# std::ranges::equal_range

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
| (1) |
template< std::forward_iterator I, std::sentinel_for <I> S, class T, class Proj = std::identity, std::indirect_strict_weak_order <const T*, std:: projected <I, Proj>> Comp = ranges::less > constexpr ranges::subrange <I> equal_range( I first, S last, const T& value, Comp comp = {}, Proj proj = {} ); |  | (since C++20) (until C++26)
template< std::forward_iterator I, std::sentinel_for <I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj>, std::indirect_strict_weak_order <const T*, std:: projected <I, Proj>> Comp = ranges::less > constexpr ranges::subrange <I> equal_range( I first, S last, const T& value, Comp comp = {}, Proj proj = {} ); |  | (since C++26)
| (2) |
template< ranges::forward_range R, class T, class Proj = std::identity, std::indirect_strict_weak_order <const T*, std:: projected < ranges::iterator_t <R>, Proj>> Comp = ranges::less > constexpr ranges::borrowed_subrange_t <R> equal_range( R&& r, const T& value, Comp comp = {}, Proj proj = {} ); |  | (since C++20) (until C++26)
template< ranges::forward_range R, class Proj = std::identity, class T = std::projected_value_t< ranges::iterator_t <R>, Proj>, std::indirect_strict_weak_order <const T*, std:: projected < ranges::iterator_t <R>, Proj>> Comp = ranges::less > constexpr ranges::borrowed_subrange_t <R> equal_range( R&& r, const T& value, Comp comp = {}, Proj proj = {} ); |  | (since C++26)

1) Returns a view 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, 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::ranges::lower_bound(), the second - with std::ranges::upper_bound().

2) Same as (1), but uses r as the source range, as if using the range ranges::begin (r) as first and ranges::end (r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | the range of elements to examine
r | - | the range of the elements to examine
value | - | value to compare the elements to
comp | - | if the first argument is less than (i.e. is ordered before) the second
proj | - | projection to apply to the elements

### Return value

std::ranges::subrange 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, the last iterator (iterator that is equal to last or ranges::end (r)) is returned as the first element. Similarly if there are no elements greater than value, the last iterator is returned as the second element.

### Complexity

The number of comparisons performed is logarithmic in the distance between first and last (at most 2 * log2(last - first) + O(1) comparisons). However, for an iterator that does not model `random_access_iterator`, the number of iterator increments is linear.

### Possible implementation

    struct equal_range_fn
    {
        template<std::forward_iterator I, std::sentinel_for<I> S,
                 class Proj = std::identity, class T = std::projected_value_t<I, Proj>,
                 std::indirect_strict_weak_order
                     <const T*, std::projected<I, Proj>> Comp = ranges::less>
        constexpr ranges::subrange<I>
            operator()(I first, S last, const T& value, Comp comp = {}, Proj proj = {}) const
        {
            return ranges::subrange
            (
                ranges::lower_bound(first, last, value, std::ref(comp), std::ref(proj)),
                ranges::upper_bound(first, last, value, std::ref(comp), std::ref(proj))
            );
        }

        template<ranges::forward_range R, class Proj = std::identity,
                 class T = std::projected_value_t<ranges::iterator_t<R>, Proj>,
                 std::indirect_strict_weak_order
                     <const T*, std::projected<ranges::iterator_t<R>,
                                               Proj>> Comp = ranges::less>
        constexpr ranges::borrowed_subrange_t<R>
            operator()(R&& r, const T& value, Comp comp = {}, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), value,
                           std::ref(comp), std::ref(proj));
        }
    };

    inline constexpr equal_range_fn equal_range;

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1,2)

### Example

Run this code

    #include <algorithm>
    #include <compare>
    #include <complex>
    #include <iostream>
    #include <vector>

    struct S
    {
        int number {};
        char name {};
        // note: name is ignored by these comparison operators
        friend bool operator== (const S s1, const S s2) { return s1.number == s2.number; }
        friend auto operator<=>(const S s1, const S s2) { return s1.number <=> s2.number; }
        friend std::ostream& operator<<(std::ostream& os, S o)
        {
            return os << '{' << o.number << ", '" << o.name << "'}";
        }
    };

    void println(auto rem, const auto& v)
    {
        for (std::cout << rem; const auto& e : v)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    int main()
    {
        // note: not ordered, only partitioned w.r.t. S defined below
        std::vector<S> vec
        {
            {1,'A'}, {2,'B'}, {2,'C'}, {2,'D'}, {4, 'D'}, {4,'G'}, {3,'F'}
        };

        const S value{2, '?'};

        namespace ranges = std::ranges;

        auto a = ranges::equal_range(vec, value);
        println("1. ", a);

        auto b = ranges::equal_range(vec.begin(), vec.end(), value);
        println("2. ", b);

        auto c = ranges::equal_range(vec, 'D', ranges::less {}, &S::name);
        println("3. ", c);

        auto d = ranges::equal_range(vec.begin(), vec.end(), 'D', ranges::less {}, &S::name);
        println("4. ", d);

        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 = ranges::equal_range(nums, {2, 0}, cmpz);
        #else
            auto p3 = ranges::equal_range(nums, CD{2, 0}, cmpz);
        #endif
        println("5. ", p3);
    }

Output:

    1. {2, 'B'} {2, 'C'} {2, 'D'}
    2. {2, 'B'} {2, 'C'} {2, 'D'}
    3. {2, 'D'} {4, 'D'}
    4. {2, 'D'} {4, 'D'}
    5. (2,2) (2,1)

### See also

ranges::lower_bound (C++20) | returns an iterator to the first element not less than the given value (algorithm function object)
ranges::upper_bound (C++20) | returns an iterator to the first element greater than a certain value (algorithm function object)
ranges::binary_search (C++20) | determines if an element exists in a partially-ordered range (algorithm function object)
ranges::partition (C++20) | divides a range of elements into two groups (algorithm function object)
ranges::equal (C++20) | determines if two sets of elements are the same (algorithm function object)
equal_range | returns range of elements matching a specific key (function template)

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