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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, which is either 1 or 0 since this container does not allow duplicates. 1) Returns the number of elements with key key. 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. They allow calling 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 is either 1 or 0 for (1). Complexity Logarithmic in the size of the container. Example // Run this code #include <set> #include <iostream> 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 2022.07.31 std::set::count(3)
NAME | Synopsis | Parameters | Return value | Complexity | Example | Output: | See also
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