std::equal_range

Defined in header `<algorithm>`
	(1)
template< class ForwardIt, class T > std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value );			(until C++20)
template< class ForwardIt, class T > constexpr std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value );			(since C++20)
		(2)
template< class ForwardIt, class T, class Compare > std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value, Compare comp );				(until C++20)
template< class ForwardIt, class T, class Compare > constexpr std::pair<ForwardIt,ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value, Compare comp );				(since C++20)

Returns a range containing all elements equivalent to value in the range [first, last).

The range [first, last) must be at least partially ordered with respect to value, i.e. it must satisfy all of the following requirements:

partitioned with respect to element < value or comp(element, value) (that is, all elements for which the expression is true precedes all elements for which the expression is false)
partitioned with respect to !(value < element) or !comp(value, element)
for all elements, if element < value or comp(element, value) is true then !(value < element) or !comp(value, element) is also true

A fully-sorted range meets these criteria.

The returned range is defined by two iterators, one pointing to the first element that is not less than value and another pointing to the first element greater than value. The first iterator may be alternatively obtained with std::lower_bound(), the second - with std::upper_bound().

The first version uses operator< to compare the elements, the second version uses the given comparison function comp.

Parameters

first, last	-	the range of elements to examine
value	-	value to compare the elements to
comp	-	binary predicate which returns true if the first argument is less than (i.e. is ordered before) the second. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for `Type1` a move is equivalent to a copy (since C++11)). The types Type1 and Type2 must be such that an object of type T can be implicitly converted to both Type1 and Type2, and an object of type ForwardIt can be dereferenced and then implicitly converted to both Type1 and Type2.
Type requirements
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `Compare` must meet the requirements of BinaryPredicate. it is not required to satisfy Compare

Return value

std::pair containing a pair of iterators defining the wanted range, the first pointing to the first element that is not less than value and the second pointing to the first element greater than value.

If there are no elements not less than value, last is returned as the first element. Similarly if there are no elements greater than value, last is returned as the second element

Complexity

The number of comparisons performed is logarithmic in the distance between first and last (At most $2 * log 2 (last - first) + O(1)$ comparisons). However, for non-LegacyRandomAccessIterators, the number of iterator increments is linear. Notably, std::set and std::multiset iterators are not random access, and so their member functions std::set::equal_range (resp. std::multiset::equal_range) should be preferred.

Possible implementation

First version

template<class ForwardIt, class T>
std::pair<ForwardIt,ForwardIt> 
    equal_range(ForwardIt first, ForwardIt last,
                const T& value)
{
    return std::make_pair(std::lower_bound(first, last, value),
                          std::upper_bound(first, last, value));
}

Second version

template<class ForwardIt, class T, class Compare>
std::pair<ForwardIt,ForwardIt> 
    equal_range(ForwardIt first, ForwardIt last,
                const T& value, Compare comp)
{
    return std::make_pair(std::lower_bound(first, last, value, comp),
                          std::upper_bound(first, last, value, comp));
}

Example

Run this code

#include <algorithm>
#include <vector>
#include <iostream>
 
struct S
{
    int number;
    char name;
    // note: name is ignored by this comparison operator
    bool operator< ( const S& s ) const { return number < s.number; }
};
 
int main()
{
    // note: not ordered, only partitioned w.r.t. S defined below
    const std::vector<S> vec = { {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4,'G'}, {3,'F'} };
 
    const S value = {2, '?'};
 
    std::cout << "Compare using S::operator<(): ";
    const auto p = std::equal_range(vec.begin(), vec.end(), value);
 
    for ( auto i = p.first; i != p.second; ++i )
        std::cout << i->name << ' ';
 
 
    std::cout << "\n" "Using heterogeneous comparison: ";
    struct Comp
    {
        bool operator() ( const S& s, int i ) const { return s.number < i; }
        bool operator() ( int i, const S& s ) const { return i < s.number; }
    };
 
    const auto p2 = std::equal_range(vec.begin(),vec.end(), 2, Comp{});
 
    for ( auto i = p2.first; i != p2.second; ++i )
        std::cout << i->name << ' ';
}

Output:

Compare using S::operator<(): B C D 
Using heterogeneous comparison: B C D

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 270	C++98	Compare was required to be a strict weak ordering	only a partitioning is needed; heterogeneous comparisons permitted

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

lower_bound	returns an iterator to the first element not less than the given value (function template)
upper_bound	returns an iterator to the first element greater than a certain value (function template)
binary_search	determines if an element exists in a partially-ordered range (function template)
partition	divides a range of elements into two groups (function template)
equal	determines if two sets of elements are the same (function template)
equal_range	returns range of elements matching a specific key (public member function of `std::set<Key,Compare,Allocator>`)
equal_range	returns range of elements matching a specific key (public member function of `std::multiset<Key,Compare,Allocator>`)
ranges::equal_range (C++20)	returns range of elements matching a specific key (niebloid)