From cppreference.com
size_typecount(constKey&key)const; (1) (since C++11)template<classK>size_typecount(constK&x)const; (2) (since C++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
macroValueStdFeature
__cpp_lib_generic_unordered_lookup
(C++20)Heterogeneous comparison lookup in
unordered associative containers
, overload (2)Example
Run this code
#include<algorithm>#include<iostream>#include<unordered_set>intmain(){std::unordered_multisetset{2,7,1,8,2,8,1,8,2,8};std::cout<<"The set is:\n";for(inte:set)std::cout<<e<<' ';constauto[min,max]=std::ranges::minmax(set);std::cout<<"\nNumbers ["<<min<<".."<<max<<"] frequency:\n";for(inti{min};i<=max;++i)std::cout<<i<<':'<<set.count(i)<<"; ";std::cout<<'\n';}Possible output:
The set is: 8 8 8 8 1 1 7 2 2 2 Numbers [1..8] frequency: 1:2; 2:3; 3:0; 4:0; 5:0; 6:0; 7:1; 8:4; See also
finds element with specific key
(public member function)
(C++20)
checks if the container contains element with specific key
(public member function)
returns range of elements matching a specific key
(public member function)