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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

Views

span (C++20)

 

std::set

Member functions

set::set

set::~set

set::operator=

set::get_allocator

Iterators

set::beginset::cbegin (C++11)

set::endset::cend (C++11)

set::rbeginset::crbegin (C++11)

set::rendset::crend (C++11)

Capacity

set::empty

set::size

set::max_size

Modifiers

set::clear

set::insert

set::emplace (C++11)

set::emplace_hint (C++11)

set::erase

set::swap

set::extract (C++17)

set::merge (C++17)

Lookup

set::count

set::find

set::contains (C++20)

set::equal_range

set::lower_bound

set::upper_bound

Observers

set::key_comp

set::value_comp

Non-member functions

std::swap

erase_if (C++20)

operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

Deduction guides(C++17)

 

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 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	-	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 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.

See also

find	finds element with specific key (public member function)
equal_range	returns range of elements matching a specific key (public member function)