NAME¶

std::equal_range - std::equal_range

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt, class T >
(constexpr since
std::pair<ForwardIt, ForwardIt> C++20)
(until C++26)
equal_range( ForwardIt first,
ForwardIt last, const T& value );
template< class ForwardIt, class T =
typename std::iterator_traits

<ForwardIt>::value_type > (since C++26)
constexpr std::pair<ForwardIt, ForwardIt>

equal_range( ForwardIt first,
ForwardIt last, const T& value );
template< class ForwardIt, class T, class
Compare >
(1)
std::pair<ForwardIt, ForwardIt> (constexpr since
equal_range( ForwardIt first, C++20)
ForwardIt last, (until C++26)

const T& value, Compare
comp );
template< class ForwardIt, class T =
typename std::iterator_traits (2)

<ForwardIt>::value_type,
class Compare > (since C++26)
constexpr std::pair<ForwardIt, ForwardIt>
equal_range( ForwardIt first,
ForwardIt last,

const T& value, Compare
comp );

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: (until C++20)

* 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).
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)).

Parameters¶

first, last - the partitioned range of elements to examine
value - value to compare the elements to
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);

comp - 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¶

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).

Complexity¶

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

1) At most \(\scriptsize 2\log_{2}(N)+O(1)\)2log
2(N)+O(1) comparisons with value using
operator<
(until C++20)
std::less{}
(since C++20).
2) At most \(\scriptsize 2\log_{2}(N)+O(1)\)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.

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)

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));
}

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)

Defect reports

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

DR Applied to Behavior as published Correct behavior
Compare was required to satisfy only a partitioning is required;
LWG 270 C++98 Compare and T was required heterogeneous comparisons
to be LessThanComparable (strict permitted
weak ordering required)
at most \(\scriptsize
2\log_{2}(N)+1\)2log corrected to \(\scriptsize
LWG 384 C++98 2(N)+1 comparisons 2\log_{2}(N)+O(1)\)2log
were allowed, which is not 2(N)+O(1)
implementable^[1]

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

Source file:	std::equal_range.3.en.gz (from stdman 2024.07.05-1.1)
Source last updated:	2024-07-08T06:08:56Z
Converted to HTML:	2024-12-31T01:00:11Z