< v2.size() << endl;cout <<  "The value is: ";for (auto i : v2)cout << i << " ";     cout << endl;//后续同理
}

Exercise 3.17

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{vector<string> text;string word;while (cin >> word) {text.push_back(word);}cout << "The original text is: " << endl;for (auto i : text) {cout << i << endl;}cout << "The changed text is: " << endl;for (auto str : text) {for (auto &c : str)  {c = toupper(c);}cout << str << endl;}return 0;
}

Exercise 3.18

不合法，不能使用无效下标访问不存在的元素。修改如下：

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{vector<int> ivec;ivec.push_back(42);cout << ivec[0] << endl;return 0;
}

Exercise 3.19

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{vector<int> ivec1(10, 42);vector<int> ivec2{42, 42, 42, 42, 42, 42, 42, 42, 42, 42};vector<int> ivec3;for (auto i = 0; i != 10; ++i)ivec3.push_back(42);vector<string> svec1{10, "42"};vector<string> svec2(10, "42");return 0;
}

ivec1的定义方式简洁且高效。

Exercise 3.20

part 1:

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{vector<int> ivec;int i = 0, sum = 0;while (cin >> i)ivec.push_back(i);if (ivec.empty())   {    //因为后续用到下标，这里必须验证是否为空！！cout << "Please input at least one integer"<< endl;return -1;}cout << "The sum is: " << endl;for (decltype(ivec.size()) index = 0; index < ivec.size() - 1; ++index)    {sum = ivec[index]+ ivec[index + 1];cout << sum << " ";}cout << endl;return 0;
}

part 2:

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{vector<int> ivec;int i = 0, sum = 0;while (cin >> i)ivec.push_back(i);if (ivec.empty())   {cout << "Please input at least one integer"<< endl;return -1;}cout << "The sum is: " << endl;for (decltype(ivec.size()) index = 0; index <= ivec.size()/2; ++index)    {if (index < ivec.size() - 1 - index)    {sum = ivec[index]+ ivec[ivec.size() - 1 - index];cout << sum << " ";}else if (index == ivec.size() - 1 - index)   {  //这里一定注意为双等号！！cout << ivec[index];}}cout << endl;return 0;
}

Exercise 3.21

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{//以v2为例，其他同理vector<int> v2(10);if (v2.empty()) {cout << "This is a empty vector " << endl;return -1;}cout << "The size of the vector is: " << v2.size() << endl;cout << "The values of the elements are: ";for (auto it = v2.cbegin(); it != v2.cend(); ++it)  {cout << *it << " ";}cout << endl;return 0;
}

Exercise 3.22

#include <iostream>
#include <string>
#include <vector>using std::string;
using std::cin;
using std::cout;
using std::endl;
using std::vector;int main()
{cout << "Please enter a text: " << endl;vector<string> svec;string str;while (getline(cin, str))svec.push_back(str);auto it = svec.begin();for (auto &c : *it)c = toupper(c);for (it;it != svec.cend(); ++it)    //注意这里it可不重新定义，但不能省略！！cout << *it << endl;return 0;
}

Exercise 3.23

#include <iostream>
#include <vector>
#include <string>using std::vector;
using std::string;
using std::cout;
using std::cin;
using std::endl;int main()
{vector<int> ivec(10);int i;cout << "Please enter ten integers: " << endl;for (auto it = ivec.begin(); it != ivec.cend(); ++it)   {cin >> *it;*it *= 2;}for (auto i : ivec)cout << i << " ";cout << endl;return 0;
}

Exercise 3.24

part 1

#include <iostream>
#include <vector>
#include <string>using std::vector;
using std::string;
using std::cout;
using std::cin;
using std::endl;int main()
{vector<int> ivec;int i;while (cin >> i)ivec.push_back(i);cout << "The sum is: " << endl;for (auto it = ivec.cbegin(); it != ivec.cend() - 1; ++it)cout << *it + *(it + 1) << " ";cout << endl;return 0;
}

part 2

法I：

#include <iostream>
#include <vector>
#include <string>using std::vector;
using std::string;
using std::cout;
using std::cin;
using std::endl;int main()
{vector<int> ivec;int i, sum;while (cin >> i)ivec.push_back(i);if (ivec.empty())cout << "Your vector is empty!" << endl;cout << "The sum is: " << endl;for (auto it = ivec.cbegin(); it != ivec.cbegin() + ivec.size()/2 +1; ++it) {if (it == ivec.cbegin() + (ivec.cend() - 1 - it))    {cout << *it;}else if (it < ivec.cbegin() + (ivec.cend() - 1 - it))   {sum = *it + *(ivec.cbegin() + (ivec.cend() - 1 - it));cout << sum << " ";}}cout << endl;return 0;
}

法II：

#include <iostream>
#include <vector>
#include <string>using std::vector;
using std::string;
using std::cout;
using std::cin;
using std::endl;int main()
{vector<int> ivec;int i, sum;while (cin >> i)ivec.push_back(i);if (ivec.empty())cout << "Your vector is empty!" << endl;cout << "The sum is: " << endl;for (auto beg = ivec.cbegin(), end = ivec.cend()  - 1; beg <= end; ++beg, --end)    {   //注意方法！！if (beg == end)cout << *beg;elsecout << *beg + *end << " ";}cout << endl;return 0;
}

Exercise 3.25

#include <iostream>
#include <vector>
#include <string>using std::vector;
using std::string;
using std::cout;
using std::cin;
using std::endl;int main()
{vector<unsigned> scores(11, 0);unsigned grade;auto it = scores.begin(); while (cin >> grade)    {if (grade <= 100)   {++*(scores.begin() + grade/10);}}for (auto i : scores)cout << i << " ";cout << endl;return 0;
}

Exercise 3.26

两个迭代器间只定义了相见，未定义相加；

所以可通过利用iterator - iterator将iterator +iterator转化为iterator + n。

Exercise 3.27