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std::equal_range - cppreference.com

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

The equivalence is checked using

:

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).

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

2) The equivalence is checked using 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)).

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. ​

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).

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

At most

comparisons with

using

.

2) At most \(\scriptsize 2\log_{2}(N)+O(1)\)2log2(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.

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.

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{});
}

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));
}

#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)

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

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

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