std::unordered_set<Key,Hash,KeyEqual,Allocator>::count - cppreference.com (original) (raw)

size_type count( const Key& key ) const; (1) (since C++11)
template< class K >size_type count( const K& x ) const; (2) (since C++20)

  1. Returns the number of elements with key that compares equal to the specified argument key, which is either ​0​ or 1 since this container does not allow duplicates.

  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 and KeyEqual are both transparent. 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.

[edit] Parameters

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

[edit] Return value

  1. Number of elements with key key, that is either 1 or ​0​.

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

[edit] Complexity

Constant on average, worst case linear in the size of the container.

[edit] Notes

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

[edit] Example

#include #include #include   int main() { std::unordered_set set{2, 7, 1, 8, 2, 8, 1, 8, 2, 8};   std::cout << "The set is: "; for (int e : set) std::cout << e << ' ';   const auto [min, max] = std::ranges::minmax(set);   std::cout << "\nNumbers from " << min << " to " << max << " that are in the set: "; for (int i{min}; i <= max; ++i) if (set.count(i) == 1) std::cout << i << ' '; std::cout << '\n'; }

Possible output:

The set is: 8 1 7 2 Numbers from 1 to 8 that are in the set: 1 2 7 8

[edit] See also

| | finds element with specific key (public member function) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | | checks if the container contains element with specific key (public member function) [edit] | | | returns range of elements matching a specific key (public member function) [edit] |