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std::set count() method

// (1) Const version
size_type count( const Key& key ) const;

// (2) Const version
template< class K >
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 qualified-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 - a value of any type that can be transparently compared with a key

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 - O(log size()).

Exceptions

(none)

Example

Main.cpp
#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.
}
Possible 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.
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.

std::set count() method

// (1) Const version
size_type count( const Key& key ) const;

// (2) Const version
template< class K >
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 qualified-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 - a value of any type that can be transparently compared with a key

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 - O(log size()).

Exceptions

(none)

Example

Main.cpp
#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.
}
Possible 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.
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.