以下程序由 Teddy van Jerry （我自己）编写并运行，基本保证正确性。（有时可能会为优化程序超前使用某些内容）

Before we comb through the codes

这一章内容仍然非常好理解，我学习花了一天半（也不是一直在学C++）。主要原因是内容在之前大多已经出现过，因而再次介绍时就尤其轻松了。
Review:
【C++ Primer(5th Edition) Exercise】练习程序 - Chapter1（第一章）
【C++ Primer(5th Edition) Exercise】练习程序 - Chapter2（第二章）
【C++ Primer(5th Edition) Exercise】练习程序 - Chapter3（第三章）
【C++ Primer(5th Edition) Exercise】练习程序 - Chapter4（第四章）

Exercise 5.3

#include <iostream>
using namespace std;int main()
{int sum = 0, val = 1;while (val <= 10)sum += val, ++val; // equivalent to sum += val, val++;cout << sum << endl;return 0;
}

其实，也可以简写成这样：

#include <iostream>
using namespace std;int main()
{int sum = 0, val = 1;while (val <= 10)sum += (val++);cout << sum << endl;return 0;
}

Exercise 5.5

#include <iostream>
#include <vector>
#include <string>
using namespace std;int main()
{unsigned short score;while (cin >> score && score <= 100){string grade;vector<string> vec{ "F","D","C","B","A","A++" };if (score < 60) grade = vec[0];else{grade = vec[(score - 50) / 10];if (score % 10 < 3 && score != 100)grade += "-";else{if (score % 10 > 7)grade += "+";}}cout << "The grade is " << grade << endl;}return 0;
}

注意：第17行score != 100很重要，要不然输入100会输出A++-的。

Exercise 5.6

#include <iostream>
#include <vector>
#include <string>
using namespace std;int main()
{unsigned short score;while (cin >> score && score <= 100){string grade;vector<string> vec{ "F","D","C","B","A","A++" };grade = (score < 60) ? vec[0] : vec[(score - 50) / 10];grade += (score % 10 > 7) ? "+" : ((score % 10 < 3 && score != 100) ? "-" : "");cout << "The grade is " << grade << endl;}return 0;
}

虽然比5.5简洁，但可读性大大下降。

Exercise 5.9

#include <iostream>
using namespace std;int main()
{char ch;unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0;while (cin >> ch){if (ch == 'a') aCnt++;if (ch == 'e') eCnt++;if (ch == 'i') iCnt++;if (ch == 'o') oCnt++;if (ch == 'u') uCnt++;}cout << "The number of 'a' is " << aCnt << ".\n";cout << "The number of 'e' is " << eCnt << ".\n";cout << "The number of 'i' is " << iCnt << ".\n";cout << "The number of 'o' is " << oCnt << ".\n";cout << "The number of 'u' is " << uCnt << ".\n";return 0;
}

Exercise 5.10

Way I:

#include <iostream>
using namespace std;int main()
{char ch;unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0;while (cin >> ch){switch (ch){case 'a': case 'A':++aCnt;break;case 'e': case 'E':++eCnt;break;case 'i': case 'I':++iCnt;break;case 'o': case 'O':++oCnt;break;case 'u': case 'U':++uCnt;break;}}cout << "The number of 'a' is " << aCnt << ".\n";cout << "The number of 'e' is " << eCnt << ".\n";cout << "The number of 'i' is " << iCnt << ".\n";cout << "The number of 'o' is " << oCnt << ".\n";cout << "The number of 'u' is " << uCnt << ".\n";return 0;
}

Way II:

#include <iostream>
using namespace std;int main()
{char ch;unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0;while (cin >> ch){ch = tolower(ch);switch (ch){case 'a':++aCnt;break;case 'e':++eCnt;break;case 'i':++iCnt;break;case 'o':++oCnt;break;case 'u':++uCnt;break;}}cout << "The number of 'a' is " << aCnt << ".\n";cout << "The number of 'e' is " << eCnt << ".\n";cout << "The number of 'i' is " << iCnt << ".\n";cout << "The number of 'o' is " << oCnt << ".\n";cout << "The number of 'u' is " << uCnt << ".\n";return 0;
}

Exercise 5.11

注：此处若不使用 Line 8 的 noskipws 将无法达到目的。

#include <iostream>
using namespace std;int main()
{char ch;unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0, spaceCnt = 0, tabCnt = 0, newlineCnt = 0;while (cin >> noskipws >> ch) // 'noskipws' means 'cin' doesn't omit space.{ch = tolower(ch);switch (ch){case 'a':++aCnt;break;case 'e':++eCnt;break;case 'i':++iCnt;break;case 'o':++oCnt;break;case 'u':++uCnt;break;case ' ':++spaceCnt;break;case '\t':++tabCnt;break;case '\n':++newlineCnt;break;}}cout << "The number of 'a' is " << aCnt << ".\n";cout << "The number of 'e' is " << eCnt << ".\n";cout << "The number of 'i' is " << iCnt << ".\n";cout << "The number of 'o' is " << oCnt << ".\n";cout << "The number of 'u' is " << uCnt << ".\n";cout << "The number of Space is " << spaceCnt << ".\n";cout << "The number of Tab is " << tabCnt << ".\n";cout << "The number of Newline is " << newlineCnt << ".\n";return 0;
}

noskipws 详见 C++ Primer 17.5.1 (English Version Page 760)

Exercise 5.12

同样要用 noskipws。

#include <iostream>
using namespace std;int main()
{char ch;char last_char = '0';unsigned ffCnt = 0, flCnt = 0, fiCnt = 0;while (cin >> noskipws >> ch) // 'noskipws' means 'cin' doesn't omit space.{if (last_char == 'f'){switch (ch){case 'f':++ffCnt;break;case 'l':++flCnt;break;case 'i':++fiCnt;break;}}last_char = ch;}cout << "The number of 'ff' is " << ffCnt << ".\n";cout << "The number of 'fl' is " << flCnt << ".\n";cout << "The number of 'fi' is " << fiCnt << ".\n";return 0;
}

Exercise 5.14

程序及启示见我的博客 调试 C++ Primer Exercise 5.14 的心得体会。

Exercise 5.17

开头部分解释详见我的博客 关于 C++中 输入多行不定数量数字 的思考。

#include <iostream>
#include <vector>
#include <string>
#include <sstream> // std::istringstream
using namespace std;int main()
{vector<int> vint1, vint2;string num1, num2;cout << " First vector:";getline(cin, num1);istringstream is1(num1);int i;while (is1 >> i)vint1.push_back(i);cout << "Second vector:";getline(cin, num2);istringstream is2(num2);int j;while (is2 >> j)vint2.push_back(j);bool result = 0;if (vint1 == vint2) result = 1;else{if (vint1.size() == vint2.size()) result = 0;else{if (vint1.size() > vint2.size()) exchange(vint1, vint2); // now vint1 is shorter than vint2unsigned s = 0;for (auto k = 0; k != vint1.size(); k++){if (vint1[k] == vint2[k]) ++s;}if (s == vint1.size()) result = 1;}}if (!result)cout << "false" << endl;else cout << "true" << endl;return 0;
}

或者使用头文件 cinvec.h，见 关于 C++中 输入多行不定数量数字 的思考。
关于编写头文件函数的一些心得见 调试 cinvec.h （C++头文件定义函数）的心得体会。

#include <iostream>
#include <vector>
#include <string>
#include "cinvec.h"
using namespace std;int main()
{string num1, num2;cout << " First vector:";vector<int>vint1 = cinvec_int(num1);cout << "Second vector:";vector<int>vint2 = cinvec_int(num2);bool result = 0;if (vint1 == vint2) result = 1;else{if (vint1.size() == vint2.size()) result = 0;else{if (vint1.size() > vint2.size()) exchange(vint1, vint2); // now vint1 is shorter than vint2unsigned s = 0;for (auto k = 0; k != vint1.size(); k++){if (vint1[k] == vint2[k]) ++s;}if (s == vint1.size()) result = 1;}}if (!result)cout << "false" << endl;else cout << "true" << endl;return 0;
}

Exercise 5.19

#include <iostream>
#include <string>
using namespace std;int main()
{string rsp;do{string str1, str2;cout << "Please enter two strings: ";cin >> str1 >> str2;string result = (str1 == str2) ? ("equal to ") : ((str1 > str2) ? "greater than " : "smaller than ");cout << str1 << " is " << result << str2 << "\nMore? Enter yes(Y) or no(N): ";cin >> rsp;} while (!rsp.empty() && rsp[0] != 'n' && rsp[0] != 'N');return 0;
}

Exercise 5.20

#include <iostream>
#include <string>
using namespace std;int main()
{string word_now;string word_last = "";string word_final = "";while (cin >> word_now){if (word_now == word_last){word_final = word_now;break;}else word_last = word_now;}if (word_final.empty()) cout << "No word was repeated." << endl;else cout << word_final << " repeats twice." << endl;return 0;
}

Exercise 5.21

#include <iostream>
#include <string>
using namespace std;int main()
{string word_now;string word_last = "";string word_final = "";while (cin >> word_now){if (islower(word_now[0]) || !isalpha(word_now[0])){word_last = word_now;continue;}if (word_now == word_last){word_final = word_now;break;}else word_last = word_now;}if (word_final.empty()) cout << "No word was repeated." << endl;else cout << word_final << " repeats twice." << endl;return 0;
}

Exercise 5.22

do
{int sz = get_size();
}
while (sz <= 0);

Exercise 5.23

#include <iostream>
using namespace std;int main()
{int a1, a2;cin >> a1 >> a2;double quotient = static_cast<double>(a1) / a2;cout << quotient << endl;return 0;
}

---

Example 1:

6 2

Output:

3

---

Example 2:

5 3

Output:

1.66667

---

Example 3:

2 0

Output:

inf

---

Example 4:

0 0

Output:

-nan(ind)

---

Example 5:

-3 0

Output:

-inf

---

Exercise 5.24

Exercise 5.25

#include <iostream>
#include <stdexcept>
#include <string>
using namespace std;int main()
{int a1, a2;while (cin >> a1 >> a2){try{if (a2 == 0) throw runtime_error("Divisor cannot be zero!");double quotient = static_cast<double>(a1) / a2;cout << quotient << endl;}catch (runtime_error err){cout << err.what() << "\nTry Again? Enter yes(Y) or no(N)." << endl;string c;cin >> c;if (!cin || c[0] == 'n' || c[0] == 'N')break;}}return 0;
}

Input & Output:

Next Chapter

【C++ Primer(5th Edition) Exercise】练习程序 - Chapter6（第六章）