C++ Primer （第五版）-第三章 字符串、向量和数组

本章介绍了字符串、向量Vector、数组之间的关系，相通而且相互类似，加上指针迭代器的加持显得更加灵活，另外C++和C之间string的标准库的区别和相同也做了一些介绍，本章依然是基础干活满满。

一、概述

二、内容题干

3.1 命名空间using声明

using生命无须专门的前缀（命名空间：：）也能使用所需的名字。 using namespace::name;

  
#include 

// using declarations for names from the standard library
using std::cin;
using std::cout; using std::endl;

int main()
{
	cout << "Enter two numbers:" << endl;

	int v1, v2;
	cin >> v1 >> v2;

	cout << "The sum of " << v1 << " and " << v2
	     << " is " << v1 + v2 << endl;

	return 0;
}

3.2、 标准库string

   #inculude     using std::string;

#include 
using std::string;

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

int main()
{
    string s;          // empty string
    cin >> s;          // read a whitespace-separated string into s
    cout << s << endl; // write s to the output
    return 0;
}

#include 
using std::string;

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

int main()
{
	string s1, s2;

	cin >> s1 >> s2; // read first input into s1, second into s2
	cout << s1 << s2 << endl; // write both strings

	return 0;
}

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

#include  
using std::string;

int main() 
{
	string word;
	while (cin >> word)       // read until end-of-file 
		cout << word << endl; // write each word followed by a new line
	return 0;
}

 string 的empty 和size操作

#include  
using std::string; using std::getline;

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

int main() 
{
	string line;

	// read input a line at a time and discard blank lines
	while (getline(cin, line))
		if (!line.empty())
			cout << line << endl;

	return 0;
}

#include 
using std::string;

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

string st1;       // empty string
string st2(st1);  // st2 is a copy of st1

int main()
{
    string st("The expense of spirit\n");
    cout << "The size of " << st << "is " << st.size()
         << " characters, including the newline" << endl;
    return 0;
}

#include 
using std::size_t;

#include  
using std::string; using std::getline;

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

int main() 
{
	string line;

	// read input a line at a time and print lines that are longer than 80 characters
	while (getline(cin, line))
		if (line.size() > 80)
			cout << line << endl;

	return 0;
}

 string::size_type类型

比较string对象

   string相加

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

#include 
using std::string;

int main() 
{

	string s1  = "hello, ", s2 = "world\n";
	string s3 = s1 + s2;   // s3 is hello, world\n
	cout << s1 << s2 << s3 << endl;

	s1 += s2;   // equivalent to s1 = s1 + s2
	cout << s1;
	
	string s4 = "hello", s5 = "world";  // no punctuation in s4 or s2
	string s6 = s4 + ", " + s5 + '\n';
	cout << s4 << s5 << "\n" << s6 << endl;
	
	return 0;
}

    处理string对象中的字符

#include 
using std::string;

#include 
using std::isupper; using std::toupper;
using std::islower; using std::tolower;
using std::isalpha; using std::isspace;

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

int main()
{
	string s("Hello World!!!");
	// punct_cnt has the same type that s.size returns
	decltype(s.size()) punct_cnt = 0; 
	
	// count the number of punctuation characters in s
	for (auto c : s)         // for every char in s
		if (ispunct(c))      // if the character is punctuation
			++punct_cnt;     // increment the punctuation counter
	
	cout << punct_cnt 
	     << " punctuation characters in " << s << endl;
	
	// convert s to uppercase
	string orig = s;
	for (auto &c : s)   // for every char in s (note: c is a reference)
		// c is a reference, so this assignment changes the char in s
		c = toupper(c);
	cout << s << endl;
	
	// convert first word in s to uppercase
	s = orig;  // restore s to original case
	decltype(s.size()) index = 0;

	// process characters in s until we run out of characters 
	// or we hit a whitespace
	while (index != s.size() && !isspace(s[index])) {

	    // s[index] returns a reference so we can change 
		// the underlying character
	    s[index] = toupper(s[index]);

		// increment the index to look at the next character 
		// on the next iteration
		++index;
	}
	cout << s << endl;
	
	return 0;
}

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

#include 
using std::string;

#include 
using std::size_t;

int main()
{
	const string hexdigits = "0123456789ABCDEF";  // possible hex digits

	cout << "Enter a series of numbers between 0 and 15"
	     << " separated by spaces.  Hit ENTER when finished: " 
	     << endl;
	string result;        // will hold the resulting hexify'd string

	string::size_type n;  // hold numbers from the input
	while (cin >> n)
		if (n < hexdigits.size())    // ignore invalid input
			result += hexdigits[n];  // fetch the indicated hex digit

	cout << "Your hex number is: " << result << endl;

	return 0;
}

3.3、标准库vector

Vector属于类模板，模板本身不是类或者函数，相反可以看作是编译器生成类或者函数编写的一份说明，编译器根据模板创建类或者函数的过程成为实例化，编译器应指出类或者函数实例化为何种类型

      定义或者初始化vector对象

#include "Version_test.h"

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

#include 
using std::string;

#include 
using std::vector;

#include "Sales_item.h"

#ifndef LIST_INIT
#include 
using std::begin; using std::end;
#endif

int main()
{
#ifdef LIST_INIT
	// list initialization, articles has 3 elements
	vector<string> articles = {"a", "an", "the"};
#else
	string temp[] = {"a", "an", "the"};
	vector<string> articles(begin(temp), end(temp));
#endif

	vector<string> svec; // default initialization; svec has no elements
	vector<int> ivec;             // ivec holds objects of type int
	vector<Sales_item> Sales_vec; // holds Sales_items
	
	vector<vector<string>> file;  // vector whose elements are vectors
	vector<vector<int>> vecOfvec; // each element is itself a vector
	
	// all five vectors have size 0
	cout << svec.size() << " " << ivec.size() << " "
	     << Sales_vec.size() << " "
	     << file.size() << " " << vecOfvec.size() << endl;
	
	vector<int> ivec2(10);     // ten elements, each initialized to 0
	vector<int> ivec3(10, -1); // ten int elements, each initialized to -1
	vector<string> svec2(10);  // ten elements, each an empty string
	vector<string> svec3(10, "hi!"); // ten strings; each element is "hi!"
	cout << ivec2.size() << " " << ivec3.size() << " "
	     << svec2.size() << " " << svec3.size() << endl;
	
	// 10 is not a string, so cannot be list initialization
	vector<string> v1(10); // construct v1 with ten value-initialized elements
#ifdef LIST_INIT
	vector<string> v2{10}; // ten elements value-initialized elements
#else
	vector<string> v2(10); 
#endif
	vector<string> v3(10, "hi");  // ten elements with value "hi"
#ifdef LIST_INIT
	// again list initialization is not viable, so ordinary construction
	vector<string> v4{10, "hi"};  // ten elements with values "hi"
#else
	vector<string> v4(10, "hi");  // ten elements with values "hi"
#endif
	
	// all four vectors have size ten
	cout << v1.size() << " " << v2.size() 
	     << " " << v3.size() << " " << v4.size() << endl;
	
#ifdef LIST_INIT
	vector<string> vs1{"hi"}; // list initialization: vs1 has 1 element 
	vector<string> vs2{10};   // ten default-initialized elements
	vector<string> vs3{10, "hi"}; // has ten elements with value "hi"
#else
	vector<string> vs1;
	vs1.push_back("hi"); // explicitly add the element; vs1 has 1 element 
	vector<string> vs2(10); // don't use curlies; 
	                        // vs2 has ten default-initialized elements
	vector<string> vs3(10, "hi"); // don't use curlies; 
	                              // vs3 has ten elements with value "hi"
#endif
	cout << vs1.size() << " " << vs2.size() << " " << vs3.size() << endl;
	
	vector<int> v5(10, 1);  // ten elements with value 1
#ifdef LIST_INIT
	vector<int> v6{10, 1};  // two elements with values 10 and 1
#else
	vector<int> v6;
	v6.push_back(10);
	v6.push_back(1);
#endif
	cout << v5.size() << " " << v6.size() << endl;
	
#ifdef LIST_INIT
	// intention is clearer
	vector<int> alt_v3 = {10};    // one element with value 10
	vector<int> alt_v4 = {10, 1}; // two elements with values 10 and 1
#else
	vector<int> alt_v3;
	alt_v3.push_back(10);    // one element with value 10

	vector<int> alt_v4;
	alt_v4.push_back(10); 
	alt_v4.push_back(1); // two elements with values 10 and 1
#endif
	cout << alt_v3.size() << " " << alt_v4.size() << endl;

	return 0;
}

          列表初始化和元素数量

向 Vector对象添加元素

![在这里插入图片描述](https://img-blog.csdnimg.cn/462285fb6b554543ad9904b392c24baf.png

include "Version_test.h"

#include 
using std::string;

#include 
using std::vector;

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

#ifndef LIST_INIT
#include 
using std::begin; using std::end;
#endif

int main()
{
#ifdef LIST_INIT
	vector<int> v = {0,1,2,3,4,5,6,7,8,9};
#else
	int temp[] = {0,1,2,3,4,5,6,7,8,9};
	vector<int> v(begin(temp), end(temp));
#endif
	auto sz = v.size(); 
	decltype(sz) i = 0;
	// duplicate contents of v onto the back of v
	while (i != sz) {
		v.push_back(*v.begin() + i);
		++i;
	}
	// prints 0...9 0...9
	for (auto it : v)
		cout << it << " ";
	cout << endl;
	
#ifdef LIST_INIT
	// alternative way to stop when we get to the original last element
	vector<int> alt_v = {0,1,2,3,4,5,6,7,8,9}; // vector with values 0...9
#else
	vector<int> alt_v(begin(temp), end(temp)); // copy the array into alt_v
#endif
	for (decltype(alt_v.size()) i = 0, sz = alt_v.size(); i != sz; ++i)
		alt_v.push_back(alt_v[i]);

	// prints 0...9 0...9
	for (auto it : alt_v)
		cout << it << " ";
	cout << endl;

#ifdef LIST_INIT
	vector<int> v2 = {0,1,2,3,4,5,6,7,8,9}; // vector with values 0...9
#else
	vector<int> v2(begin(temp), end(temp)); 
#endif
	decltype(v2.size()) ix = 0;   // we'll use ix to index the vector

	// set the elements with values less than 5 to 0
	while (ix != v2.size() && v2[ix] < 5) {
		v2[ix] = 0; // changes the value of the element in v
		++ix;       // increment the index so the next iteration fetches the next element
	}

	// print the elements using subscripts
	for (unsigned i = 0; i != v2.size(); ++i)
		cout << v2[i] << " ";
	cout << endl;

#ifdef LIST_INIT
	// equivalent but using iterators
	vector<int> alt_v2 = {0,1,2,3,4,5,6,7,8,9}; // vector with values 0 ...9
#else
	vector<int> alt_v2(begin(temp), end(temp));
#endif
	// set the elements to 0 up to the first one that is 5 or greater
	auto it = alt_v2.begin();
	while (it != alt_v2.end() && *it < 5) {
		*it = 0;   // changes the value of the element in alt_v2
		++it;      // advance the iterator to denote the next element
	}
	for (auto it = alt_v2.begin(); // it denotes first element in alt_v2
	          it != alt_v2.end();  // so long as it denotes an element
	          ++it)          // increment the iterator to next element
		cout << *it << " ";  // print element denoted by it from alt_v2
	cout << endl;

	return 0;
}

#include 
using std::string;

#include 
using std::vector;

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

int main()
{
	// hold the grades we read from the standard input
	vector<unsigned> grades;

	// count the number of grades by clusters of ten: 
	// 0--9, 10--19, . ..  90--99, 100
	vector<unsigned> scores(11, 0); // 11 buckets, all initially 0
	unsigned grade;
	while (cin >> grade) {      // read the grades
		if (grade <= 100)       // handle only valid grades
			grades.push_back(grade);
			++scores[grade/10]; // increment the counter for the current cluster
	}
	cout << "grades.size = " << grades.size() << endl;
	for (auto it : grades)
		cout << it << " " ;
	cout << endl;

	cout << "scores.size = " << scores.size() << endl;
	for (auto it : scores)
		cout << it << " " ;
	cout << endl;

	// equivalent program using iterators instead of subscripts
	vector<unsigned> alt_scores(11, 0);  // 11 buckets, all initially 0
	// for each grade in the input
	for (auto it = grades.begin(); it != grades.end(); ++it) {
		unsigned i = *it;
		// increment the counter for the current cluster
		++(*(alt_scores.begin() + i/10));
	}

	cout << "alt_scores.size = " << alt_scores.size() << endl;
	for (auto it = alt_scores.begin(); it != alt_scores.end(); ++it) 
		cout << *it << " " ;
	cout << endl;

}

3.4 、迭代器的介绍

 迭代器的运算

include "Version_test.h"

#include 
using std::string;

#include 
using std::vector;

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

#ifndef LIST_INIT
#include 
using std::begin; using std::end;
#endif

int main()
{
#ifdef LIST_INIT
	vector<int> v = {0,1,2,3,4,5,6,7,8,9};
#else
	int temp[] = {0,1,2,3,4,5,6,7,8,9};
	vector<int> v(begin(temp), end(temp));
#endif
	auto sz = v.size(); 
	decltype(sz) i = 0;
	// duplicate contents of v onto the back of v
	while (i != sz) {
		v.push_back(*v.begin() + i);
		++i;
	}
	// prints 0...9 0...9
	for (auto it : v)
		cout << it << " ";
	cout << endl;
	
#ifdef LIST_INIT
	// alternative way to stop when we get to the original last element
	vector<int> alt_v = {0,1,2,3,4,5,6,7,8,9}; // vector with values 0...9
#else
	vector<int> alt_v(begin(temp), end(temp)); // copy the array into alt_v
#endif
	for (decltype(alt_v.size()) i = 0, sz = alt_v.size(); i != sz; ++i)
		alt_v.push_back(alt_v[i]);

	// prints 0...9 0...9
	for (auto it : alt_v)
		cout << it << " ";
	cout << endl;

#ifdef LIST_INIT
	vector<int> v2 = {0,1,2,3,4,5,6,7,8,9}; // vector with values 0...9
#else
	vector<int> v2(begin(temp), end(temp)); 
#endif
	decltype(v2.size()) ix = 0;   // we'll use ix to index the vector

	// set the elements with values less than 5 to 0
	while (ix != v2.size() && v2[ix] < 5) {
		v2[ix] = 0; // changes the value of the element in v
		++ix;       // increment the index so the next iteration fetches the next element
	}

	// print the elements using subscripts
	for (unsigned i = 0; i != v2.size(); ++i)
		cout << v2[i] << " ";
	cout << endl;

#ifdef LIST_INIT
	// equivalent but using iterators
	vector<int> alt_v2 = {0,1,2,3,4,5,6,7,8,9}; // vector with values 0 ...9
#else
	vector<int> alt_v2(begin(temp), end(temp));
#endif
	// set the elements to 0 up to the first one that is 5 or greater
	auto it = alt_v2.begin();
	while (it != alt_v2.end() && *it < 5) {
		*it = 0;   // changes the value of the element in alt_v2
		++it;      // advance the iterator to denote the next element
	}
	for (auto it = alt_v2.begin(); // it denotes first element in alt_v2
	          it != alt_v2.end();  // so long as it denotes an element
	          ++it)          // increment the iterator to next element
		cout << *it << " ";  // print element denoted by it from alt_v2
	cout << endl;

	return 0;
}

3.5、数组

#include "Version_test.h"

#include 
using std::vector;

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

#ifndef LIST_INIT
#include 
using std::begin; using std::end;
#endif

int main()
{
#ifdef LIST_INIT
	vector<int> v{1,2,3,4,5,6,7,8,9};
#else
	int temp[] = {1,2,3,4,5,6,7,8,9};
	vector<int> v(begin(temp), end(temp));
#endif
	for (auto &i : v) // for each element in v (note: i is a reference)
		i *= i;           // square the element value
	for (auto i : v)      // for each element in v
		cout << i << " "; // print the element
	cout << endl;

	return 0;
}

          指针和数组

                c语言风格字符串

#include 
using std::string;

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

#include 

#include 
using std::size_t;

int main() {
	string s1 = "A string example";
	string s2 = "A different string";
	
	if (s1 < s2)  // false: s2 is less than s1
		cout << s1 << endl;
	else
		cout << s2 << endl;
	const char ca1[] = "A string example";
	const char ca2[] = "A different string";
	
	if (strcmp(ca1, ca2) < 0) // same effect as string comparison s1 < s2
		cout << ca1 << endl;
	else
		cout << ca2 << endl;
	
	const char *cp1 = ca1, *cp2 = ca2;
	cout << strcmp(cp1, cp2) << endl; // output is positive
	cout << strcmp(cp2, cp1) << endl; // output is negative
	cout << strcmp(cp1, cp1) << endl; // output is zero
	
	
	cout << strlen(cp1) << endl; // prints 16; strlen ignores the null
	
	const unsigned sz = 16 + 18 + 2;
	char largeStr[sz];      // will hold the result
	// disastrous if we miscalculated the size of largeStr
	strcpy(largeStr, ca1);  // copies ca1 into largeStr
	strcat(largeStr, " ");  // adds a space at the end of largeStr
	strcat(largeStr, ca2);  // concatenates ca2 onto largeStr
	// prints A string example A different string
	cout << largeStr << endl;       
	
	strncpy(largeStr, ca1, sz); // size to copy includes the null
	if (strlen(ca1) > sz)
		largeStr[sz-1] = '\0';
	strncat(largeStr, " ", 2);  // pedantic, but a good habit
	strncat(largeStr, ca2, sz - strlen(largeStr)); 
	cout << largeStr << endl;       
	
	// initialize large_string as a concatenation of s1, a space, and s2
	string large_string = s1 + " " + s2; 
	cout << large_string << endl;       
	
	return 0;
}

3.6多维数组

#include "Version_test.h"

#include 
using std::begin; using std::end;

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

#include 
using std::size_t;

int main()
{
	// array of size 3; each element is an array of 4 uninitailzed ints 
	int ia1[3][4];
	
	// array of size 10; each element is a 20-element array 
	// whose elements are arrays of 30 ints
	int arr[10][20][30] = {0}; // initialize all elements to 0
	
	// assigns the first element of arr to the last element 
	// in the last row of ia
	ia1[2][3] = arr[0][0][0];
	
	// binds row to the second four-element array in ia 
	int (&row)[4] = ia1[1];
	
	// three elements, each element is an array of size 4
	int ia2[3][4] = { 
	    {0, 1, 2, 3},   // initializers for the row indexed by 0
	    {4, 5, 6, 7},   // initializers for the row indexed by 1
	    {8, 9, 10, 11}  // initializers for the row indexed by 2
	};
	
	// equivalent initialization without the optional 
	// nested braces for each row
	int ia3[3][4] = {0,1,2,3,4,5,6,7,8,9,10,11};
	
	// explicitly initialize only element 0 in each row
	int ia4[3][4] = {{ 0 }, { 4 }, { 8 }};
	
	// explicitly initialize row 0; the remaining elements 
	// are value initialized
	int ix[3][4] = {0, 3, 6, 9};
	
	// prints 9 0 0 
	cout << ix[0][3] << ' ' << ix[1][0] << ' ' << ix[2][0] << endl;
	
#ifdef CONSTEXPR_VARS
	constexpr size_t rowCnt = 3, colCnt = 4;
#else
	const size_t rowCnt = 3, colCnt = 4;
#endif
	int ia[rowCnt][colCnt];   // 12 uninitialized elements 
	
    // for each row
    for (size_t i = 0; i != rowCnt; ++i) {
        // for each column within the row
        for (size_t j = 0; j != colCnt; ++j) {
            // assign the element's positional index as its value
            ia[i][j] = i * colCnt + j;   
		}
	}

	// four ways to print the contents of ia
	// 1. using nested range for loops
	for (const auto &row : ia) // for every element in the outer array
		for (auto col : row)   // for every element in the inner array
			cout << col << endl; // print the element's value
    cout << ia[0][0] << ' ' << ia[2][3] << endl; // prints 0 11


	// 2. using pointers and a traditional for loop
	//    with pointer arithmetic to calculate the end pointers 
	for (auto p = ia; p != ia + rowCnt; ++p) {
		// q points to the first element of an array of four ints; 
		// that is, q points to an int
	    for (auto q = *p; q != *p + colCnt; ++q)
	         cout << *q << ' ';
		cout << endl;
	}
	
	// 3. using pointers and a traditional for loop
	//    with the library begin and end functions to manage the pointers
	for (auto p = begin(ia); p != end(ia); ++p) {
		// q points to the first element in an inner array
		for (auto q = begin(*p); q != end(*p); ++q)
			cout << *q << ' ';  // prints the int value to which q points
		cout << endl;
	}
	
	// 4. using a type alias to declare the loop control variable
#ifdef TYPE_ALIAS_DECLS
	using int_array = int[4]; // new style type alias declaration 
#else
	typedef int int_array[4]; // equivalent typedef declaration
#endif

	for (int_array *p = ia; p != ia + 3; ++p) {
	    for (int *q = *p; q != *p + 4; ++q)
	         cout << *q << ' ';
		cout << endl;
	}

	// alternative way to assign positional index to elements 
	// in a two-dimensional array
	int alt_ia[rowCnt][colCnt]; // 12 uninitialized elements 
	size_t cnt = 0;
	for (auto &row : alt_ia)    // for every element in the outer array
		for (auto &col : row) { // for every element in the inner array
			col = cnt;          // give this element the next value
			++cnt;              // increment cnt
		}
	// now print the value of the array
	for (const auto &row : alt_ia) // for every element in the outer array
		for (auto col : row)     // for every element in the inner array
			cout << col << endl;

	return 0;
}

3.7、小结

3.8、术语表