std::equal_range() algorithm

since C++20

// (1)
template< class ForwardIt, class T >
constexpr std::pair<ForwardIt, ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value );

// (2)
template< class ForwardIt, class T, class Compare >
constexpr std::pair<ForwardIt, ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value, Compare comp );

until C++20

// (1)
template< class ForwardIt, class T >
std::pair<ForwardIt, ForwardIt> equal_range( ForwardIt first, ForwardIt last, const T& value );

// (2)
template< class ForwardIt, class T, class Compare >
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 view is constructed from two iterators:

1. Pointing to the first element that is not less than value.
2. Pointing to the first element greater than value.

The first iterator may be alternatively obtained with std::ranges::lower_bound(), the second - with std::ranges::upper_bound().

• (1) Uses operator< to compare the elements.
• (2) Uses the given comparison function comp.

Parameters

first last	The partially-ordered range to examine.
value	The value to compare the elements to.
comp	Comparison function object (i.e. an object that satisfies the requirements of Compare). The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const&, but must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so 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 RandomIt can be implicitly converted to both of them.

Type requirements

ForwardIt	LegacyForwardIterator
Compare	BinaryPredicate

Compare is not required to satisfy Compare.

Return value

A std::pair containing a pair of iterators defining the wanted range:

1. Pointing to the first element that is not less than value.
2. 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 `log^2(last - first) + O(1) comparisons).

However, for non-LegacyRandomAccessIterators, the number of iterator increments is linear.

Notably, std::map, std::multimap, std::set, and std::multiset iterators are not random access, and so their member equal_range() functions should be preferred.

Exceptions

(none)

Possible implementation

equal_range (1)

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));
}

equal_range (2)

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));
}

Examples

Main.cpp

#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 << ' ';
}

Output

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