map容器里面所有的元素都是pair对组。对组中第一个元素叫key(键值)，第二个元素叫value(实值)。和set容器一样，map容器也属于关联式容器，底层用二叉树实现，且所有的元素都会在插入时按键值自动排序。map和multimap的主要区别在于是否允许有重复的键值，map容器不允许有重复的键值，而multimap允许有重复的键值。

构造和赋值

函数原型

map<T1,T2>mp; //map默认构造函数
map<T1,T2>(const map &mp);  //拷贝构造函数

map& operator=(const map &mp); //重载赋值操作符

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

void test01()
{
    //创建map容器
    map<int, int > m1;//第一个int代表key 第二个int代表value
    m1.insert(pair<int, int>(1, 10));
    m1.insert(pair<int, int>(2, 20));
    m1.insert(pair<int, int>(4, 40));
    m1.insert(pair<int, int>(5, 50));
    m1.insert(pair<int, int>(3, 30));
    m1.insert(pair<int, int>(6, 60));

    printMap(m1);  /*key = 1 value = 10
                     key = 2 value = 20
                     key = 3 value = 30
                     key = 4 value = 40
                     key = 5 value = 50
                     key = 6 value = 60*/

    //拷贝构造
    map<int, int> m2(m1);

    //赋值
    map<int, int> m3;
    m3 = m2;
}

map容器中所有的元素都是对组，对组中第一个元素是.first表示key值，第二个元素是.second表示value。

大小和交换

函数原型

size();  //返回容器大小
empty();  //判断容器是否为空
swap(sw);   //交换两个集合容器

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

void test01()
{

    map<int, int > m1;
    m1.insert(pair<int, int>(1, 10));
    m1.insert(pair<int, int>(2, 20));
    m1.insert(pair<int, int>(4, 40));
    m1.insert(pair<int, int>(5, 50));
    m1.insert(pair<int, int>(3, 30));
    m1.insert(pair<int, int>(6, 60));

    if(m1.empty())
    {
        cout << "m1为空！" << endl;
    }
    else
    {
        cout << "m1不为空！" <<"大小为："<<m1.size()<< endl;
    }

    map<int, int> m2;
    m2.insert(pair<int, int>(6, 60));
    m2.insert(pair<int, int>(7, 70));
    m2.insert(pair<int, int>(10, 100));
    m2.insert(pair<int, int>(8, 80));
    m2.insert(pair<int, int>(9, 90));

    cout << "交换前：" << endl;
    cout << "m1:" << endl;
    printMap(m1);  /*key = 1 value = 10
                     key = 2 value = 20
                     key = 3 value = 30
                     key = 4 value = 40
                     key = 5 value = 50
                     key = 6 value = 60*/
    cout << "m2:" << endl;
    printMap(m2);     /*key = 6 value = 60
                        key = 7 value = 70
                        key = 8 value = 80
                        key = 9 value = 90
                        key = 10 value = 100*/
    
    //交换
    m1.swap(m2);

    cout << "交换后：" << endl;
    cout << "m1:" << endl;
    printMap(m1);     /*key = 6 value = 60
                        key = 7 value = 70
                        key = 8 value = 80
                        key = 9 value = 90
                        key = 10 value = 100*/
    cout << "m2:" << endl;
    printMap(m2);    /*key = 1 value = 10
                       key = 2 value = 20
                       key = 3 value = 30
                       key = 4 value = 40
                       key = 5 value = 50
                       key = 6 value = 60*/
}

插入和删除

函数原型

insert(elem); //在容器中插入元素
clear(); //清空所有元素
erase(pos);  //删除pos迭代器所指的元素，返回下一个元素的迭代器
erase(begin,end)  //删除[begin,end)区间所有的元素，返回下一个元素的迭代器
erase(key); //删除容器中值为key的元素

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

void test01()
{

    map<int, int > m1;
    m1.insert(pair<int, int>(1, 10)); //插入

    m1.insert(make_pair(2, 20));

    m1.insert(map<int, int>::value_type(3, 30));
    
    m1[4] = 40;  //key = 4 value = 40

    printMap(m1);  /*key = 1 value = 10
                     key = 2 value = 20
                     key = 3 value = 30
                     key = 4 value = 40*/
    cout << "++++++++++++++++++++++++++++++++++++++" << endl;
    //删除
    m1.erase(m1.begin());
    printMap(m1);  /*  
                    key = 2 value = 20
                    key = 3 value = 30
                    key = 4 value = 40*/
    cout << "++++++++++++++++++++++++++++++++++++++" << endl;

    m1.erase(3);  //删除key为3的元素
    printMap(m1);  /*key = 2 value = 20
                    key = 4 value = 40*/
    cout << "++++++++++++++++++++++++++++++++++++++" << endl;

    m1.clear();  //清空
    cout << "容器大小为：" << m1.size() << endl;
}

erase有一个重载版本，erase(key)，要注意只能按key值删除，不能按value值删除。

查找和统计

函数原型

find(key); //查找key是否存在，若存在，返回该元素的迭代器；若不存在，返回end迭代器
count(key);  //统计key的个数

void test01()
{

    map<int, int > m1;
    m1.insert(pair<int,int>(1,10));
    m1.insert(pair<int,int>(2,20));
    m1.insert(pair<int,int>(3,30));
    m1.insert(pair<int,int>(4,40));

    map<int, int>::iterator pos = m1.find(3);  //查找key值为3的元素。
    if (pos != m1.end())
    {
        cout << "找到元素key = " << (*pos).first << " value = " <<(*pos).second << endl;
    }
    else
    {
        cout << "未找到元素！" << endl;
    }

    int num = m1.count(2);  //查找key值为2的元素
    cout << " num = " << num << endl;  //1 对于map容器而言，查找结果只能为1或0
}

对于map容器而言，查找到的结果只能是1或0；对于multimap而言，查找结果可以是其他数字。

map容器排序

map容器默认升序排列，但是也可以通过仿函数来修改默认排序规则为降序。

class Compare
{
public:
    bool operator()(int val1, int val2) const
    {
        //降序
        return val1 > val2;
    }
};

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

void test01()
{

    map<int, int,Compare > m1;
    m1.insert(pair<int,int>(1,10));
    m1.insert(pair<int,int>(2,20));
    m1.insert(pair<int,int>(3,30));
    m1.insert(pair<int,int>(4,40));

    printMap(m1);  /*key = 4 value = 40
                     key = 3 value = 30
                     key = 2 value = 20
                     key = 1 value = 10*/
}