1 Introduction
Multimap is an associative container that contains sorted key-value pairs, namely (key, value), allowing the same key value , internally sorted according to the key value; adding, modifying, and querying have logarithmic time complexity, Its storage structure is a red-black tree;
header files and definitions
#include <map>
template<
class Key,
class T,
class Compare = std::less<Key>,
class Allocator = std::allocator<std::pair<const Key, T>>
> class multimap;
2. Initialization
The initialization method is as follows
#include <iostream>
#include <string>
#include <map>
template<typename T>
void showInfo(T &t)
{
auto iter = t.begin();
while (iter != t.end())
{
std::cout<<"["<<iter->first<<", "<<iter->second<<"]; ";
iter++;
}
std::cout<<std::endl;
}
int main()
{
//允许重复的数据出现，但是内部会对其进行排序
std::multimap<int32_t, std::string> mm1{
{
1,"c"},{
2,"linux"},{
1,"c++"},{
1,"c++"},{
2,"linux"}};
showInfo(mm1);
std::multimap<int32_t, std::string> mm2 = mm1;
showInfo(mm1);
std::multimap<int32_t, std::string> mm3;
mm3.emplace(std::make_pair(1, "teacher"));
mm3.emplace(std::make_pair(1, "student"));
mm3.insert({
3, "boy"});
mm3.emplace(std::pair<int32_t, std::string>(4, "girl"));
mm3.emplace_hint(mm3.begin(), std::make_pair(5, "class"));
showInfo(mm3);
return 0;
}
output
[1, c]; [1, c++]; [1, c++]; [2, linux]; [2, linux];
[1, c]; [1, c++]; [1, c++]; [2, linux]; [2, linux];
[1, teacher]; [1, student]; [3, boy]; [4, girl]; [5, class];
3. use
Most of the operations it supports are the same as map , but the following points are different;
but compared with set, count() is useful; find() is to find the time when the first element appears;
- Multimap does not support at() and operator[], because data duplication is allowed in multimap and cannot be accessed through subscripts
- multimap does not support try_emplace (C++17)
Since multimap does not support subscript access, how to access elements? At this time find(), count(), equal_range(), lower_bound() and upper_bound() will come into play;
example
int main()
{
std::multimap<int32_t, std::string> mm1{
{
1,"c"},{
2,"linux"},{
1,"c++"},{
1,"c++"},{
2,"linux"}, {
2,"python"},{
1,"matlab"}};
showInfo(mm1);
//1. 通过lower_bound和upper_bound输出元素相同的数据
std::cout<<"[1]\n";
auto lower_iter = mm1.lower_bound(1);
auto upper_iter = mm1.upper_bound(1);
decltype(lower_iter) m; //decltype自动推导出m的类型
for(m = lower_iter; m != upper_iter; m++)
{
std::cout<<"["<<m->first<<", "<<m->second<<"]; ";
}
std::cout<<std::endl<<std::endl;
//2. 通过equal_range来查找元素
std::cout<<"[2]\n";
auto equal_iter = mm1.equal_range(1);
auto lower_iter_1 = equal_iter.first;
auto upper_iter_2 = equal_iter.second;
decltype(lower_iter_1) m_1;
for(m_1 = lower_iter_1; m_1 != upper_iter_2; m_1++)
{
std::cout<<"["<<m_1->first<<", "<<m_1->second<<"]; ";
}
std::cout<<std::endl<<std::endl;
//3. find() 和 count()
std::cout<<"[3 ]\n";
auto iter = mm1.find(1);
if(iter != mm1.end())
{
for(size_t i = 0;i <mm1.count(1);i++)
{
std::cout<<"["<<iter->first<<", "<<iter->second<<"]; ";
iter++;
}
}
std::cout<<std::endl;
return 0;
}
output
[1, c]; [1, c++]; [1, c++]; [1, matlab]; [2, linux]; [2, linux]; [2, python];
[1]
[1, c]; [1, c++]; [1, c++]; [1, matlab];
[2]
[1, c]; [1, c++]; [1, c++]; [1, matlab];
[3]
[1, c]; [1, c++]; [1, c++]; [1, matlab];