Namespaces
Variants
Views
Actions

std::set<Key,Compare,Allocator>::count

From cppreference.com
< cpp‎ | container‎ | set
size_type count( const Key& key ) const;
(1)
template< class K >
size_type count( const K& x ) const;
(2) (since C++14)

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 only participates in overload resolution if the qualified-id Compare::is_transparent is valid and denotes a type. They allow calling this function without constructing an instance of Key.

Contents

[edit] Parameters

key - key value of the elements to count
x - alternative value to compare to the keys

[edit] Return value

Number of elements with key that compares equivalent to key or x, which is either 1 or 0 for (1).

[edit] Complexity

Logarithmic in the size of the container.

[edit] Example

#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 created.
        // 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.

[edit] See also

finds element with specific key
(public member function) [edit]
returns range of elements matching a specific key
(public member function) [edit]