1. Basic concepts
Introduction:
- All elements in the map are pairs
- The first element in the pair is key (key value) , which acts as an index, and the second element is value (real value)
- All elements are automatically sorted according to the key value of the element
Nature:
- Map/multimap is an associative container , and the underlying structure is implemented with a binary tree.
advantage:
- The value value can be quickly found based on the key value.
The difference between map and multimap :
- The map does not allow duplicate key value elements in the container
- multimap allows duplicate key elements in the container
Two, construction and assignment
Function description:
- Construct and assign values to the map container
Function prototype:
structure:
map<T1, T2> mp;//map default constructor:map(const map &mp);//Copy constructor
Assignment:
map& operator=(const map &mp);//Overload the equal sign operator
Example:
#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>m; //默认构造
m.insert(pair<int, int>(1, 10));
m.insert(pair<int, int>(2, 20));
m.insert(pair<int, int>(3, 30));
printMap(m);
map<int, int>m2(m); //拷贝构造
printMap(m2);
map<int, int>m3;
m3 = m2; //赋值
printMap(m3);
}
int main() {
test01();
system("pause");
return 0;
}
Summary: All elements in the map appear in pairs, and pairs should be used when inserting data
Three, size and exchange
Function description:
- Count the size of the map container and exchange the map container
Function prototype:
size();//Returns the number of elements in the containerempty();//Determine whether the container is emptyswap(st);//Exchange two collection containers
Example:
#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>m;
m.insert(pair<int, int>(1, 10));
m.insert(pair<int, int>(2, 20));
m.insert(pair<int, int>(3, 30));
if (m.empty())
{
cout << "m为空" << endl;
}
else
{
cout << "m不为空" << endl;
cout << "m的大小为： " << m.size() << endl;
}
}
//交换
void test02()
{
map<int, int>m;
m.insert(pair<int, int>(1, 10));
m.insert(pair<int, int>(2, 20));
m.insert(pair<int, int>(3, 30));
map<int, int>m2;
m2.insert(pair<int, int>(4, 100));
m2.insert(pair<int, int>(5, 200));
m2.insert(pair<int, int>(6, 300));
cout << "交换前" << endl;
printMap(m);
printMap(m2);
cout << "交换后" << endl;
m.swap(m2);
printMap(m);
printMap(m2);
}
int main() {
test01();
test02();
system("pause");
return 0;
}
to sum up:
- Statistics size — size
- Determine whether it is empty — empty
- Exchange container — swap
Four, insert and delete
Function description:
- The map container inserts data and deletes data
Function prototype:
insert(elem);//Insert elements in the container.clear();//Clear all elementserase(pos);//Delete the element pointed to by the pos iterator and return the iterator of the next element.erase(beg, end);//Delete all elements in the interval [beg,end), and return the iterator of the next element.erase(key);//Delete the element whose value is key in the container.
Example:
#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()
{
//插入
map<int, int> m;
//第一种插入方式
m.insert(pair<int, int>(1, 10));
//第二种插入方式
m.insert(make_pair(2, 20));
//第三种插入方式
m.insert(map<int, int>::value_type(3, 30));
//第四种插入方式
m[4] = 40;
printMap(m);
//删除
m.erase(m.begin());
printMap(m);
m.erase(3);
printMap(m);
//清空
m.erase(m.begin(),m.end());
m.clear();
printMap(m);
}
int main() {
test01();
system("pause");
return 0;
}
to sum up:
- There are many ways to insert map, just remember one
- Insert — insert
- Delete — erase
- Clear — clear
Five, search and statistics
Function description:
- Find data and statistics on the map container
Function prototype:
find(key);//Find whether the key exists, if it exists, return an iterator of the key's elements; if it does not exist, return set.end();count(key);//Count the number of elements of the key
Example:
#include <map>
//查找和统计
void test01()
{
map<int, int>m;
m.insert(pair<int, int>(1, 10));
m.insert(pair<int, int>(2, 20));
m.insert(pair<int, int>(3, 30));
//查找
map<int, int>::iterator pos = m.find(3);
if (pos != m.end())
{
cout << "找到了元素 key = " << (*pos).first << " value = " << (*pos).second << endl;
}
else
{
cout << "未找到元素" << endl;
}
//统计
int num = m.count(3);
cout << "num = " << num << endl;
}
int main() {
test01();
system("pause");
return 0;
}
to sum up:
- Find — find (returns an iterator)
- Statistics — count (for map, the result is 0 or 1)
- The map does not allow the insertion of repeated key elements, and only takes the element whose key is x the first time.
Six, sort
learning target:
- The default sorting rule of the map container is sorting from small to large according to the key value. Master how to change the sorting rule
Main technical points:
- Using functors, you can change the sorting rules
Example:
#include <map>
class MyCompare {
public:
bool operator()(int v1, int v2) {
return v1 > v2;
}
};
void test01()
{
//默认从小到大排序
//利用仿函数实现从大到小排序
map<int, int, MyCompare> m;
m.insert(make_pair(1, 10));
m.insert(make_pair(2, 20));
m.insert(make_pair(3, 30));
m.insert(make_pair(4, 40));
m.insert(make_pair(5, 50));
for (map<int, int, MyCompare>::iterator it = m.begin(); it != m.end(); it++) {
cout << "key:" << it->first << " value:" << it->second << endl;
}
}
int main() {
test01();
system("pause");
return 0;
}
to sum up:
- Use functors to specify the sorting rules of the map container
- For custom data types, the map must specify the sorting rules, the same as the set container