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Showing content from https://en.cppreference.com/w/cpp/container/unordered_multiset/equal_range.html below:

std::unordered_multiset<Key,Hash,KeyEqual,Allocator>::equal_range - cppreference.com

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

and

KeyEqual

are both

transparent

. 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] Notes [edit] Example

Estimates the characters frequency for given string.

#include <iostream>
#include <iterator>
#include <string>
#include <unordered_set>
 
int main()
{
    std::string sentence{"cppreference.com"};
    std::cout << "The sentence: " << sentence << '\n';
 
    std::unordered_multiset<char> sequence;
    for (char x : sentence)
        sequence.insert(x);
 
    std::cout << "The sequence: { ";
    for (char x : sequence)
        std::cout << x << ' ';
 
    std::cout << "}\n" "Symbol:Frequency: ";
    for (auto it = sequence.begin(); it != sequence.end();)
    {
        if (auto [first, last] = sequence.equal_range(*it); first != last)
        {
            std::cout << *first << ":" << std::distance(first, last) << "  ";
            it = last;
        }
        else
            ++it;
    }
}

Possible output:

The sentence: cppreference.com
The sequence: { m o c c c p p r r e e e e f n . }
Symbol:Frequency: m:1  o:1  c:3  p:2  r:2  e:4  f:1  n:1  .:1

[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]

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