C++Primer第五版 第五章习题答案

练习5.1

      空语句：最简单的语句，空语句只有一个单独的分号。当程序中在语法上需要一条语句但是逻辑上却不需要时，此时应该使用空语句。

//不断的读取输入流的内容，直到输入中遇到s。
while (cin >> a && a != s)
    ; //使用空语句时，应该加上注释，使得代码阅读者知道语句是有意义的。

 

练习5.2

      块(也称复合语句)是指用花括号括起来的(可能为空的)语句和声明序列。

      当语言需要一个语句而程序逻辑需要多个语句时，可能会使用一个块。例如:

while (val <= 10)
{
    sum += val;
    ++val;
}

 

练习5.3

#include 
int main()
{
    int sum = 0, val = 1;
    while (val <= 10)
        sum += val, ++val;
    std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;
    // 改写之后的代码可读性降低了

    return 0;
}

 

练习5.4

(a) while (string::iterator iter != s.end()) { /* . . . */ }
// iter变量未初始化,且初始化应该在while外部，修改：
    std::string::iterator iter = s.begin();
    while (string::iterator iter != s.end()) { /* . . . */ }

(b) while (bool status = find(word)) { /* . . . */ }
        if (!status) { /* . . . */ }
// status声明在while的控制结构里，只能在while循环内部使用，if语句不在while块中，所以status应该在外部声明
// 赋值运算作为条件，通常应该加上括号。如果像这题这样没加，当find(word)返回false赋值给status时，循环将一直持续下去，修改:
    bool status;
    while ((status = find(word))) {/* ... */}
    if (!status) {/* ... */}

 

练习5.5

#include 
#include 
#include 

using std::vector;
using std::string;
using std::cout;
using std::endl;
using std::cin;

int main()
{
    vector scores = {"F", "D", "C", "B", "A", "A++"};

    int grade{0};
    while (cin >> grade) {
        string lettergrade;
        if (grade < 60)
            lettergrade = scores[0];
        else {
            lettergrade = scores[(grade - 50) / 10];
            if (grade != 100) {
                if (grade % 10 > 7)
                    lettergrade += "+";
                else if (grade % 10 < 3)
                    lettergrade += "-";
            }
        }

        cout << lettergrade << endl;
    }

    return 0;
}

 

练习5.6

#include 
#include 
#include 

using std::vector;
using std::string;
using std::cout;
using std::endl;
using std::cin;

int main()
{
    vector scores = {"F", "D", "C", "B", "A", "A++"};

    int grade{0};
    while (cin >> grade) {
        string lettergrade = grade < 60 ? scores[0] : scores[(grade - 50) / 10];
        lettergrade +=
            (grade == 100 || grade < 60)
                ? ""
                : (grade % 10 > 7) ? "+" : (grade % 10 < 3) ? "-" : "";
        cout << lettergrade << endl;
    }

    return 0;
}

 

练习5.7

(a) if (ival1 != ival2) 
        ival1 = ival2;                 // 少了分号
    else ival1 = ival2 = 0;
(b) if (ival < minval)
    {
        minval = ival;                 // 少了花括号
        occurs = 1;
    }
(c) int val;                           // val定义要在外部，才能在两个if语句中都使用它
    if (ival = get_value())
        cout << "ival = " << ival << endl;
    if (!ival)
        cout << "ival = 0\n";
(d) if (ival == 0)                     // 判断相等的是==，不是赋值的=
    ival = get_value();

 

练习5.8

      一个口语术语，用来指如何处理嵌套的if语句，在c++中，else总是与其最近的尚未匹配的if相匹配。

 

练习5.9

#include 

using std::cout;
using std::endl;
using std::cin;

int main()
{
    unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0;
    char ch;
    while (cin >> ch) {
        if (ch == 'a')
            ++aCnt;
        else if (ch == 'e')
            ++eCnt;
        else if (ch == 'i')
            ++iCnt;
        else if (ch == 'o')
            ++oCnt;
        else if (ch == 'u')
            ++uCnt;
    }
    cout << "Number of vowel a: \t" << aCnt << '\n' << "Number of vowel e: \t"
         << eCnt << '\n' << "Number of vowel i: \t" << iCnt << '\n'
         << "Number of vowel o: \t" << oCnt << '\n' << "Number of vowel u: \t"
         << uCnt << endl;

    return 0;
}

 

练习5.10

#include 
using std::cin;
using std::cout;
using std::endl;

int main()
{
    unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0;
    char ch;
    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 << "Number of vowel a(A): \t" << aCnt << '\n'
         << "Number of vowel e(E): \t" << eCnt << '\n'
         << "Number of vowel i(I): \t" << iCnt << '\n'
         << "Number of vowel o(O): \t" << oCnt << '\n'
         << "Number of vowel u(U): \t" << uCnt << endl;

    return 0;
}

 

练习5.11

#include 

using std::cin;
using std::cout;
using std::endl;

int main()
{
    unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0, spaceCnt = 0,
             tabCnt = 0, newLineCnt = 0;
    char ch;
    while (cin >> std::noskipws >> 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;
        case ' ':
            ++spaceCnt;
            break;
        case '\t':
            ++tabCnt;
            break;
        case '\n':
            ++newLineCnt;
            break;
        }

    cout << "Number of vowel a(A): \t" << aCnt << '\n'
         << "Number of vowel e(E): \t" << eCnt << '\n'
         << "Number of vowel i(I): \t" << iCnt << '\n'
         << "Number of vowel o(O): \t" << oCnt << '\n'
         << "Number of vowel u(U): \t" << uCnt << '\n' << "Number of space: \t"
         << spaceCnt << '\n' << "Number of tab char: \t" << tabCnt << '\n'
         << "Number of new line: \t" << newLineCnt << endl;

    return 0;
}

 

练习5.12

#include 

using std::cin;
using std::cout;
using std::endl;

int main()
{
    unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0, spaceCnt = 0,
             tabCnt = 0, newLineCnt = 0, ffCnt = 0, flCnt = 0, fiCnt = 0;
    char ch, prech = '\0';
    while (cin >> std::noskipws >> ch) {
        switch (ch) {
        case 'a':
        case 'A':
            ++aCnt;
            break;
        case 'e':
        case 'E':
            ++eCnt;
            break;
        case 'i':
            if (prech == 'f') ++fiCnt;
        case 'I':
            ++iCnt;
            break;
        case 'o':
        case 'O':
            ++oCnt;
            break;
        case 'u':
        case 'U':
            ++uCnt;
            break;
        case ' ':
            ++spaceCnt;
            break;
        case '\t':
            ++tabCnt;
            break;
        case '\n':
            ++newLineCnt;
            break;
        case 'f':
            if (prech == 'f') ++ffCnt;
            break;
        case 'l':
            if (prech == 'f') ++flCnt;
            break;
        }
        prech = ch;
    }

    cout << "Number of vowel a(A): \t" << aCnt << '\n'
         << "Number of vowel e(E): \t" << eCnt << '\n'
         << "Number of vowel i(I): \t" << iCnt << '\n'
         << "Number of vowel o(O): \t" << oCnt << '\n'
         << "Number of vowel u(U): \t" << uCnt << '\n' << "Number of space: \t"
         << spaceCnt << '\n' << "Number of tab char: \t" << tabCnt << '\n'
         << "Number of new line: \t" << newLineCnt << '\n' << "Number of ff: \t"
         << ffCnt << '\n' << "Number of fl: \t" << flCnt << '\n'
         << "Number of fi: \t" << fiCnt << endl;

    return 0;
}

 

练习5.13

      case关键字和它对应的值一起被称为case标签，case标签必须是整型常量表达式。

(a) // 应该有break语句
    unsigned aCnt = 0, eCnt = 0, iouCnt = 0;
    char ch = next_text();
    switch (ch) {
        case 'a': aCnt++; break;
        case 'e': eCnt++; break;
        default : iouCnt++; break;
    }
(b) // 如果在某处一个带有初值的变量位于作用域之外，在另一处该变量位于作用域之内，则从前一处跳到后一处是非法行为
    // 这里ix定义不可以放在switch作用域内
    unsigned index = some_value();
    int ix;
    switch (index) {
        case 1:
            ix = get_value();
            ivec[ ix ] = index;
            break;
        default:
            ix = static_cast(ivec.size())-1;
            ivec[ ix ] = index;
    }
(c) // case标签语法错误
    unsigned evenCnt = 0, oddCnt = 0;
    int digit = get_num() % 10;
    switch (digit) {
        case 1: case 3: case 5: case 7: case 9:
            oddcnt++;
            break;
        case 2: case 4: case 6: case 8: case 0:
            evencnt++;
            break;
    }
(d) // case标签必须是一个常量表达式
    const unsigned ival=512, jval=1024, kval=4096;
    unsigned bufsize;
    unsigned swt = get_bufCnt();
    switch(swt) {
        case ival:
            bufsize = ival * sizeof(int);
            break;
        case jval:
            bufsize = jval * sizeof(int);
            break;
        case kval:
            bufsize = kval * sizeof(int);
            break;
    }

 

练习5.14

#include 
#include 

using std::string;
using std::cin;
using std::cout;
using std::endl;

int main()
{
    string pre_word, word, max_repeat_word;
    int repeat_times = 0, max_repeat_times = 0;
    
    while (cin >> word) {
        if (word == pre_word) {
            ++repeat_times;
        } else {
            repeat_times = 1;
            pre_word = word;
        }
        
        if (max_repeat_times < repeat_times) {
            max_repeat_times = repeat_times;
            max_repeat_word = pre_word;
        }
    }
    
    if (max_repeat_times <= 1){
        cout << "no word was repeated" << endl;
    } else {
        cout << "the word '" << max_repeat_word << "' occurred " << max_repeat_times << " times" << endl;
    }
}

 

练习5.15

(a) // ix在for循环的语句头中定义，if语句在for循环的语句块外，所以ix时未定义的
    int ix;
    for (ix = 0; ix != sz; ++ix)  { /* ... */ }
    if (ix != sz)

(b) // for循环语句头中某些部分可以省略，但是分号不能省略
    int ix;
    for (; ix != sz; ++ix) { /* ... */ }

(c) // ix和sz同时加1，循环永远也结束不了
    for (int ix = 0; ix != sz; ++ix) { /*...*/ }

 

练习5.16

// while循环
int i;
while ( cin >> i )
    // ...

// 等同的for循环
for (int i = 0; cin >> i;)
    // ...

// for循环
for (int i = 0; i != size; ++i)
    // ...

// 等同的while循环
int i = 0;
while (i != size)
{
    // ...
    ++i;
}

 

练习5.17

#include 
#include 

using std::cout;
using std::endl;
using std::vector;

int main()
{
    vector vec1{0, 1, 1, 2};
    vector vec2{0, 1, 1, 2, 3, 5, 8};

    auto size = vec1.size() < vec2.size() ? vec1.size() : vec2.size();
    for (decltype(vec1.size()) i = 0; i != size; ++i) {
        if (vec1[i] != vec2[i]) {
            cout << "false" << endl;
            return 0;
        }
    }
    cout << "true" << endl;
    return 0;
}

 

练习5.18

(a) // 添加括号
    do {
        int v1, v2;
        cout << "Please enter two numbers to sum:" ;
        if (cin >> v1 >> v2)
            cout << "Sum is: " << v1 + v2 << endl;
    }while (cin);
(b) int ival;
    do {
        // . . .
    } while (ival = get_response());      // 不应在此范围内声明
(c) int ival = get_response();
    do {
        ival = get_response();            // ival不应在此范围内声明
    } while (ival);

 

练习5.19

#include 
#include 

using std::cout;
using std::cin;
using std::endl;
using std::string;

int main()
{
    string rsp;
    do {
        cout << " 输入两个字符串: ";
        string str1, str2;
        cin >> str1 >> str2;
        cout << (str1 <= str2 ? str1 : str2) << " 比另一个小。 "
             << "\n\n"
             << " 再一次? 输入yes或者no: ";
        cin >> rsp;
    } while (!rsp.empty() && rsp[0] == 'y');
    return 0;
}

 

练习5.20

#include 
#include 

using std::cout;
using std::cin;
using std::endl;
using std::string;

int main()
{
    string read, tmp;
    while (cin >> read)
        if (read == tmp)
            break;
        else
            tmp = read;

    if (cin.eof())
        cout << "一个单词也没有重复." << endl;
    else
        cout << read << "连续出现两次." << endl;
    return 0;
}

 

练习5.21

#include 
using std::cin;
using std::cout;
using std::endl;

#include 
using std::string;

int main() {
  string curr, prev;
  bool no_twice = false;

  while (cin >> curr) {
    if (!isupper(curr[0])) {
      prev = "";
      continue;
    }
    if (prev == curr) {
      cout << curr << " 连续出现两次。" << endl;
      no_twice = true;
      break;
    }
    prev = curr;
  }

  if (!no_twice) cout << " 没有单词重复 " << endl;
}

 

练习5.22

for (int sz = get_size(); sz <=0; sz = get_size())
    ; // 空语句

 

练习5.23

#include 

int main(void)
{
    int a, b;
    std::cin >> a >> b;
    std::cout << static_cast(a) / b << std::endl;

    return 0;
}

 

练习5.24

#include 
#include 

int main(void)
{
    int a, b;
    std::cin >> a >> b;

    if (b == 0) throw std::runtime_error("divisor is 0");

    std::cout << static_cast(a) / b << std::endl;

    return 0;
}

 

练习5.25

#include 
#include 
using std::cin;
using std::cout;
using std::endl;
using std::runtime_error;

int main(void)
{
    int a, b;
    cout << "Input two integers: ";
    while (cin >> a >> b) {
        try {
            if (b == 0) throw runtime_error("divisor is 0");
            cout << static_cast(a) / b << endl;
            cout << " 输入两个整数: ";
        }
        catch (runtime_error err) {
            cout << err.what() ;
            cout << "\n再试一次? 输入 y 或 n:" << endl;
            char c;
            cin >> c;
            if (!cin || c == 'n')
                break;
            else
                cout << " 输入两个整数: ";
        }
    }

    return 0;
}