std::unordered_multiset::count(3)

NAME
std::unordered_multiset::count - std::unordered_multiset::count

Synopsis
   size_type count( const Key& key ) const; (1) (since C++11)
   template< class K > (2) (since C++20)
   size_type count( const K& x ) const;
   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
                        Heterogeneous comparison lookup
   __cpp_lib_generic_unordered_lookup 201811L (C++20) in unordered associative
                        containers, overload (2)

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <unordered_set>
 int main()
 {
     std::unordered_multiset set{2, 7, 1, 8, 2, 8, 1, 8, 2, 8};
     std::cout << "The set is:\n";
     for (int e : set)
         std::cout << e << ' ';
     const auto [min, max] = std::ranges::minmax(set);
     std::cout << "\nNumbers [" << min << ".." << max << "] frequency:\n";
     for (int i{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
   find finds element with specific key
               (public member function)
   contains checks if the container contains element with specific key
   (C++20) (public member function)
   equal_range returns range of elements matching a specific key
               (public member function)