C++Primer第五版习题答案详解(三)

第3章字符串、向量和数组

本章代码开头需要以下部分

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

练习3.2

int main()
{
  string line;
  //一次读入一行
	while (getline(cin, line))
		cout << line << endl;
  //上下两部分代码不能一起用
	string word;
  //一次读入一个词
	while (cin >> word)
		cout << word << endl;
	return 0;
}

练习3.3

输入运算符会自动忽略字符串前的空白（空格、制表符、换行等），从第一个真正的字符到下一个空白；
getline函数会保存字符串中的空白，读入数据直到遇见换行符，如果第一个就是换行符那就输出一个空string；

练习3.4

int main()
{
	string line1, line2;
	cin >> line1 >> line2;
	if (line1 == line2)
		cout << "Equal " << line1 << endl;
	else
	{
		if (line1 > line2)
			cout <<"Different "<< line1 << endl;
		else
			cout <<"Different "<< line2 << endl;
	}
	if(line1.size() == line2.size())
		cout << "Size equal " << line1 << endl;
	else
	{
		if (line1.size() > line2.size())
			cout << "Size different " << line1 << endl;
		else
			cout << "Size different " << line2 << endl;
	}
	return 0;
}

练习3.5

int main()
{
	string line, temp;
	while (getline(cin, temp)) {

		line += temp;//line += (temp +" ");
	}
	cout << line << endl;
	return 0;
}

练习3.6

int main()
{
	string str;
	getline(cin, str);
	for (auto &c : str)
		c = 'X';
	cout << str << endl;
}

练习3.7

无影响，因为输入的字符串内字符类型为char

练习3.8

都要先知道字符串的长度，相比较而言，范围for语句更加简洁一点

int main()
{
	string str;
	getline(cin, str);
	int len = str.size();
	//for (int i = 0; i < len; i++)
	//	str[i] = 'X';
	int i = 0;
	while (i<len)
	{
		str[i] = 'X';
		i++;
	}
	cout << str << endl;
}

练习3.9

不合法，使用超出范围的下标将引发不可预知的结果，由此推断，使用下标访问空string也会引发不可预知的结果

练习3.10

int main()
{
	string str, result;
	getline(cin, str);
	for (auto c : str) {
		if (ispunct(c))
			continue;
		else
			result += c;
	}
	cout << result << endl;
}

练习3.11

不合法，设置一个auto类型的引用时，初始值中的顶层const依旧保留，c的类型为const string&。如果在for循环中要改变c的值，则语句不合法；

练习3.12

1
2
3

vector<vector<int>> ivec;    //合法，创建类型为vector<int>的对象。
vector<string> svec = ivec;    //不合法，svec是string类型，而ivec是vector<int>类型。
vector<string>svec(10, "null");  //合法，创建10个string类型的元素，每个都被初始化为null

练习3.13

(a)vector<int> v1;                     //不包含元素
(b)vector<int> v2(10);                 //包含10个元素，每个都初始化为0
(c)vector<int> v3(10, 42);             // 包含10个元素，每个都初始化为42
(d)vector<int> v4{10};                 // 包含1个元素，值为10 
(e)vector<int> v5{10, 42};             // 包含2个元素，值分别是10和42
(f)vector<string> v6{10};              // 10个默认初始化的元素，初始化为空
(g)vector<string> v7{10, "hi"};        // 10个值为hi的元素

练习3.14,3.15

int main()
{
	vector<int> sum;
	int number;
	while (cin >> number) {
		sum.push_back(number);
	}
	vector<string> str;
	string word;
	while (cin >> word) {
		str.push_back(word);
	}
}

练习3.17

int main()
{
	vector<string> sen;
	string word;
	while (cin >> word) {
		sen.push_back(word);
	}
	for (auto &temp:sen)
		for (auto& str : temp) {
			str = toupper(str);
		}
	for (auto i : sen)
		cout << i << endl;
	return 0;
}

练习3.18

不合法，ivec为空，不包含任何元素，不能通过下标去访问元素

应改为 ivec.push_back(42)；或者给ivec加一个大小；

练习3.19

int main()
{
	vector<int> number(10,42);
	vector<int> number1(number);
	vector<int> number2{ 42,42,42,42,42,42,42,42,42,42 };
}

练习3.20

int main()
{
	vector<int> num;
	int i;
	while (cin >> i)
		num.push_back(i);
	for (i = 0; i < num.size()-1; i++)
		cout << num[i] + num[i + 1]<<" ";
  cout << endl;
	return 0;
}

int main()
{
	vector<int> num;
	int i;
	while (cin >> i)
		num.push_back(i);
	int size = num.size();
	for (i = 0; i <size; i++)
		cout << num[i] + num[size -1 - i]<<" ";
  cout << endl;
	return 0;	
}

练习3.23

int main()
{
	vector<int> res = { 1,2,3,4,5,6,7,8,9,10 };
	for (auto it = res.begin(); it != res.end(); it++) {
		*it = (*it) * 2;
	}
	for (auto i : res) {
		cout << i <<" ";
	}
  cout << endl;
	return 0;
}

练习3.24

int main()
{
	vector<int> text(10);
	for (int i = 0; i < 10; i++)
	{
		cin >> text[i];
	}
	for (auto vector_begin = text.begin(), vector_end = text.end(); vector_begin != vector_end-1; vector_begin++)			//这里减1是因为end指向text尾元素的下一位
		cout << *vector_begin+ * (vector_begin + 1) << endl;
  
	for (auto vector_begin = text.begin(), vector_end = text.end(); vector_begin != vector_end; vector_begin++)
	{
		vector_end--;
		cout << *vector_begin + *vector_end << endl;
	}
}

练习3.25

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

练习3.26

因为end指的是最后一个元素的后一个位置！！！

练习3.27

unsigned buf_size = 1024;
int ia[buf_size];                               //非法，buf_size不是常量
int ia[4*7-14];                                 // 合法
int ia[txt_size()];                             // 非法，函数返回值不是常量
char str[11] = "fundamental";         					// 非法，没有空间可放空字符

练习3.28

int main()
{
	string sa[10];
	int ia[10];
	for (auto i:ia)
		cout << i << " ";
		//cout << i << " ";
   cout << endl;
}

ia[10]中含有10个未知数，sa[10]中均为空对象

练习3.29

数组长度不可扩展，一旦定义，不能再更改长度，vector则比较灵活

练习3.30

constexpr size_t array_size = 10;
int ia[array_size];
for (size_t ix = 1; ix <= array_size; ix ++) 
{ 
  	// 下标越界，下标范围为0--9，而不是1--10
    ia[ix] = ix;
}

练习3.31

int main()
{
	int num[10]={};
	for (int i = 0; i < 10; i++)
		num[i] = i;
	for (auto j : num)
		cout << j << " ";
	cout << endl;
	return 0;
}

练习3.32

int main()
{
	int num[10]={};
	for (int i = 0; i < 10; i++)
		num[i] = i;
	//for (auto j : num)
	//	cout << j << " ";
	//return 0;
	int a[10];
	for (int i = 0; i < 10; i++)
		a[i] = num[i];
	for (auto j : a)
		cout << j << " ";
	cout << endl;
}

int main()
{
    vector<int> vec(10);
    for (int i = 0; i < 10; i++) 
    {
        vec[i] = i;
    }

    for (auto iter : vec) 
    {
        cout << iter << " ";
    }
    cout << endl;

    vector<int> vec1(vec);
    for (auto iter1 : vec1) {
        cout << iter1 << " ";
    }
    cout << endl;
}

练习3.33
不初始化scores，则其中内容未知，大概率输出结果为错误；

练习3.34

p1和p2是指针，相减为其地址相减，因为指向的是同一元素，地址相同，因此p1地址不变；

只要p1和p2合法，这个语句就是合法的；

练习3.35

int main()
{
	int a[10] = { };
	int* p = a;
	for (int i = 0; i < 10; i++)
	{
		*p = 0;
		p++;
	}
	for (auto i : a)
		cout << i << " ";
	cout << endl;
}

练习3.36

int main()
{
	int a[10] = { 0,1,2,3,4,5,6,7,8,9 };
	int b[10] = { 0,1,2,3,4,5,6,7,8,9 };
	int flag = 0;
	if (sizeof(a) == sizeof(b))
	{
		int len = sizeof(a) / sizeof(a[0]);
		for (int i = 0; i <len; i++)
			if (a[i] == b[i])
				flag++;
		if (flag == len)
			cout << "Equal" << endl;
		else
		{
			cout << "Not equal" << endl;
		}
	}
	else
	{
		cout << "Couldn't be comapred" << endl;
	}
}

vector的比较直接用等号就可以；

int main()
{
    vector<int> vec1 = { 1,2,3,4,5,6,7,8,9 };
    vector<int> vec2 = { 1,2,3,4,5,6,7,8,9 };
    if (vec1 == vec2) {
        cout << "equal" << endl;
    }
    else {
        cout << "not equal" << endl;
    }
}

练习3.37

int main()
{
    const char ca[] = { 'h', 'e', 'l', 'l', 'o' };
    const char* cp = ca;
    while (*cp) {
        cout << *cp << endl;
        ++cp;
    }
}
//ca初始化时，并未加‘\0’,因此其长度未知，while循环会一直继续知道遇见'\0'。输出“hello+未知字符”

练习3.38

两个指针相加，相当于两个地址值相加，比如0x849248 + 0x328134.并没有实际意义

练习3.39

int main()
{
    string str1 = "hello world";
    string str2 = "hello world";
    if (str1 == str2)
    {
        cout << str1 << " is equal with " << str2 << endl;
    }
    else
    {
        cout << str1 << " is not equal with " << str2 << endl;
    }
    char c1[] = { 'c','+','+','\0' };
    char c2[] = { 'c','+','+','\0' };
    if (strcmp(c1, c2))
    {
        cout << "c1>c2" << endl;
    }
    else if (strcmp(c1, c2) == 0)
    {
        cout << "c1=c2" << endl;
    }
    else
    {
        cout << "c1<c2" << endl;
    }
}

练习3.40

不知道为啥一直报错，逻辑上写的和网络上的方法是一样的

int main()
{
    char s1[] = { 'U','E','S','T','C','\0'};
    char s2[] = { 'I',' ','L','O','V','E',' ','\0' };
    char s3[100];
    strcpy(s3, s2);
    strcat(s3, " ");
    strcat(s3, s1);
    cout << s3 << endl;
}

练习3.41

int main()
{
    int num[10] = { 1, 3, 5, 7, 9, 2, 4, 6, 8, 10 };
    vector<int> vec(num, num + 10);
}

练习3.42

int main()
{
    vector<int> ivec = { 1, 3, 5, 7, 9, 2, 4, 6, 8, 10 };
    int c[10];

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

练习3.43

int main()
{
    int ia[3][4] = {
                    {0, 1, 2, 3},
                    {4, 5, 6, 7},
                    {8, 9, 10, 11}
    };

    // version1
    // 要用int引用，是因为防止数组被自动转化成指针。int row[4],就会变成int指针。
    for (const int(&row)[4] : ia) {
        for (int col : row)
            cout << col << " ";
    }
    cout << endl;

    // version 2
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++)
            cout << ia[i][j] << " ";
    }
    cout << endl;

    // version 3
    // p指向含有4个整数的数组，q指向p的首元素
    for (int(*p)[4] = ia; p != ia + 3; p++) {
        for (int* q = *p; q != *p + 4; q++)
            cout << *q << " ";
    }
    cout << endl;

}

练习3.44

记得在开头加上：

using std::begin;
using std::end;

int main()
{
    int ia[3][4] = {
                    {0, 1, 2, 3},
                    {4, 5, 6, 7},
                    {8, 9, 10, 11}
    };
    //使用别名
    using ci = const int[4];
    // version 1
    for (ci& row : ia) {
        for (int col : row) {
            cout << col << " ";
        }
    }
    cout << endl;

    // version 2
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++)
            cout << ia[i][j] << " ";
    }
    cout << endl;

    // version 3
    // 使用别名
    using ip = int[4];
    for (ip* p = begin(ia); p != end(ia); p++) {
        for (auto q = begin(*p); q != end(*p); q++) {
            cout << *q << " ";
        }
    }
    cout << endl;
}

练习3.45

int main()
{
    int ia[3][4] = {
                    {0, 1, 2, 3},
                    {4, 5, 6, 7},
                    {8, 9, 10, 11}
    };
    // version 1
    for (auto& row : ia) {
        for (auto col : row) {
            cout << col << " ";
        }
    }
    cout << endl;

    // version 2
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++)
            cout << ia[i][j] << " ";
    }
    cout << endl;

    // version 3
    // p指向含有4个整数的数组，q指向p的首元素
    for (auto p = begin(ia); p != end(ia); p++) {
        for (auto q = begin(*p); q != end(*p); q++) {
            cout << *q << " ";
        }
    }
    cout << endl;
}

C++Primer第五版习题答案详解(三)

第3章 字符串、向量和数组

第3章字符串、向量和数组