Containers library
Unordered associative
std::pair<iterator,iterator> equal_range( const Key& key );
(1) (since C++11)
std::pair<const_iterator,const_iterator> equal_range( const Key& key ) const;
(2) (since C++11)
template< class K >
std::pair<iterator,iterator> equal_range( const K& x );
(3) (since C++20)
template< class K >
std::pair<const_iterator,const_iterator> equal_range( const K& x ) const;
(4) (since C++20)
1,2) Returns a range containing all elements with key key in the container. The range is defined by two iterators, the first pointing to the first element of the wanted range and the second pointing past the last element of the range.
3,4) Returns a range containing all elements in the container with key equivalent to 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.


[edit] Parameters

key - key value to compare the elements to
x - a value of any type that can be transparently compared with a key

[edit] Return value

std::pair containing a pair of iterators defining the wanted range. If there are no such elements, past-the-end (see end()) iterators are returned as both elements of the pair.

[edit] Complexity

Average case linear in the number of elements with the key key, worst case linear in the size of the container.

[edit] Example

Estimates the numbers frequency for given numeric sequence.

#include <iostream>
#include <iterator>
#include <unordered_set>
int main()
    std::unordered_multiset sequence{3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9};
    std::cout << "The sequence:\n";
    for (int x : sequence) { std::cout << x << ' '; }
    std::cout << "\nNumber:Frequency:\n";
    for (int number{0}; number != 10; ++number) {
        const auto r = sequence.equal_range(number);
        std::cout << number << ":" << std::distance(r.first, r.second) << "; ";

Possible output:

The sequence:
7 8 6 2 9 9 9 5 5 5 4 3 3 1 1 
0:0; 1:2; 2:1; 3:2; 4:1; 5:3; 6:1; 7:1; 8:1; 9:3;

[edit] See also

finds element with specific key
(public member function) [edit]
checks if the container contains element with specific key
(public member function) [edit]
returns the number of elements matching specific key
(public member function) [edit]
returns range of elements matching a specific key
(function template) [edit]