std::equal_range
| Defined in header <algorithm>
|
||
template< class ForwardIt, class T >
std::pair<ForwardIt, ForwardIt>
equal_range( ForwardIt first, ForwardIt last, const T& value );
|
(1) | (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) | |
template< class ForwardIt, class T, class Compare >
std::pair<ForwardIt, ForwardIt>
equal_range( ForwardIt first, ForwardIt last,
const T& value, Compare comp );
|
(2) | (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) | |
Searches for the range containing all elements equivalent to value in the partitioned source range [first, last). An element is considered equivalent to value if it neither orders before nor orders after value.
operator<(until C++20)std::less{}(since C++20).bool(e < value) and !bool(value < e)(until C++20)bool(std::less{}(e, value)) and !bool(std::less{}(value, e))(since C++20). If any of the following conditions is satisfied, the behavior is undefined:
- The elements
eof the source range are not partitioned with respect to these two expressions at the same time. - There exists an element
ein the source range such that these two expressions yield different values.
comp.bool(comp(e, value)) and !bool(comp(value, e)). If any of the following conditions is satisfied, the behavior is undefined:
- The elements
eof the source range are not partitioned with respect to these two expressions at the same time. - There exists an element
ein the source range such that these two expressions yield different values.
Parameters
| first, last | - | the pair of iterators defining the source range |
| 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:
While the signature does not need to have |
| 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 object, where:
- The data member
firstholds an iterator to the first element which is not ordered beforevalue, orlastif no such element is found. - The data member
secondholds an iterator to the first element which is ordered aftervalue, orlastif no such element is found.
Complexity
Given N as std::distance(first, last):
value using operator<(until C++20)std::less{}(since C++20).comp.Notes
Although std::equal_range only requires the source range to be partitioned, this algorithm is usually used in the case where the source range 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.
If ForwardIt is not a LegacyRandomAccessIterator, the number of iterator increments is linear in std::distance(first, last). 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.
| 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
#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 |
|---|---|---|---|
| LWG 270 | C++98 | Compare was required to satisfy Compare and T was requiredto be LessThanComparable (strict weak ordering required) |
only a partitioning is required; heterogeneous comparisons permitted |
| LWG 384 | C++98 | at most 2log2(N)+1 comparisons were allowed, which is not implementable[1] |
corrected to 2log2(N)+𝓞(1) |
- ↑ Applying
equal_rangeto a single-element range requires 2 comparisons, but at most 1 comparison is allowed by the complexity requirement.
See also
(C++20) |
finds the range of elements matching the given value using binary search (algorithm function object) |
| finds the first element not less than the given value using binary search (function template & algorithm function object) | |
(C++20) |
|
| finds the first element greater than the given value using binary search (function template & algorithm function object) | |
(C++20) |
|
| determines if an element exists in a range using binary search (function template & algorithm function object) | |
(C++20) |
|
| divides a range of elements into two groups (function template & algorithm function object) | |
(C++20) |
|
| determines if two sets of elements are the same (function template & algorithm function object) | |
(C++20) |
|
| returns range of elements matching a specific key (public member function of std::set<Key,Compare,Allocator>)
| |
| returns range of elements matching a specific key (public member function of std::multiset<Key,Compare,Allocator>)
|