C + + associative containers <map> Simple Summary

C + + associative containers <map> Simple Summary

Map provides variable-size associative containers that efficiently retrieve element values based on associated key Values. When you are working with data on Key-value pairs, you can consider using the Map association Container.

Characteristics:

• A variable-sized associative container that efficiently retrieves element values based on associated key Values.

• Reversible because it provides a bidirectional iterator to access its elements.

• Ordered because its elements are sorted according to the key values of the specified comparison Function.

• Only. because each of its elements must have a unique key.

• The associative container pair, because its element data value differs from its key Value.

• The template class, because it provides functionality that is generic, regardless of the element or key Type. the data type used for elements and keys is specified as a class template and as a parameter in the comparison function and Allocator.

Header Files: #include <map>

Namespaces: using namespace std:map;

Constructor function:

map<key_type, Value_type>map_show//key_type represents a key type, Value_type represents a value type, such as: map<string, int>map_show; means string as key , the associated map of the shaping position

map<int, int>map_int;//similar Arrays

map<string, int>map_str; Similar to string-subscript arrays

Basic Maintenance Operations 1. access

The subscript accesses the "[]" and the at () methods, and the second method is recommended, which checks the Boundary.

For map<string, int>map_str; is accessed in the following ways:

Map_str["firstel") or map_str.at ("firstel");//return key value Firstel corresponding value

int size () const; returns the number of elements in the MAP.

BOOL Empty () const; Determines whether the map is empty, true indicates null, false is Non-null

Iterator Find (const key& Key); returns an iterator in the map that has the corresponding element position of the key equivalent to the specified key.

note: The Count method:map_str.count ("first")// Returns the number of elements in the map whose keys match the key specified by the Parameter. returns 1 if the map contains an element whose sort key matches the parameter key, or 0 if the map does not contain an element with a matching key.

2. add

For map<string, int>map_str, There are several ways to Insert:

Map_str.insert (make_pair ("key", 23)); Insert key value pair {"key", 23} into the map

Map_str.insert ({"key", 23}); Insert key value pair {"key", 23} into the map

Map_str.insert (map_str.begin () +2, make_pair ("key", 23)); Insert {"key" in the second position, 23} key-value pairs

3. Delete

Iterator Erase (const_iterator Where); Remove a key-value pair from where iterator position

int erase (const key_type& key); Remove key value pairs where key values and key are equal

Iterator erase (const_iterator first, const_iterator last); Removes key-value pairs between first and last iterators

void Clear (); clears all key-value pairs within the map element

4. Traverse

It is very convenient to traverse with Iterators.

Iterator begin (); returns an iterator that points to the first element in a map

Iterator End (); Returns an iterator that points to the last element in the map

 for (map<stringint>::iterator map_it = map_str.begin (); map_it! = map_str.end (); map_it++) {      " key:   " " value:    " << map_it->second;   
Map_it->first represents the first value of a key-value pair, which is the value of the key, Map_it->second represents the second value of a key-value pair, which is the value of the key}

member functions

At	Finds the element with the specified key Value.
Begin	Returns an iterator that points to the first element in the MAP.
Cbegin	Returns a const iterator that points to the first element in a map.
Cend	Returns a const iterator that exceeds the End.
Clear	Clears all the mapped elements.
Count	Returns the number of elements in the map whose keys match the key specified in the Parameter.
Crbegin	Returns a const iterator that points to the first element in a reverse map.
Crend	Returns a const iterator that points to the position after the last element in the reverse map.
Emplace	Inserts an element into the map that is constructed on the Ground.
Emplace_hint	Inserts the element into the map, with a location hint.
Empty	Returns true if the map is Empty.
End	Returns an iterator that exceeds the End.
Equal_range	Returns a pair of Iterators. The first iterator in this iterator pair points to the first element in a map whose key is greater than the specified key. The second iterator in this iterator pair points to the first element in a map whose key is equal to or greater than the specified key.
Erase	Removes the element or range of elements from the map at the specified Location.
Find	Returns an iterator that points to the position of the element whose key is equal to the specified key in the MAP.
Get_allocator	Returns a copy of the allocator object used to construct the map .
Insert	Inserts an element or range of elements into the map at the specified Location.
Key_comp	Returns a copy of the comparison object used to sort the keys in the map.
Lower_bound	Returns an iterator that points to the first element in the map whose key value is equal to or greater than the key value of the specified key.
Max_size	Returns the maximum length of the map.
Rbegin	Returns an iterator that points to the first element in the reverse map.
Rend	Returns an iterator that points to the position after the last element in the reverse map.
Size	Returns the number of elements in the MAP.
Swap	Swaps two elements of a map.
Upper_bound	Returns an iterator that points to the first element in the map whose key value is greater than the key value of the specified key.
Value_comp	Retrieves a copy of the comparison object used to sort the element values in the MAP.
Shrink_to_fit	Discard additional Capacity.
Size	Returns the number of vector elements
Swap	Swaps the elements of two Vectors.

C + + associative containers <map> Simple Summary