std::equal_range

template< class ForwardIt, class T >
std::pair<ForwardIt, ForwardIt> 
    equal_range( ForwardIt first, ForwardIt last, const T& value );

template< class ForwardIt, class T >
constexpr std::pair<ForwardIt, ForwardIt> 
    equal_range( ForwardIt first, ForwardIt last, const T& value );

template< class ForwardIt, class T, class Compare >
std::pair<ForwardIt, ForwardIt> 
    equal_range( ForwardIt first, ForwardIt last,
                 const T& value, Compare comp );

template< class ForwardIt, class T, class Compare >
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 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 range is defined by 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::lower_bound(), the second - with std::upper_bound().

The first version uses operator< to compare the elements, the second version uses the given comparison function comp.

Parameters

Return value

A std::pair 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, last is returned as the first element. Similarly if there are no elements greater than value, last is returned as the second element.

Complexity

The number of comparisons performed is logarithmic in the distance between first and last (At most 2 * log
2(last - first) + O(1) comparisons). However, for non-LegacyRandomAccessIterators, the number of iterator increments is linear. 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.

Possible implementation

template<class ForwardIt, class T>
std::pair<ForwardIt, ForwardIt> 
    equal_range(ForwardIt first, ForwardIt last, const T& value)
{
    return std::make_pair(std::lower_bound(first, last, value),
                          std::upper_bound(first, last, value));
}

template<class ForwardIt, class T, class Compare>
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 <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 i = p.first; i != p.second; ++i)
        std::cout << i->name << ' ';
 
    std::cout << "\n" "Using heterogeneous comparison: ";
    const auto p2 = std::equal_range(vec.begin(), vec.end(), 2, Comp{});
 
    for (auto i = p2.first; i != p2.second; ++i)
        std::cout << i->name << ' ';
}

Compare using S::operator<(): B C D 
Using heterogeneous comparison: B C D

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