unordered_map

百度都是关于unorder_map和map的差别等，主要讲效率什么的，而我只想快速的使用unorder_map而已。

简要说明unorder_map

1. 效率高
2. 内部使用harsh原理构造
3. 多用在关联性比较高的数据结构中
4. 多用在查找、对比等算法中......

函数简介

使用类似python的字典，也就是key和value对应关系。

unordered_map<Key,T>

遍历unorder_map中的key和value：

   unordered_map<key,T>::iterator it;
    (*it).first;        //the key value
    (*it).second   //the mapped value
    for(unordered_map<key,T>::iterator iter=mp.begin();iter!=mp.end();iter++)
          cout<<"key value is"<<iter->first<<" the mapped value is "<< iter->second;

    也可以这样：
    for(auto& v : mp)
        print v.first and v.second

• empty

  Test whether container is empty (public member function)

  true if the container size is 0, false otherwise.

  判断内部是否为空，返回bool类型

• size

  Return container size (public member function)

  返回容器大小，类型unsigned integral

• max_size

  Return maximum size (public member function)

  返回最大的数据endReturn const_iterator to end (public member function)

---

• begin

  Return iterator to beginning (public member function)

  返回类型为迭代器，和vector的begin类似。

• end

  Return iterator to end (public member function)

• cbegin

  Return const_iterator to beginning (public member function)

  于上面begin的却别在于cbegin=const begin 也就是不可改变的，其实用法一样的。

• cend

  Return const_iterator to end (public member function)

---

• find

  Get iterator to element (public member function)

  注意这里返回是迭代器，可以和emplace_hint等使用迭代器做参数的函数一起使用。

• count

  Count elements with a specific key (public member function )

  1 if an element with a key equivalent to k is found, or zero otherwise.

  存在返回1，不存在返回0.

• equal_range

  Get range of elements with specific key (public member function)

    若有unordered_map<int, int> mp;查找x是否在map中
    //方法1:  若存在  mp.find(x)!=mp.end()
    //方法2:  若存在  mp.count(x)!=0

---

• emplace

  Construct and insert element (public member function )

  插入数据：mymap.emplace ("NCC-1701", "J.T. Kirk")；

• emplace_hint

  Construct and insert element with hint (public member function )

  这里和上面的find结合使用，出入的值包括一个迭代器。

  mymap.emplace_hint (iter,"NCC-1701", "J.T. Kirk")；

• insert

  Insert elements (public member function )

   myrecipe.insert (std::make_pair<std::string,double>("eggs",6.0)); // move insertion
   myrecipe.insert (mypantry.begin(), mypantry.end());  // range insertion
  
  

• erase

  Erase elements (public member function )

  // erase examples:
    mymap.erase ( mymap.begin() );      // erasing by iterator
    mymap.erase ("France");             // erasing by key
    mymap.erase ( mymap.find("China"), mymap.end() ); // erasing by range
  

• clear

  Clear content (public member function )

  All the elements in the unordered_map container are dropped: their destructors are called, and they are removed from the container, leaving it with a size of 0.

  全部清除！

• swap

  Swap content (public member function)

  std::unordered_map<std::string,std::string>
       first = {{"Star Wars","G. Lucas"},{"Alien","R. Scott"},{"Terminator","J. Cameron"}},
       second  = {{"Inception","C. Nolan"},{"Donnie Darko","R. Kelly"}};
  
    first.swap(second);
  
    std::cout << "first: ";
    for (auto& x: first) std::cout << x.first << " (" << x.second << "), ";
    std::cout << std::endl;
  
    std::cout << "second: ";
    for (auto& x: second) std::cout << x.first << " (" << x.second << "), ";
    std::cout << std::endl;
  

  交换全部key和value，很少用到。

---

• bucket_count

  Return number of buckets (public member function)

• max_bucket_count

  Return maximum number of buckets (public member function)

• bucket_size

  Return bucket size (public member type)

• bucket

  Locate element's bucket (public member function)

---

Hash policy

• load_factor

  Return load factor (public member function)

• max_load_factor

  Get or set maximum load factor (public member function )

• rehash

  Set number of buckets (public member function )

• reserve

  Request a capacity change (public member function)

后面两个暂时还不懂什么意思，之后用到再进行总结。

参考链接

这几次写文章都忘记写参考链接，非常抱歉！！！

一个简单介绍文章

官网