C++ associative containers (mainly introduces ordered associative containers)

Some points to note in the use of associative containers

• There are mainly map and set, and unordered and multi versions are derived.

map

• Each element in the map is a [key,value]pair object, and the key and value can be accessed through first and second. During the operation, the value can be modified, but the key cannot be modified.

multiset/multimap

• For map and set, there are no repeated elements, so you can directly determine whether the container contains a specific element through find
• For multimap and multiset, the same elements will be stored in adjacent positions, so if you find the first position, you can find the values ​​stored later in turn

• code

  void test()
  {
      multimap<int, string> maps;
      maps.insert( make_pair(1, "a") );
      maps.insert(make_pair(1, "b"));
      maps.insert(make_pair(2, "d"));
      maps.insert(make_pair(1, "c"));
      maps.insert(make_pair(3, "e"));
  
      int cnt = maps.count( 1 );
      auto it = maps.find( 1 );
      while (cnt-- != 0)
          cout << it++->second << endl;
  }
  

lower_bound,upper_bound

• lower_boundis an iterator that returns the first greater than or equal to the container position where the specified element is located
• upper_boundis an iterator that returns the first greater than the container position where a particular element is located
• The above 2 functions can also be used to find out those values ​​in a (sorted) container that are equal to a specific element
• For the container of the set category, you can use the generic algorithm or the function implemented by yourself to implement lower_bound/upper_bound, but for the container of the map category, it is recommended to use the function implemented by the map class; if you want to use the generic algorithm of stl to implement lower_bound/upper_bound, then Need to give pair

unordered container

• Those classes at the beginning of unordered, implemented with a hash table, can be O(1)searched.