STL commonly used containers - introduction and use of map containers

1. Introduction to map container

The map container is an associative container that stores pair-type key-value pairs (pair objects created by the pair class template).

pair key-value pair (pair<const K, T>): the first element in the key-value pair is key (key), which is used for searching, and the second element is value (value).

Using each key-value pair stored in the map container, the value of the key can neither be repeated nor modified.

When using the map container to store multiple key-value pairs, by default, all key-value pairs will be sorted in ascending order automatically according to the size of the key of each key-value pair.

2. Map container construction and assignment

Default construction: map<T1,T2> m;

When creating a map container with default construction, it can also be initialized

Copy construction: map(const map &m);

By calling the copy constructor of the map container, the same map container can be successfully created

Move constructor: Pass the temporarily created map object as a parameter to the map container to be initialized, and then call the move constructor to create the map container.

//返回临时map对象的函数
map<string,int> disMap() {
    map<string, int>tempMap{ {"C语言",1},{"STL",2} };
    return tempMap;
}
//用临时map对象做为参数调用移动构造函数创建map容器
map<string, int>newMap(disMap());
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Interval construction: take the key-value pairs in the specified area in the existing map container, create and initialize a new map container

map<string, int>oldMap{ {"C语言",10},{"STL",20} };
map<string, int>newMap(++myMap.begin(), myMap.end());
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code example

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

void printMap(map<int, int>& m) {
	for (map<int, int>::iterator it = m.begin(); it != m.end(); it++) {
		cout << (*it).first << " " << (*it).second << endl;
	}
	cout << endl;
}
//返回临时map对象的函数
map<int, int> disMap() {
	map<int, int>tempMap{ {5,40},{4,50} };
	return tempMap;
}

void test01() {
	//默认构造
	map<int, int> m;
	map<int, int> m1{{3,10},{1,20},{2,30}};
	printMap(m1); 

	//拷贝构造
	map<int, int> m3(m1);
	printMap(m3);

	//赋值
	map<int, int> m4;
	m4 = m3;
	printMap(m4);

	//区间构造
	map<int, int> m5(++m3.begin(), m3.end());
	printMap(m5);

	//移动构造
	map<int, int> m6(disMap());
	printMap(m6);
}
int main() {
	test01();
	system("pause");
	return 0;
}
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3. Map container size and exchange

member method	Function
empty()	Returns true if the container is empty; otherwise, false.
size()	Returns the number of key-value pairs stored in the current map container.
swap()	Exchange the key-value pairs stored in two map containers, and the two key-value pairs must be of the same type.

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

void printMap(map<int, int>& m) {
	for (map<int, int>::iterator it = m.begin(); it != m.end(); it++) {
		cout << "key=" << (*it).first << " value=" << (*it).second << endl;
	}
	cout << endl;
}
void test01() {
	//默认构造
	map<int, int> mp{{1, 10},{3, 20},{4, 30},{2, 40}};
	if (!mp.empty()) {
		cout << "容器mp不为空" << endl;
		cout << "容器的大小为：" << mp.size();
		cout << endl;
	}
	else {
		cout << "容器为空" << endl;
	}
	map<int, int> m1{{1, 1},{3, 2},{4, 3},{2, 4}};
	cout << "交换前：" << endl;
	printMap(mp);
	printMap(m1);
	cout << "交换后： " << endl;
	mp.swap(m1);
	printMap(mp);
	printMap(m1);
}
int main() {
	test01();
	system("pause");
	return 0;
}
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4. Add and delete map containers

member method	Function
insert(elem)	Add key-value pairs to the container.
clear()	Clear all key-value pairs in the map container, and the size() of the map container is 0.
erase(pos)	Delete the key-value pair at position pos and return an iterator to the next element.
erase(beg,end)	Delete key-value pairs in the range [ beg,end ), and return an iterator to the next element.
erase(key)	Delete the key-value pair of the specified key in the container.

Use of the [] operator

Use the [ ] operator to obtain the value corresponding to the specified key, and also modify the value corresponding to the specified key;

如果修改时 map 容器内部没有存储以 [ ] 运算符内指定数据为键的键值对，则使用 [ ] 运算符会向当前 map 容器中添加一个新的键值对。

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

void printMap(map<string, int>& m) {
	for (map<string, int>::iterator it = m.begin(); it != m.end(); it++) {
		cout << "key=" << (*it).first << " value=" << (*it).second << endl;
	}
	cout << endl;
}
void test01() {

	map<string, int> m1{ {"STL",5} };
	cout <<  "获取指定键对应的值：" << m1["STL"] << endl;

	//向 map 容器添加新键值对
	m1["Python"] = 6;
	//修改 map 容器中指定键对应的值，如果指定键不存在则会执行添加操作
	m1["STL"] = 50;
	printMap(m1);

}
int main() {
	test01();
	system("pause");
	return 0;
}
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4.1、insert插入数据的四种方式

当使用 insert() 方法向 map 容器的指定位置插入新键值对时，其底层会先将新键值对插入到容器的指定位置，如果其破坏了 map 容器的有序性，该容器会对新键值对的位置进行调整。

• 如果插入成功，insert() 方法会返回一个指向 map 容器中已插入键值对的迭代器；
• 如果插入失败，insert() 方法同样会返回一个迭代器，该迭代器指向 map 容器中和 val 具有相同键的那个键值对。

1、不指定位置，直接添加键值对

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

void printMap(map<string, int>& m) {
	for (map<string, int>::iterator it = m.begin(); it != m.end(); it++) {
		cout << "key=" << (*it).first << " value=" << (*it).second << endl;
	}
	cout << endl;
}

void test01() {

	map<string, int> m;

	//创建一个pair键值对
	pair<string, int> pair1 = { "STL",20 };
	//添加pair到map中
	m.insert(pair1);
	m.insert({ "C",40 });

	//调用 pair 类模板的构造函数添加
	m.insert(pair<string, int>("JAVA", 10));
	//调用 make_pair() 函数添加
	m.insert(make_pair("python", 20));
	printMap(m);
}
int main() {
	test01();
	system("pause");
	return 0;
}
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2、向指定位置插入键值对

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

void printMap(map<string, int>& m) {
	for (map<string, int>::iterator it = m.begin(); it != m.end(); it++) {
		cout << "key=" << (*it).first << " value=" << (*it).second << endl;
	}
	cout << endl;
}

void test01() {

	map<string, int> m;

	//创建一个pair键值对
	pair<string, int> pair1 = { "STL",20 };
	//添加pair到map中
	m.insert(m.begin(),pair1);

	//调用 pair 类模板的构造函数添加
	m.insert(m.begin(),pair<string, int>("JAVA", 10));
	//调用 make_pair() 函数添加
	m.insert(m.end(), make_pair("python", 20));
	printMap(m);
}
int main() {
	test01();
	system("pause");
	return 0;
}
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3、向 map 容器中插入其它 map 容器区间内的所有键值对

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

void printMap(map<string, int>& m) {
	for (map<string, int>::iterator it = m.begin(); it != m.end(); it++) {
		cout << "key=" << (*it).first << " value=" << (*it).second << endl;
	}
	cout << endl;
}

void test01() {

    map<string, int> m;
	map<string, int> m1{ {"STL教程",20},{"C语言教程",50},{"Java教程",30} };
	printMap(m1);
    //将<first,last>区域内的键值对插入到 copymap 中
	m.insert(++m1.begin(), m1.end());
	printMap(m);
}
int main() {
	test01();
	system("pause");
	return 0;
}
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4、一次向 map 容器中插入多个键值对

map<string, int> m;
m.insert({ {"STL",1},{ "C语言",2},{ "Java",30} });
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4.2、删除键值对

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

void printMap(map<string, int>& m) {
	for (map<string, int>::iterator it = m.begin(); it != m.end(); it++) {
		cout << "key=" << (*it).first << " value=" << (*it).second << endl;
	}
	cout << endl;
}

void test01() {

    map<string, int> m;
	m.insert({ { "STL",1}, {"C语言",2}, {"Java",30}, {"MySQL",5}, {"Docker",70}, {"Redis",3} });
	printMap(m);

	//清除
	m.erase(++m.begin());
	printMap(m);
    
    //清除区间
	m.erase(++m.begin(), --m.end());
	printMap(m);

    //按照key删除
	m.erase("C语言");
	printMap(m);

    //清除全部
	m.clear();
	printMap(m);

}
int main() {
	test01();
	system("pause");
	return 0;
}
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5、map容器查找和统计

成员方法	功能
find(key)	在 map 容器中查找键为 key 的键值对，如果成功找到，则返回指向该键值对的双向迭代器；反之，则返回和 end() 方法一样的迭代器。
count(key)	在当前 map 容器中，查找键为 key 的键值对的个数并返回。由于 map 容器中各键值对的键的值是唯一的，因此该函数的返回值最大为 1。

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

void printMap(map<int, int>& m) {
	for (map<int, int>::iterator it = m.begin(); it != m.end(); it++) {
		cout << "key=" << (*it).first << " value=" << (*it).second << endl;
	}
	cout << endl;
}
void test01() {
	map<int, int> mp;
	mp.insert({ {1,10},{3, 20},{4, 30} });

	map<int, int>::iterator it = mp.find(4);
	if (it != mp.end()) {
		cout << "查到了元素的key值为: " << (*it).first << " 元素的value值为： " << (*it).second << endl;
	}
	else {
		cout << "没有查到这个key的对组" << endl;
	}
	it = mp.find(5);
	if (it != mp.end()) {
		cout << "查到了元素的key值为 " << (*it).first << " 元素的value值为： " << (*it).second << endl;
	}
	else {
		cout << "没有查到这个key的对组" << endl;
	}
	//map的统计值只有0和1
	int num1 = mp.count(1);
	int num2 = mp.count(6);
	cout << num1 << "  " << num2 << endl;
}
int main() {
	test01();
	system("pause");
	return 0;
}
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6、map容器排序

map容器默认是升序排序的，利用仿函数可以改变map的排序规则

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

class MyCompare {
public:
	bool operator()(int v1, int v2) const {
		return v1 > v2;
	}
};
//map降序排序
void printMap(map<int, int, MyCompare>& m) {
	for (map<int, int, MyCompare>::iterator it = m.begin(); it != m.end(); it++) {
		cout << "key=" << (*it).first << " value=" << (*it).second << endl;
	}
	cout << endl;
}
void test01() {
	map<int, int, MyCompare> mp;
	mp.insert({ {1,10},{3, 20},{4, 30} });
	printMap(mp);
}
int main() {
	test01();
	system("pause");
	return 0;
}
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