std:: equal_range

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

Parameters

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 ) :

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.

Possible implementation

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));
}

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';
}

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.

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

See also