Difference between revisions of "cpp/container/unordered multimap/count"
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Latest revision as of 10:55, 4 December 2021
| size_type count( const Key& key ) const; |
(1) | (since C++11) |
| 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.
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.Contents |
[edit] Parameters
| key | - | key value of the elements to count |
| x | - | a value of any type that can be transparently compared with a key |
[edit] Return value
1) Number of elements with key key.
2) Number of elements with key that compares equivalent to x.
[edit] Complexity
linear in the number of elements with key key on average, worst case linear in the size of the container.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_generic_unordered_lookup |
201811L | (C++20) | Heterogeneous comparison lookup in unordered associative containers, overload (2) |
[edit] Example
Run this code
#include <iostream> #include <string> #include <unordered_map> int main() { std::unordered_multimap<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 [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) = 2
dict.count(7) = 0[edit] See also
| finds element with specific key (public member function) | |
| (C++20) |
checks if the container contains element with specific key (public member function) |
| returns range of elements matching a specific key (public member function) |
