NAME¶

std::unordered_map::count - std::unordered_map::count

Synopsis¶

size_type count( const Key& key ) const; (1) (since C++11)
template< class K > (2) (since C++20)
size_type count( const K& x ) const;

1) Returns the number of elements with key that compares equal to the specified
argument key, which is either 0 or 1 since this container does not allow duplicates.
2) Returns the number of elements with key that compares equivalent to the specified
argument x. This overload participates in overload resolution only if
Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type.
This assumes that such Hash is callable with both K and Key type, and that the
KeyEqual is transparent, which, together, allows 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¶

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

Complexity¶

Constant on average, worst case linear in the size of the container.

Notes¶

Feature-test macro Value Std Feature
Heterogeneous comparison lookup
__cpp_lib_generic_unordered_lookup 201811L (C++20) in unordered associative
containers, overload (2)

Example¶

// Run this code

#include <iostream>
#include <string>
#include <unordered_map>

int main()
{
std::unordered_map<int, std::string> dict = {
{1, "one"}, {6, "six"}, {3, "three"}
};
dict.insert({4, "four"});
dict.insert({5, "five"});
dict.insert({6, "six"});

std::cout << "dict: { ";
for (auto const& [key, value] : dict)
std::cout << '[' << key << "]=" << value << ' ';
std::cout << "}\n\n";

for (int i{1}; i != 8; ++i)
std::cout << "dict.count(" << i << ") = " << dict.count(i) << '\n';
}

Possible output:¶

dict: { [5]=five [4]=four [1]=one [6]=six [3]=three }

dict.count(1) = 1
dict.count(2) = 0
dict.count(3) = 1
dict.count(4) = 1
dict.count(5) = 1
dict.count(6) = 1
dict.count(7) = 0

See also¶

find finds element with specific key
(public member function)
contains checks if the container contains element with specific key
(C++20) (public member function)
equal_range returns range of elements matching a specific key
(public member function)