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C++ Usage and examples of map

map is C ++ Part of STL (Standard Template Library). map is an associative container that stores sorted key-value pairs, where each key is unique, can be inserted or deleted, but cannot be changed. However, the value associated with the key can be changed.

For example:An employee map container where the employee ID is the key and the name is the value, can be represented as:

Key	Value
101	Nikita
102	Robin
103	Deep
104	John

Syntax

Example

template < class Key,</　　　　　　　　　　　　　　　　　　　　　//map::key_type
　　　　　　　　　　　class T,</　　　　　　　　　　　　　　　　　//map::mapped_type
　　　　　　　　　　　class Compare = less,</　　　　　　　　　//map::key_compare
　　　　　　　　　　　class Alloc = allocator<pair>　　　　//map::allocator_type
　　　　　　　　　　　> class map;

parameters

key:The data type of the key to be stored in the map.

type:The data type of the value to be stored in the map.

compare:A comparison class that accepts two bool type parameters and returns a value. This parameter is optional, and the binary predicate less <" key"> is the default value.

alloc:The type of the allocator object. This parameter is optional, and the default value is the allocator.

Create map

You can easily create a map with the following statement:

typedef pair<const Key, T> value_type;

The following statement will be used to create a key type ofKey type,andvaluetypeis of typeThe map.An important point is that the keys and corresponding values in the map are always inserted in pairs, you cannot insert only keys or only values in the map.

Instance1

Example

#include <string.h>　　
#include <iostream>　　
#include <map>　　
#include <utility>　　
using namespace std;　　
int main()
{
　　　mapEmployees;
　　　//　1) using array index notation for assignment
　　　Employees[101] = "Nikita";
　　　Employees[105] = "John";
　　　Employees[103] = "Dolly";
　　　Employees[104] = "Deep";
　　　Employees[102] = "Aman";
　　　cout << "Employees[104] = << Employees[104] << endl << endl;
　　　cout << "Map size: " << Employees.size() << endl;
　　　cout << endl << "Natural order:" << endl;
　　　for (map::iterator ii = Employees.begin(); ii != Employees.end();　++ii)
　　　{
　　　　　　　cout << (*ii).first << ": " << (*ii).second << endl;
　　　}
　　　cout << endl << "Reverse order:" << endl;
　　　for(map::reverse_iterator ii=Employees.rbegin(); ii!=Employees.rend();　++ii)
　　　{
　　　　　　　cout << (*ii).first << ": " << (*ii).second << endl;
　　　}
}

Output:

Employees[104=Deep
Map size:　5
Natural order:
101:　Nikita
102:　Aman
103:　Dolly
104:　Deep
105:　John
Reverse order:
105:　John
104:　Deep
103:　Dolly
102:　Aman
101:　Nikita

Member functions

The following is a list of all member functions of map:

Constructor/Destructor

Function	Description
constructors	Construct map
destructors	Map destructor
operator=	Copy the elements of the map to another map container.

Iterator

Function	Description
begin	Return an iterator pointing to the first element in the map.
cbegin	Return a const iterator pointing to the first element in the map.
end	Return an iterator pointing to the end.
cend	Return a constant iterator pointing to the end.
rbegin	Return a reverse iterator pointing to the end.
rend	Return a reverse iterator pointing to the beginning.
crbegin	Return a constant reverse iterator pointing to the end.
crend	Return a constant reverse iterator pointing to the beginning.

Capacity

Function	Description
empty	Return true if the map is empty.
size	Return the number of elements in the map.
max_size	Return the maximum capacity of the map.

Element access

Function	Description
operator[]	Retrieve the element with the given key.
at	Retrieve the element with the given key.

Modifier

Function	Description
insert	Insert elements into the map.
erase	Erase elements from the map.
swap	Swap the contents of the map.
clear	Delete all elements of the map.
emplace	Construct a new element and insert it into the map.
emplace_hint	Construct a new element through hint and insert it into the map.

Observer

Function	Description
key_comp	Return a copy of the key comparison object.
value_comp	Return a copy of the value comparison object.

Operation mode

Function	Description
find	Search for an element with the given key.
count	Get the number of elements that match the given key.
lower_bound	Return the lower bound of the iterator.
upper_bound	Return an iterator to the upper bound.
equal_range	Return the range of elements that match the given key.

Allocator

Function	Description
get_allocator	Return the allocator object used to construct the map.

Non-member Overloaded Function

Function	Description
operator==	Check if two maps are equal.
operator!=	Check if two maps are equal.
operator<	Check if the first map is less than other maps.
operator<=	Check if the first map is less than or equal to other maps.
operator>	Check if the first map is greater than other maps.
operator>=	Check if the first map is greater than other maps.
swap()	Swap elements of two maps.

C++ <cmath>c++ Priority Queue (priority_queue)