[C++] Use of associative containers (map and multimap)

1. Introduction to map

In C++, std::mapand multimapare both associative containers , and are included in header files #include <map>, and their underlying structures are implemented using binary trees .

• mapAll elements in are pair, pairthe first element is the key value ( key), which acts as an index; the second element is the real value ( value), which is the mapped value
• The elements in this type of container will be automatically sorted according to the key value ( ) of the element when inserted.key

2. map constructor

	Constructor prototype	explain
1	map<T1, T2> mp	default construction
2	map(const map &mp)	copy construction

3. Map assignment operation

	Function prototype: =	explain
1	set& operator=(const set &st)	Overloaded = operator

4. Map data insertion and deletion

	Function prototype: insert, erase, clear	explain
1	insert(elem)	Insert element into container
2	erase(key)	Delete the element whose key value is key in the container
3	erase(pos)	Delete the element pointed to by the pos iterator and return the iterator of the next element
4	erase(beg, end)	Delete all elements in the interval [beg, end) and return an iterator of the next element
5	clear()	Clear all elements

Example 1:map<string, int>

#include <iostream>
#include <string>
#include <map>       //必须包含该头文件
using namespace std;

void printmap(map<string,int>& s)
{
    
    
	for (map<string,int>::iterator it = s.begin(); it != s.end(); it++)
	{
    
    
		cout << "键值：" << it->first << "\t实值：" << it->second << endl;
	}
	cout << endl;
}

void test01()
{
    
    
	map<string,int> s1;
	//第1种插入方式
	s1["娃哈哈"] = 2;

	//第2种插入方式
	s1.insert(pair<string,int>("可乐", 3));

	//第3种插入方式
	s1.insert(make_pair("脉动", 4));

	//第4种插入方式
	s1.insert(map<string, int>::value_type("红牛", 5));
	
	cout << "插入后的元素：" << endl;
	printmap(s1);      //string类型key会根据首字母顺序

	s1.erase("娃哈哈");
	cout << "删除娃哈哈后的元素：" << endl;
	printmap(s1);

	s1.clear();
	cout << "清空后的元素：" << endl;
	printmap(s1);
}

int main()
{
    
    
	test01();
	system("pause");
	return 0;
}

//result
插入后的元素：
键值：红牛      实值：5
键值：可乐      实值：3
键值：脉动      实值：4
键值：娃哈哈    实值：2

删除娃哈哈后的元素：
键值：红牛      实值：5
键值：可乐      实值：3
键值：脉动      实值：4

清空后的元素：

Notice:

• mapThe data inserted into the container will be keyautomatically sorted according to the type, stringand the type keywill be sorted according to the first letter.
• mapInserting elements with duplicate key values ​​into the container will fail, and the number of elements will not change.

Example 2:map<int, double>

#include <iostream>
#include <string>
#include <map>       //必须包含该头文件
using namespace std;

void printmap(map<int, double>& s)
{
    
    
	for (map<int, double>::iterator it = s.begin(); it != s.end(); it++)
	{
    
    
		cout << "键值：" << it->first << "\t实值：" << it->second << endl;
	}
	cout << endl;
}

void test01()
{
    
    
	map<int, double> s1;
	//第1种插入方式
	s1[1] = 1.001;

	//第2种插入方式
	s1.insert(pair<int, double>(4, 1.004));

	//第3种插入方式
	s1.insert(make_pair(2, 1.002));

	//第4种插入方式
	s1.insert(map<int, double>::value_type(3, 1.003));

	cout << "插入后的元素：" << endl;
	printmap(s1);      //string类型key会根据首字母顺序

	s1.erase(1);
	cout << "删除key=1的元素：" << endl;
	printmap(s1);

	s1.clear();
	cout << "清空后的元素：" << endl;
	printmap(s1);
}

int main()
{
    
    
	test01();
	system("pause");
	return 0;
}

//result
插入后的元素：
键值：1 实值：1.001
键值：2 实值：1.002
键值：3 实值：1.003
键值：4 实值：1.004

删除key=1的元素：
键值：2 实值：1.002
键值：3 实值：1.003
键值：4 实值：1.004

清空后的元素：

5. Map search and statistics

	Function prototype: find, count	explain
1	find(key)	Find whether the key exists. If it exists, return the iterator of the key value element; if it does not exist, return map.end()
2	count(key)	Count the number of elements of key (for map, the result is 0 or 1)

Notice:

• find(key)What is returned is an iterator, iterator->firstand the sum can iterator->secondbe obtained bykeyvalue
• Because mapthe elements are not repeated, count()0 or 1 is returned.

Example:

#include <iostream>
#include <string>
#include <map>       //必须包含该头文件
using namespace std;

void test01()
{
    
    
	map<string, int> s1;
	//插入insert
	s1["娃哈哈"] = 2;
	s1.insert(pair<string, int>("可乐", 3));
	s1.insert(make_pair("脉动", 4));
	s1.insert(map<string, int>::value_type("红牛", 5));

	//查找find
	map<string, int>::iterator pos = s1.find("脉动");
	if (pos != s1.end())
	{
    
    
		cout << "找到元素: key = " << pos->first << " value = " << pos->second << endl;
	}
	else
	{
    
    
		cout << "未到元素key!";
	}

	//统计count
	int num1 = s1.count("可乐");
	cout << "可乐 num = " << num1 << endl;
	int num2 = s1.count("雪碧");
	cout << "雪碧 num = " << num2 << endl;
}

int main()
{
    
    
	test01();
	system("pause");
	return 0;
}

//result
找到元素: key = 脉动 value = 4
可乐 num = 1
雪碧 num = 0

6. Number of map elements and exchange

	Function prototype: empty, size, swap	explain
1	empty()	Determine whether the container is empty
2	size()	Returns the number of elements in the container
3	swap(set<T> & st)	Swap the elements in two containers

Note:swap() The elements contained in the two containers exchanged must be of the same type.

7. map iterator

	Function prototype: begin, end	explain
1	begin()	Returns an iterator pointing to the first element of the set container
2	end()	Returns an iterator pointing to the next position of the last element of the set container
3	rbegin()	Returns a reverse iterator pointing to the last element of the set container
4	rend()	Returns a reverse iterator pointing to the previous position of the first element of the set container
5	cbegin()	Returns a const iterator pointing to the first element of the set container
6	a few()	Returns a const iterator pointing to the next position of the last element of the set container
7	crbegin()	Returns a const reverse iterator pointing to the last element of the set container
8	crend()	Returns a const reverse iterator pointing to the previous position at the beginning of the set container

Note:const iterator It is an constiterator pointing to a value. The iterator itself can be modified, but it cannot be used to modify the value it points to.

8. Map container sorting

• The default sorting rule of the map container is keyfrom small to large values.
• Using functors, you can change the sorting rules
• For custom data types, value sorting rules mapmust be specified , the same as [ set usage method ]keyset

Example:

#include <iostream>
#include <string>
#include <map>       //必须包含该头文件
using namespace std;

//仿函数，重载operator()运算符
class Compare
{
    
    
public:
    //Visual Studio的编译器此处需添加const，具有指定const-volatile类型的变量只能调用具有相同或更大const-volatile限定定义的成员函数
	bool operator()(int v1, int v2)const  
	{
    
    
		return v1 > v2;
	}
};

void test01()
{
    
    
	map<int,string,Compare> s1;  //指定排序规则, 从大到小
	s1.insert(pair<int, string>(1, "李四"));
	s1.insert(pair<int, string>(3, "张三"));
	s1.insert(pair<int, string>(4, "王五"));
	s1.insert(pair<int, string>(2, "赵六"));
	
	for (map<int, string>::iterator it = s1.begin(); it != s1.end(); it++)
	{
    
    
		cout << "键值：" << it->first << "\t实值：" << it->second << endl;
	}
	cout << endl;
}

int main()
{
    
    
	test01();
	system("pause");
	return 0;
}

//result
键值：4 实值：王五
键值：3 实值：张三
键值：2 实值：赵六
键值：1 实值：李四

9. The difference between map and multimap

• mapDo not allow duplicate elements in the container
• multimapAllow duplicate elements in the container
• mapWhen inserting data, the insertion result will be returned, indicating whether the insertion is successful.
• multimapData is not detected so duplicate data can be inserted

Example :multimap methodfind

#include <iostream>
#include <string>
#include <map>       //必须包含该头文件
using namespace std;

void test01()
{
    
    
	multimap<int, string> s1;
	s1.insert(pair<int, string>(1, "李四"));
	s1.insert(pair<int, string>(3, "张三"));
	s1.insert(pair<int, string>(1, "丁一"));
	s1.insert(pair<int, string>(4, "王五"));
	s1.insert(pair<int, string>(2, "赵六"));
	s1.insert(pair<int, string>(1, "吴二"));

	int sum = s1.count(1);
	cout << "key = 1的元素数量：" << sum << endl;

	//因为map会自动排列，key相同的元素必定连续排列，所以从第一个find(1)位置遍历count(1)次
	multimap<int, string>::iterator it = s1.find(1);
	for (int i = 0; i != s1.count(1); i++,it++)
	{
    
    
		cout << "键值：" << it->first << "\t实值：" << it->second << endl;
	}
	cout << endl;
}

int main()
{
    
    
	test01();
	system("pause");
	return 0;
}

//result
key = 1的元素数量：3
键值：1 实值：李四
键值：1 实值：丁一
键值：1 实值：吴二

Note: Because multimapit will be automatically arranged, keythe same elements must be arranged continuously, so by find(1)traversing from the first position count(1), you can obtain all key = 1the key-value pair elements.

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