home | help 
std::unordered_map::count(3)   C++ Standard Libary  std::unordered_map::count(3)

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 de-
     notes 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 compar-
     ison lookup
	__cpp_lib_generic_unordered_lookup 201811L (C++20) in unordered associa-
     tive
							   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)

http://cppreference.com 	   2024.06.10	    std::unordered_map::count(3)
home | help