C++ Primer课后习题解答（第三章）

Exercises Section 3.2.2

Ex3.2

// 一次读取一行
#include<iostream>
#include<string>

using namespace std;

int main()
{
    string line;

    while (getline(cin, line))
    {
        cout << line << endl;
    }
    system("pause");
    return 0;
}

// 一次读取一个 word
#include<iostream>
#include<string>

using namespace std;

int main()
{
    string word;

    while (cin >> word)
    {
        cout << word << endl;
    }
    system("pause");
    return 0;
}

Ex3.3

在 string 输入操作中， whitespace characters 被忽略；在  getline 函数中，whitespace characters 正常输出。

Ex3.4

#include<iostream>
#include<string>

using namespace std;

int main()
{
    string s1;
    string s2;
    
    cout << "Please enter two string: " << endl;
    getline(cin, s1);
    getline(cin, s2);
    //比较 s1 和 s2 的大小
    if (s1 == s2)       // s1 和 s2 的长度和内容都相等
    {
        cout << "s1 and s2 are equal" << endl;
    }
    else if (s1 < s2)
    {
        cout << "The larger string is s2" << endl;
    }
    else 
    {
        cout << "The larger string is s1" << endl;
    }

    // 比较 s1 和 s2 长度的大小
    if (s1.size() == s2.size())
    {
        cout << "The length of s1 equals the length of s2" << endl;
    }
    else if (s1.size() < s2.size())
    {
        cout << "s2 is longer" << endl;
    }
    else 
    {
        cout << "s1 is longer" << endl;
    }
    system("pause");
    return 0;
}

Ex3.5

// 拼接 string 
#include<iostream>
#include<string>

using namespace std;

int main()
{
    string s;
    string result = "";

    while (getline(cin, s))
    {
        result += s;
    }
    cout << result << endl;
    system("pause");
    return 0;
}

// 分别输出 string
#include<iostream>
#include<string>
#include<vector>

using namespace std;

int main()
{
    string s;
    vector<string> result;
    while (getline(cin, s))
    {
        result.push_back(s);
    }

    for (string i : result)
    {
        cout << i << " ";
    }
    cout << endl;
    system("pause");
    return 0;
}

Exercises Section 3.2.3

Ex3.6

#include<iostream>
#include<string>

using namespace std;

int main()
{
    string s;
    getline(cin, s);
    for (auto &c : s)
    {
        c = 'X';
    }
    cout << s << endl;
    system("pause");
    return 0;
}

Ex3.7

不能得到预期的结果，这是因为首先复制了 string 的一个副本，只是对其副本进行了修改。

Ex3.8

#include<iostream>
#include<string>

using namespace std;

int main()
{
    string s;
    getline(cin, s);
    auto it = s.begin();
    while (it != s.end())
    {
        (*it) = 'X';
        ++it;
    }
    cout << s << endl;
    system("pause");
    return 0;
}

Ex3.9

    string s;
    cout << s[0] << endl;

输出 string s 的第一个字符；有效

Ex3.10

#include<iostream>
#include<string>

using namespace std;

int main()
{
    string s;
    getline(cin, s);
    for (auto &c : s)
    {
        if (!ispunct(c))
        {
            cout << c;
        }
    }
    cout << endl;
    system("pause");
    return 0;
}

Ex3.11

    const string s = "Keep out!";
    for (auto &c : s){ /* ... */ }

合法；c 的类型为 const char&

Exercises Section 3.3.1

Ex3.12

a) vector<vector<int>> ivec;	// 合法；ivec 是一个 vector，里面的元素是 存储 int 类型的 vector
b) vector<string> svec = ivec;	// 不合法； 利用一个 vector 给另外一个 vector 赋值，必须保证两个 vector 内的元素类型一样
c) vector<string> svec(10, "null");		// 合法；svec是一个 vector，有 10 个元素，每个元素为字符串值"null" 

Ex3.13

a) vector<int> v1;	// 没有元素
b) vector<int> v2(10);	// 10 个元素，每个元素值为 0
c) vector<int> v3(10, 42);	// 10 个元素，每个元素值为 42
d) vector<int> v4{10};		// 1 个元素，值为 10
e) vector<int> v5{10, 42};	// 2 个元素，值为 10 和 42
f) vector<string> v6{10};	// 10 个元素，每个元素值为 ""
g) vector<string> v7{10, "hi"};	// 10 个元素，每个元素值为 "hi"

Exercises Section 3.3.2

Ex3.14

#include<iostream>
#include<vector>
using namespace std;

int main()
{
    int value;
    vector<int> result;
    while (cin >> value)
    {
        result.push_back(value);
    }
    for (auto i : result)
    {
        cout << i << " ";
    }
    cout << endl;
    system("pause");
    return 0;
}

Ex3.15

#include<iostream>
#include<vector>
#include<string>
using namespace std;

int main()
{
    string value;
    vector<string> result;
    while (getline(cin, value))
    {
        result.push_back(value);
    }
    for (auto s : result)
    {
        cout << s << " ";
    }
    cout << endl;
    system("pause");
    return 0;
}

Exercises Section 3.3.3

Ex3.17

#include<iostream>
#include<vector>
#include<string>

using namespace std;

int main()
{
    string word;
    vector<string> result;

    while (cin >> word)
    {
        result.push_back(word);
    }

    for (auto & s : result)
    {
        for (auto &c : s)
        {
            c = toupper(c);
        }
    }

    for (auto i = 0; i != result.size(); ++i)
    {
        if (i % 8 != 0)
            cout << result[i] << " ";
        else 
        {
            cout << endl;
            cout << result[i] << " ";
        }
    }
    cout << endl;
    system("pause");
    return 0;
}

Ex3.18

vector<int> ivec;
   ivec[0] = 42;

不合法；修改如下：

vector<int> ivec;
ivec.push_back(42);

Ex3.19

vector<int> v1 = {42, 42, 42, 42, 42, 42, 42, 42, 42, 42};
vector<int> v2(10, 42);
vector<int> v3 = v2;

Ex3.20

#include<iostream>
#include<vector>

using namespace std;

int main()
{
    int value;
    vector<int> result;
    int sum = 0;
    while (cin >> value)
    {
        result.push_back(value);
    }

    for (auto i = result.begin(), j = i + 1; i != result.end(); ++i)
    {
            sum = (*i) + (*j);
            cout << sum << " ";
    }
    cout << endl;

    for (auto i = result.begin(), j = result.end() - 1; i <= j; ++i, --j)
    {
        if (i != j)
        {
            sum = (*i) + (*j);
            cout << sum << " ";
        }
        else 
        {
            sum = (*i);
            cout << sum << " ";
        }
    }
    cout << endl;
    
    system("pause");
    return 0;
}

Exercises Section 3.4.1

Ex3.23

#include<iostream>
#include<vector>

using namespace std;

int main()
{
    int value;
    vector<int> result;

    for (int i = 0; i < 10; ++i)
    {
        cin >> value;
        result.push_back(2 * value);
    }

    for (auto item = result.begin(); item != result.end(); ++item)
    {
        cout << *item << " ";
    }
    cout << endl;

    system("pause");
    return 0;
}

Exercises Section 3.4.2

Ex3.26

mid = beg + (end - beg) / 2;	// 推荐
mid = (beg + end) / 2;		// 不推荐

原因：第二种可能造成整数溢出

Exercises Section 3.5.1

Ex3.27

unsigned buf_size = 1024;
a) int ia[buf_size];		// 非法；buf_size 不是一个常量
b) int ia[4 * 7 - 14];		// 合法
c) int ia[txt_size()];		// 如果 txt_size() 返回的是常量 int 类型则合法
d) char st[11] = "fundamental";		// 非法；最后的'\0'占一个字符，故而数组大小不够

Ex3.28

string sa[10];			// 元素值全为空字符串
int ia[10];				// 元素值全为0
int main()
{
	string sa2[10];		// 元素值全为空字符串
    int ia2[10];		// 元素值不确定，取决于编译器
}

Ex3.29

array 大小确定，而 vector 大小可以不确定。

Exercises Section 3.5.2

Ex3.30

    constexpr size_t array_size = 10;		
    int ia[array_size];
    for (size_t ix = 1; ix <= array_size; ++ix)		// 下标为0-9，没有 10 下标
        ia[ix] = ix;

Ex3.31

#include<iostream>

using namespace std;

int main()
{
    int a[10];

    for (int i = 0; i < 10; ++i)
    {
        a[i] = i;
    }

    for (int i = 0; i < 10; ++i)
    {
        cout << a[i] << " ";
    }
    cout << endl;

    system("pause");
    return 0;
}

Ex3.32

#include<iostream>
#include<vector>

using namespace std;

int main()
{
    int a[10], b[10];
    vector<int> s1;
    vector<int> s2;
    for (int i = 0; i < 10; ++i)
    {
        a[i] = i;
    }
    for (int i = 0; i < 10; ++i)
    {
        b[i] = a[i];
    }
    for (auto i = 0; i < 10; ++i)
    {
        s1.push_back(i);
    }
    for (vector<int>::const_iterator it = s1.begin(); it != s1.end(); ++it)
    {
        s2.push_back(*it);
    }
    for (vector<int>::const_iterator it = s2.begin(); it != s2.end(); ++it)
    {
        cout << *it << " ";
    }
    
    cout << endl;
    system("pause");
    return 0;
}

Ex3.33

未初始化的数组的元素值取决于编译器

Exercises Section 3.5.3

Ex3.34

p1 += p2 - p1;	// 将指针 p2 的值赋给指针 p1；如果 p1 被 const 修饰则非法

Ex3.35

#include<iostream>

using namespace std;

int main()
{
    int a[10];
    int *p = &a[10];

    for (int *it = a; it != p; ++it)
    {
        *it = 0;
    }
    for (int x : a)
    {
        cout << x << " ";
    }
    cout << endl;
    system("pause");
    return 0;
}

Ex3.36

// 比较数组是否相等
#include<iostream>

using namespace std;

int main()
{
    int arr1[10];
    int arr2[10];
    
    int size_arr1 = end(arr1) - begin(arr1);
    int size_arr2 = end(arr2) - begin(arr2);

    if (size_arr1 != size_arr2) {
        cout << "Differen. " << endl;
        return 0;
    }

    for (int i = 0; i != size_arr1; ++i) {
        if (arr1[i] != arr2[i]) {
            cout << "Different. " << endl;
            return 0;
        }
    }
    cout << "Same arr. " << endl;
    system("pause");
    return 0;
}

// 比较vector 是否相等
#include<iostream>
#include<vector>

using namespace std;

int main()
{
    vector<int> ivec1{1, 2, 3};
    vector<int> ivec2{1, 2, 3};
    if (ivec1 == ivec2) 
    {
        cout << "Same vector. " << endl;
    }
    else 
    {
        cout << "Different. " << endl;
    }
    system("pause");
    return 0;
}

Exercises Section 3.5.4

Ex3.37

#include<iostream>

using namespace std;

int main()
{
    const char ca[] = {'h', 'e', 'l', 'l', 'o'};
    const char *cp = ca;

    while (*cp)
    {
        cout << *cp << endl;
        ++cp;
    }
    system("pause");
    return 0;
}

Ex3.38

C++ 中没有指针加法

Ex3.39

// C char[]
#include<iostream>
#include<cstring>

using namespace std;

int main()
{
    const char ca1[] = "string-A";
    const char ca2[] = "string-B";
    if (strcmp(ca1 , ca2) > 0) 
    {
        cout << "string-A big" << endl;
    } 
    else 
    {
        cout << "string-B big" << endl;
    }
    system("pause");
    return 0;
}

// C++ string
#include<iostream>
#include<string>

using namespace std;

int main()
{
    string s1 = "string-A";
    string s2 = "string-B";
    cout << "Big one is: ";
    if (s1 > s2) 
    {
        cout << s1 << endl;
    } 
    else 
    {
        cout << s2 << endl;
    }
    system("pause");
    return 0;
}

Ex3.40

#include<cstring>
#include<iostream>

using namespace std;

int main()
{
    char s1[100] = "stringA";
    char s2[] = "stringB";
    strcat(s1 , s2);
    char s[100];
    strcpy(s , s1);
    cout << s << endl;
    system("pause");
    return 0;
}

Exercises Section 3.5.5

Ex3.41

#include<iostream>
#include<vector>

using namespace std;

int main()
{
    int value;
    vector<int> v1;
    int a[10];

    for (int i = 0; i < 10; ++i)
    {
        cin >> value;
        a[i] = value;
    }

    for (int i = 0; i < 10; ++i)
    {
        v1.push_back(a[i]);
    }

    for (auto i : v1)
    {
        cout << i << " ";
    }
    cout << endl;

    system("pause");
    return 0;
}

Ex3.42

#include<iostream>
#include<vector>

using namespace std;

int main()
{
    int value;
    vector<int> v;
    while (cin >> value)
    {
        v.push_back(value);
    }
    int a[v.size()];
    for (int i = 0; i < v.size(); ++i)
    {
        a[i] = v[i];
    }
    for (int i = 0; i < v.size(); ++i)
    {
        cout << a[i] << " ";
    }

    system("pause");
    return 0;
}