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std::set::count(3) C++ Standard Libary std::set::count(3) NAME std::set::count - std::set::count Synopsis size_type count( const Key& key ) const; (1) template< class K > (2) (since C++14) size_type count( const K& x ) const; Returns the number of elements with key that compares equivalent to the specified argument. 1) Returns the number of elements with key key. This 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 value x. This overload participates in overload resolution only if the quali- fied-id Compare::is_transparent is valid and denotes a type. It allows call- ing this function without constructing an instance of Key. Parameters key - key value of the elements to count x - alternative value to compare to the keys Return value Number of elements with key that compares equivalent to key or x, which, for overload (1), is either 1 or 0. Complexity Logarithmic in the size of the container plus linear in the number of elements found. Notes Feature-test macro Value Std Feature Heterogeneous comparison lookup __cpp_lib_generic_associative_lookup 201304L (C++14) in associative containers; overload (2) Example // Run this code #include <functional> #include <iostream> #include <set> struct S { int x; S(int i) : x{i} { std::cout << "S{" << i << "} "; } bool operator<(S const& s) const { return x < s.x; } }; struct R { int x; R(int i) : x{i} { std::cout << "R{" << i << "} "; } bool operator<(R const& r) const { return x < r.x; } }; bool operator<(R const& r, int i) { return r.x < i; } bool operator<(int i, R const& r) { return i < r.x; } int main() { std::set<int> t{3, 1, 4, 1, 5}; std::cout << t.count(1) << ", " << t.count(2) << ".\n"; std::set<S> s{3, 1, 4, 1, 5}; std::cout << ": " << s.count(1) << ", " << s.count(2) << ".\n"; // Two temporary objects S{1} and S{2} were created. // Comparison function object is defaulted std::less<S>, // which is not transparent (has no is_transparent member type). std::set<R, std::less<>> r{3, 1, 4, 1, 5}; std::cout << ": " << r.count(1) << ", " << r.count(2) << ".\n"; // C++14 heterogeneous lookup; temporary objects were not cre- ated. // Comparator std::less<void> has predefined is_transparent. } Output: 1, 0. S{3} S{1} S{4} S{1} S{5} : S{1} 1, S{2} 0. R{3} R{1} R{4} R{1} R{5} : 1, 0. See also find finds element with specific key (public member function) equal_range returns range of elements matching a specific key (public member function) http://cppreference.com 2024.06.10 std::set::count(3)
NAME | Synopsis | Parameters | Return value | Complexity | Notes | Example | Output: | See also
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