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 );			(constexpr since C++20) (until C++26)
template< class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > constexpr std::pair<ForwardIt, ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value );			(since C++26)
		(2)
template< class ForwardIt, class T, class Compare > std::pair<ForwardIt, ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value, Compare comp );				(constexpr since C++20) (until C++26)
template< class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type, class Compare > constexpr std::pair<ForwardIt, ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value, Compare comp );				(since C++26)

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

1) The equivalence is checked using operator<:

Returns the results of std::lower_bound(first, last, value) and std::upper_bound(first, last, value).

If any of the following conditions is satisfied, the behavior is undefined:

For any element elem of [first, last), bool(elem < value) does not imply !bool(value < elem).
The elements elem of [first, last) are not partitioned with respect to expressions bool(elem < value) and !bool(value < elem).

(until C++20)

Equivalent to std::equal_range(first, last, value, std::less{}).

(since C++20)

2) The equivalence is checked using comp:

Returns the results of std::lower_bound(first, last, value, comp) and std::upper_bound(first, last, value, comp).

If any of the following conditions is satisfied, the behavior is undefined:

For any element elem of [first, last), bool(comp(elem, value)) does not imply !bool(comp(value, elem)).
The elements elem of [first, last) are not partitioned with respect to expressions bool(comp(elem, value)) and !bool(comp(value, elem)).

first, last	-	the partitioned range of elements to examine
value	-	value to compare the elements to
comp	-	binary predicate which returns true if the first argument 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.

[edit] Return value

A std::pair containing a pair of iterators, where

first is an iterator to the first element of the range [first, last) not ordered before value (or last if no such element is found), and
second is an iterator to the first element of the range [first, last) ordered after value (or last if no such element is found).

[edit] Complexity

Given $\scriptsize N$ $N$ as std::distance(first, last):

1) At most

2log 2 (N)+O(1)

comparisons with value using operator<(until C++20)std::less{}(since C++20).

2) At most

2log 2 (N)+O(1)

applications of the comparator comp.

However, if ForwardIt is not a LegacyRandomAccessIterator, the number of iterator increments is linear in $\scriptsize N$ $N$ . 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.

[edit] Notes

Although std::equal_range only requires [first, last) to be partitioned, this algorithm is usually used in the case where [first, last) is sorted, so that the binary search is valid for any value.

On top of the requirements of std::lower_bound and std::upper_bound, std::equal_range also requires operator< or comp to be asymmetric (i.e., a < b and b < a always have different results).

Therefore, the intermediate results of binary search can be shared by std::lower_bound and std::upper_bound. For example, the result of the std::lower_bound call can be used as the argument of first in the std::upper_bound call.

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	202403	(C++26)	List-initialization for algorithms (1,2)

[edit] Possible implementation

equal_range (1)

template<class ForwardIt,
         class T = typename std::iterator_traits<ForwardIt>::value_type>
constexpr std::pair<ForwardIt, ForwardIt> 
    equal_range(ForwardIt first, ForwardIt last, const T& value)
{
    return std::equal_range(first, last, value, std::less{});
}

equal_range (2)

template<class ForwardIt,
         class T = typename std::iterator_traits<ForwardIt>::value_type,
         class Compare>
constexpr 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));
}

[edit] Example

Run this code

#include <algorithm>
#include <complex>
#include <iostream>
#include <vector>
 
struct S
{
    int number;
    char name;
    // note: name is ignored by this comparison operator
    bool operator<(const S& s) const { return number < s.number; }
};
 
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; }
};
 
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 it = p.first; it != p.second; ++it)
        std::cout << it->name << ' ';
    std::cout << '\n';
 
    std::cout << "Using heterogeneous comparison: ";
    const auto p2 = std::equal_range(vec.begin(), vec.end(), 2, Comp{});
 
    for (auto it = p2.first; it != p2.second; ++it)
        std::cout << it->name << ' ';
    std::cout << '\n';
 
    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 0}, {2, 2}, {2, 1}, {3, 0}, {3, 1}};
    auto cmpz = [](CD x, CD y) { return x.real() < y.real(); };
    #ifdef __cpp_lib_algorithm_default_value_type
        auto p3 = std::equal_range(nums.cbegin(), nums.cend(), {2, 0}, cmpz);
    #else
        auto p3 = std::equal_range(nums.cbegin(), nums.cend(), CD{2, 0}, cmpz);
    #endif
 
    for (auto it = p3.first; it != p3.second; ++it)
        std::cout << *it << ' ';
    std::cout << '\n';
}

Output:

Compare using S::operator<(): B C D 
Using heterogeneous comparison: B C D
(2,2) (2, 1)

[edit] 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 satisfy Compare and `T` was required to be LessThanComparable (strict weak ordering required)	only a partitioning is required; heterogeneous comparisons permitted
LWG 384	C++98	at most $\scriptsize 2\log_{2}(N)+1$ $2log 2 (N)+1$ comparisons were allowed, which is not implementable^[1]	corrected to $\scriptsize 2\log_{2}(N)+O(1)$ $2log 2 (N)+O(1)$

↑ Applying equal_range to a single-element range requires 2 comparisons, but at most 1 comparison is allowed by the complexity requirement.

[edit] See also

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

[1] Applying equal_range to a single-element range requires 2 comparisons, but at most 1 comparison is allowed by the complexity requirement.

[1]

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std::equal_range

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[edit] Parameters