std::ranges::equal_range

 

C++

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Feature test macros (C++20)

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Containers library

Iterators library

Ranges library (C++20)

Algorithms library

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Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Constrained algorithms

All names in this menu belong to namespace std::ranges

Non-modifying sequence operations

all_ofany_ofnone_of for_each for_each_n    countcount_if adjacent_find mismatch equal lexicographical_compare    containscontains_subrange (C++23)(C++23) findfind_iffind_if_not find_lastfind_last_iffind_last_if_not (C++23)(C++23)(C++23)   find_end find_first_of search search_n starts_with (C++23) ends_with (C++23)

Modifying sequence operations

copycopy_if copy_n copy_backward    move move_backward shift_leftshift_right (C++23)(C++23)   fill fill_n generate generate_n    transform swap_ranges    shuffle sample removeremove_if replacereplace_if reverse rotate remove_copyremove_copy_if    unique replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy

Partitioning operations

is_partitioned    partition_point    partition    partition_copy    stable_partition

Sorting operations

is_sorted is_sorted_until     sort stable_sort         partial_sort       nth_element partial_sort_copy

Binary search operations (on sorted ranges)

lower_bound         upper_bound         binary_search      equal_range

Set operations (on sorted ranges)

merge inplace_merge       set_difference set_intersection    set_union          includes set_symmetric_difference

Heap operations

is_heap is_heap_until       make_heap           sort_heap push_heap pop_heap

Minimum/maximum operations

max max_element         min min_element         minmax minmax_element     clamp

Permutation operations

is_permutation      next_permutation    prev_permutation

Fold operations

fold_left (C++23) fold_left_first (C++23)   fold_right (C++23) fold_right_last (C++23)   fold_left_with_iter (C++23) fold_left_first_with_iter (C++23)

Numeric operations iota (C++23)             Random number generation generate_random (C++26)

Operations on uninitialized storage

construct_at    destroy destroy_n destroy_at uninitialized_copy    uninitialized_move uninitialized_fill uninitialized_copy_n    uninitialized_move_n uninitialized_fill_n uninitialized_default_construct uninitialized_default_construct_n uninitialized_value_construct uninitialized_value_construct_n

Return types

in_fun_result in_in_result in_found_result    in_out_result in_in_out_result    in_out_out_result    min_max_result out_value_result (C++23) in_value_result (C++23)

 

Defined in header <algorithm>
Call signature
	(1)
template< std::forward_iterator I, std::sentinel_for<I> S,           class T, class Proj = std::identity,           std::indirect_strict_weak_order               <const T*, std::projected<I, Proj>> Comp = ranges::less > constexpr ranges::subrange<I> equal_range( I first, S last, const T& value,                                            Comp comp = {}, Proj proj = {} );			(since C++20) (until C++26)
template< std::forward_iterator I, std::sentinel_for<I> S,           class Proj = std::identity,           class T = std::projected_value_t<I, Proj>,           std::indirect_strict_weak_order               <const T*, std::projected<I, Proj>> Comp = ranges::less > constexpr ranges::subrange<I> equal_range( I first, S last, const T& value,                                            Comp comp = {}, Proj proj = {} );			(since C++26)
		(2)
template< ranges::forward_range R,           class T, class Proj = std::identity,           std::indirect_strict_weak_order               <const T*, std::projected<ranges::iterator_t<R>,                                         Proj>> Comp = ranges::less > constexpr ranges::borrowed_subrange_t<R>     equal_range( R&& r, const T& value, Comp comp = {}, Proj proj = {} );				(since C++20) (until C++26)
template< ranges::forward_range R,           class Proj = std::identity,           class T = std::projected_value_t<ranges::iterator_t<R>, Proj>,           std::indirect_strict_weak_order               <const T*, std::projected<ranges::iterator_t<R>,                                         Proj>> Comp = ranges::less > constexpr ranges::borrowed_subrange_t<R>     equal_range( R&& r, const T& value, Comp comp = {}, Proj proj = {} );				(since C++26)

1) Returns a view 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 view is constructed from 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::ranges::lower_bound(), the second - with std::ranges::upper_bound().

2) Same as (1), but uses r as the source range, as if using the range ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first, last	-	the range of elements to examine
r	-	the range of the elements to examine
value	-	value to compare the elements to
comp	-	if the first argument is less than (i.e. is ordered before) the second
proj	-	projection to apply to the elements

Return value

std::ranges::subrange 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, the last iterator (iterator that is equal to last or ranges::end(r)) is returned as the first element. Similarly if there are no elements greater than value, the last iterator is returned as the second element.

Complexity

The number of comparisons performed is logarithmic in the distance between first and last (at most 2 * log2(last - first) + O(1) comparisons). However, for an iterator that does not model random_access_iterator, the number of iterator increments is linear.

Possible implementation

struct equal_range_fn { template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj>, std::indirect_strict_weak_order <const T*, std::projected<I, Proj>> Comp = ranges::less> constexpr ranges::subrange<I> operator()(I first, S last, const T& value, Comp comp = {}, Proj proj = {}) const { return ranges::subrange ( ranges::lower_bound(first, last, value, std::ref(comp), std::ref(proj)), ranges::upper_bound(first, last, value, std::ref(comp), std::ref(proj)) ); }   template<ranges::forward_range R, class Proj = std::identity, class T = std::projected_value_t<ranges::iterator_t<R>, Proj>, std::indirect_strict_weak_order <const T*, std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less> constexpr ranges::borrowed_subrange_t<R> operator()(R&& r, const T& value, Comp comp = {}, Proj proj = {}) const { return (*this)(ranges::begin(r), ranges::end(r), value, std::ref(comp), std::ref(proj)); } };   inline constexpr equal_range_fn equal_range;

Notes

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

Example

Run this code

#include <algorithm>
#include <compare>
#include <complex>
#include <iostream>
#include <vector>
 
struct S
{
    int number {};
    char name {};
    // note: name is ignored by these comparison operators
    friend bool operator== (const S s1, const S s2) { return s1.number == s2.number; }
    friend auto operator<=>(const S s1, const S s2) { return s1.number <=> s2.number; }
    friend std::ostream& operator<<(std::ostream& os, S o)
    {
        return os << '{' << o.number << ", '" << o.name << "'}";
    }
};
 
void println(auto rem, const auto& v)
{
    for (std::cout << rem; const auto& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    // note: not ordered, only partitioned w.r.t. S defined below
    std::vector<S> vec
    {
        {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4, 'D'}, {4,'G'}, {3,'F'}
    };
 
    const S value{2, '?'};
 
    namespace ranges = std::ranges;
 
    auto a = ranges::equal_range(vec, value);
    println("1. ", a);
 
    auto b = ranges::equal_range(vec.begin(), vec.end(), value);
    println("2. ", b);
 
    auto c = ranges::equal_range(vec, 'D', ranges::less {}, &S::name);
    println("3. ", c);
 
    auto d = ranges::equal_range(vec.begin(), vec.end(), 'D', ranges::less {}, &S::name);
    println("4. ", d);
 
    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 = ranges::equal_range(nums, {2, 0}, cmpz);
    #else
        auto p3 = ranges::equal_range(nums, CD{2, 0}, cmpz);
    #endif
    println("5. ", p3);
}

Output:

1. {2, 'B'} {2, 'C'} {2, 'D'}
2. {2, 'B'} {2, 'C'} {2, 'D'}
3. {2, 'D'} {4, 'D'}
4. {2, 'D'} {4, 'D'}
5. (2,2) (2,1)

See also

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