std::unordered_set<Key,Hash,KeyEqual,Allocator>::count
From cppreference.com
< cpp | container | unordered set
| 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 1 or 0 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::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, that is either 1 or 0.2) Number of elements with key that compares equivalent to
x.Complexity
Constant on average, worst case linear in the size of the container.
Notes
| Feature-test macro | Comment |
|---|---|
__cpp_lib_generic_unordered_lookup |
for overload (2) |
Example
Run this code
#include <algorithm> #include <iostream> #include <unordered_set> 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 << ' '; } } }
Possible output:
The set is: 8 1 7 2 Numbers from 1 to 8 that are in the set: 1 2 7 8
See also
| (C++11) |
finds element with specific key (public member function) |
| (C++20) |
checks if the container contains element with specific key (public member function) |
| (C++11) |
returns range of elements matching a specific key (public member function) |