std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::count
From cppreference.com
< cpp | container | unordered map
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size_type count( const Key& key ) const;
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(1) | (since C++11) |
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template
<
class K >
size_type count( const K& x ) const ; |
(2) | (since C++20) |
1) Returns the number of elements with key that compares equal to the specified argument key, which is either 0 or 1
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 |
|---|---|---|---|
__cpp_lib_generic_unordered_lookup |
201811L |
(C++20) | Heterogeneous comparison lookup 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) = 0See also
| finds element with specific key (public member function) |
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(C++20)
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checks if the container contains element with specific key (public member function) |
| returns range of elements matching a specific key (public member function) |