std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::count

From cppreference.com

 
 
 
std::unordered_multimap
Member types
Member functions
Iterators
Capacity
Modifiers
Lookup
unordered_multimap::count
Bucket interface
Hash policy
Observers
Non-member functions
(哋它亢++11)(哋它亢++11)(until 哋它亢++20)
Deduction guides (哋它亢++17)
 
size_type count( const Key& key ) const;
(1) (since 哋它亢++11)
template< class K >
size_type count( const K& x ) const;
(2) (since 哋它亢++20)
1) Returns the number of elements with key that compares equal to the specified argument key.
2) Returns the number of elements with key that compares equivalent to the specified argument x. This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.

Parameters

key - key value of the elements to count
x - a value of any type that can be transparently compared with a key

Return value

1) Number of elements with key key.
2) Number of elements with key that compares equivalent to x.

Complexity

linear in the number of elements with key key on average, worst case linear in the size of the container.

Notes

Feature-test macro Value Std Feature
__cpp_lib_generic_unordered_lookup 201811L (哋它亢++20) Heterogeneous comparison lookup in unordered associative containers, overload (2)

Example

#include <iostream>
#include <string>
#include <unordered_map>
 
int main()
{
    std::unordered_multimap<int, std::string> dict = {
        {1, "one"}, {6, "six"}, {3, "three"}
    };
    dict.insert({4, "four"});
    dict.insert({5, "five"});
    dict.insert({6, "six"});
 
    std::cout << "dict: { ";
    for (auto const& [key, value] : dict)
        std::cout << '[' << key << "]=" << value << ' ';
    std::cout << "}\n\n";
 
    for (int i{1}; i != 8; ++i)
        std::cout << "dict.count(" << i << ") = " << dict.count(i) << '\n';
}

Possible output:

dict: { [5]=five [4]=four [1]=one [6]=six [6]=six [3]=three }
 
dict.count(1) = 1
dict.count(2) = 0
dict.count(3) = 1
dict.count(4) = 1
dict.count(5) = 1
dict.count(6) = 2
dict.count(7) = 0

See also

finds element with specific key
(public member function)
(哋它亢++20)
checks if the container contains element with specific key
(public member function)
returns range of elements matching a specific key
(public member function)