C++ Primer课后习题解答（第五章）

Exercises Section 5.1

Ex5.1

;		// null statement

Ex5.2

// block
{

}

Exercises Section 5.2

Ex5.4

a) while (string::iterator iter != s.end())  { /*... */ }	// 未初始化 iter 的值
// 修改如下
string::iterator iter = s.begin();
while (iter != s.end()) { /*... */ }

b) while (bool status = find(word)) { /*... */ }	
   if (!status)  { /*... */ }       // status 不能在 while 循环外获取
// 修改如下
while (bool status = find(word)) 
{ 
	if (!status)  { /*... */ } 
	/*... */ 
}

Exercises Section 5.3.1

Ex5.7

a) if (ival1 != ival2)
	   ival1 = ival2
   else ival1 = ival2 = 0;
// 修改如下
if (ival1 != ival2)
	ival1 = ival2;
else 
	ival1 = ival2 = 0;
    
b) if (ival < minval)
	   minval = ival;
       occurs = 1;     
// 修改如下
if (ival < minval)
	minval = ival;
	occurs += 1;
    
c) if (int ival = get_value())
	   cout << "ival = " << ival << endl;
   if (!ival)
   	   cout << "ival = 0\n";      
// 修改如下
int ival = get_value();
if (!ival)
	cout << "ival = 0\n";
else 
	cout << "ival = " << ival << endl;
    
d) if (ival = 0)
	   ival = get_value();
// 修改如下
if (ival == 0)
	ival = get_value();

Ex5.8

"dangling else"：当有多个 if 嵌套时，如果只有一个 else，那么该 else 在没有明确表示匹配哪个 if 时，会自动匹配离它最近的 if。

Exercises Section 5.3.2

Ex5.9

#include<iostream>
#include<string>

using namespace std;

int main()
{
    int a_count = 0;
    int e_count = 0;
    int i_count = 0;
    int o_count = 0;
    int u_count = 0;
    string s = "";
    cout << "Please enter a string: " << endl;
    getline(cin, s);

    for (char c : s)
    {
        if (c == 'a' || c == 'A')
            ++a_count;
        if (c == 'e' || c == 'E') 
            ++e_count;
        if (c == 'i' || c == 'I')
            ++i_count;
        if (c == 'o' || c == 'O')
            ++o_count;
        if (c == 'u' || c == 'U')
            ++u_count;
    }
    cout << a_count << " " << e_count << " " << i_count << " "  << o_count << " " << u_count << endl;
    system("pause");
    return 0;
}

Ex5.10

#include<iostream>

using namespace std;

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: \t" << aCnt << endl;
    cout << "Number of vowel e: \t" << eCnt << endl;
    cout << "Number of vowel i: \t" << iCnt << endl;
    cout << "Number of vowel o: \t" << oCnt << endl;
    cout << "Number of vowel u: \t" << uCnt << endl;
    system("pause");
    return 0;
}

Ex5.11

#include<iostream>

using namespace std;

int main()
{
    unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0, blackCnt = 0;
    char ch;
    while ((ch = cin.get()) != EOF)     // cin 会自动忽略所有空白字符，故而使用cin.get()
    {
        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 ' ': case '\t': case '\n':
                ++blackCnt;
                break;
        }
    }
    cout << "Number of vowel a: \t" << aCnt << endl;
    cout << "Number of vowel e: \t" << eCnt << endl;
    cout << "Number of vowel i: \t" << iCnt << endl;
    cout << "Number of vowel o: \t" << oCnt << endl;
    cout << "Number of vowel u: \t" << uCnt << endl;
    cout << "Number of blank spaces, tabs and newlines: \t" << blackCnt << endl;
    system("pause");
    return 0;
}

Ex5.13

a) 
	unsigned aCnt = 0, eCnt = 0, iouCnt = 0;
    char ch = next_text();
    switch(ch) 			// 缺少 break
    {
        case 'a':   aCnt++;
        case 'e':   eCnt++;
        default:    iouCnt++;
    }
// 修改如下
	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)  
	unsigned index = some_value();
    switch(index) 
    {
        case 1:
            int ix = get_value();
            ivec[ix] = index;
            break;
        default:
            ix = ivec.size() - 1;   // 上个 case 初始化的变量不能在下一个case 中使用
            ivec[ix] = index;
    }
	// 修改如下
	unsigned index = some_value();
    switch(index) 
 	{
    	case 1:
        	int ix = get_value();
        	ivec[ix] = index;
        	break;
    	default:
        	int ix = ivec.size() - 1;   
        	ivec[ix] = index;
        	break;
    	}
    
c) 
    unsigned evenCnt = 0, oddCnt = 0;
    int digit = get_num() % 10;
    switch(digit)
    {
        case 1, 3, 5, 7, 9:		// 必须分开写，不能使用逗号
            oddCnt++;
            break;
        case 2, 4, 6, 8, 10:
            evenCnt++;
            break;
    }
// 修改如下
	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 10:
            evenCnt++;
            break;
    }

d)
	unsigned ival = 512, jval = 1024, kval = 4096;
    unsigned bufsize;
    unsigned swt = get_bufCnt();
    switch(swt)
    {
        case ival:				// case 后面不能跟变量
            bufsize = ival + sizeof(int);
            break;
        case jval:
            bufsize = jval + sizeof(int);
            break;
        case kval:
            bufsize = kval + sizeof(int);
            break;
    }
// 修改如下
    unsigned ival = 512, jval = 1024, kval = 4096;
    unsigned bufsize;
    unsigned swt = get_bufCnt();
    switch(swt)
    {
        case 512:
            bufsize = ival + sizeof(int);
            break;
        case 1024:
            bufsize = jval + sizeof(int);
            break;
        case 4096:
            bufsize = kval + sizeof(int);
            break;
    }

Exercises Section 5.4.1

Ex5.14

#include<iostream>
#include<string>

using namespace std;

int main()
{
    int max_cnt = 0, cnt = 0;
    string max_word, word, front_word;
    while (cin >> word)
    {
        if (front_word.empty())
        {
            max_word = word;
            front_word = word;
            
            ++cnt;
            max_cnt = cnt;
        }
        else 
        {
            if (front_word == word)
            {
                ++cnt;
                front_word = word;
                if (max_cnt < cnt)
                {
                    max_cnt = cnt;
                    max_word = word;
                }
            }
            else 
            {
                front_word = word;
                cnt = 1;
            }
        }
    }
    if (max_cnt > 1)
        cout << "The " << max_word << " occurred " << max_cnt << " times" << endl;
    else 
        cout << "No word occurred more than two times." << endl;
    system("pause");
    return 0;
}

Exercises Section 5.4.2

Ex5.15

a)
	for (int ix = 0; ix != sz; ++ix) { /*... */ }
    if (ix != sz)	// ix 不能在 for 循环外使用
    	// ...
// 修改如下
	int ix = 0;
    for (; ix != sz; ++ix) { /* ... */ }
    if (ix != sz)
    	// ...

b)
	int ix;
    for (ix != sz; ++ix)  { /*... */ }	// for 循环少了一条语句且 ix 未被初始化
// 修改如下
	for (int ix = 0; ix != sz; ++ix) { /*... */ }

c)
	for (int ix = 0; ix != sz; ++ix, ++sz) { /*...*/ }	// for 循环永远不会停止
// 修改如下
	for (int ix = 0; ix != sz; ++ix) { /*...*/ }

Ex5.17

#include<iostream>
#include<vector>

using namespace std;

int main()
{
    vector<int> v1,v2;
    int a=0,b=0;
    while(cin>>a)
       v1.push_back(a);
    cin.clear();      // 清除ctrl z 状态 让 cin >> b 能进行下去 
    while(cin>>b)
       v2.push_back(b);
    if(v1.size()<v2.size())
    {    
        int i=0;
        while (i != v1.size() && v1[i] == v2[i])
        {
            ++i;
        }

        if(i==v1.size())
        {
            cout<<" v1 is prefix of v2 "<<endl;
            cout<<" true "<<endl; 
        }
        else  
            cout<<" false "<<endl; 
    }
    else
    {
        int i=0;
        while (i != v2.size() && v1[i] == v2[i])
        {
            ++i;
        }

        if(i==v2.size())
        {
            cout<<"v2 is prefix of v1"<<endl; 
            cout<<" false "<<endl;
        }
        else
            cout<<" flase "<<endl; 
    }
    system("pause");
    return 0;
}

Exercises Section 5.4.4

Ex5.18

a) // 忘记花括号了；修正如下
	do
    {
    	int v1, v2;
        cout << "Please enter two numbers to sum:";
        if (cin >> v1 >> v2)
        	cout << "Sum is: " << v1 + v2 << endl;
    } while (cin);
   
b) // while 括号后面的变量必须是事先声明了的； 修改如下
	int ival = get_response();
	do
    {
    	// ...
    } while (ival);

c)	// ival 不能在 while 循环外使用；修改如下
	int ival = get_response();
	do 
    {
    } while (ival);

Ex5.19

#include<iostream>
#include<string>

using namespace std;

int main()
{
    do 
    {
        string s1, s2;
        cout << "Please enter two string :" << endl;
        if (cin >> s1 >> s2)
        {
            if (s1 > s2)
                cout << "s2 is less  than s1" << endl;
            else if (s1 < s2)
                cout << "s1 is less than s2" << endl;
            else 
                cout << "two string are equal" << endl;
        }
    } while (cin);
    system("pause");
    return 0;
}

Exercises Section 5.5.1

Ex5.20

#include<iostream>
#include<string>

using namespace std;

int main()
{
    string str, tmpstr;
    unsigned cnt = 0;
    cin >> str;
    tmpstr = str;
    while (cin >> str)
    {
        if (str == tmpstr)
        {
            ++cnt;
            cout << str << " occurs twice in succession" << endl;
            break;
        }
        tmpstr = str;
    }
    if (cnt == 0)
    {
        cout << "No word occurs twice in succession" << endl;
    }
    system("pause");
    return 0;
}

Exercises Section 5.5.2

Ex5.21

#include<iostream>
#include<string>

using namespace std;

int main()
{
    string str, tmpstr;
    unsigned cnt = 0;
    cin >> str;
    tmpstr = str;
    while (cin >> str)
    {
        if (str[0] >= 'A' && str[0] <= 'Z' && str == tmpstr)
        {
            ++cnt;
            cout << str << " occurs twice in succession" << endl;
            break;
        }
        tmpstr = str;
    }
    if (cnt == 0)
    {
        cout << "No word occurs twice in succession" << endl;
    }
    system("pause");
    return 0;
}

Exercises Section 5.5.3

Ex5.22

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

Exercises Section 5.6.3

Ex5.23

#include<iostream>

using namespace std;

int main()
{
    int v1, v2;
    cout << "Please enter two numbers: ";
    cin >> v1 >> v2;
    while (v2 == 0)
    {
        cout << "The second number is zero, please enter another value: ";
        cin >> v2;
    }
    cout << "The result is " << v1 / v2 << endl;
    system("pause");
    return 0;
}

Ex5.24

#include<iostream>

using namespace std;

int main()
{
    int v1, v2;
    cout << "Please enter two numbers: ";
    cin >> v1 >> v2;
    if (v2 != 0)
    {
        cout << "The result is " << v1 / v2 << endl;
        system("pause");
        return 0;
    }
    else 
    {
        cerr << "The second number is zero！" << endl;
        system("pause");
        return -1;
    }
}

Ex5.25

#include<iostream>

using namespace std;

int main()
{
    int v1, v2;
    cout << "Please enter two numbers: ";
    cin >> v1 >> v2;
    try
    {
        if (v2 == 0)
            throw runtime_error("The second number is zero!");

    }
    catch(runtime_error err)
    {
        cout << err.what() << " Please enter another value: ";
        cin >> v2;
    }
    cout << "The result is " << v1 / v2 << endl;
    system("pause");
    return 0;
}
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