# std::ranges::adjacent_find

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::forward_iterator I, std::sentinel_for <I> S, class Proj = std::identity, std::indirect_binary_predicate < std:: projected <I, Proj>, std:: projected <I, Proj>> Pred = ranges::equal_to > constexpr I adjacent_find( I first, S last, Pred pred = {}, Proj proj = {} ); | (1) | (since C++20)
template< ranges::forward_range R, class Proj = std::identity, std::indirect_binary_predicate < std:: projected < ranges::iterator_t <R>, Proj>, std:: projected < ranges::iterator_t <R>, Proj>> Pred = ranges::equal_to > constexpr ranges::borrowed_iterator_t <R> adjacent_find( R&& r, Pred pred = {}, Proj proj = {} ); | (2) | (since C++20)

Searches the range `[`first`,` last`)` for the first two consecutive equal elements.

1) Elements are compared using pred (after projecting with the projection proj).
2) Same as (1), but uses r as the source range, as if using 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
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

An iterator to the first of the first pair of identical elements, that is, the first iterator `it` such that bool( std::invoke (pred, std::invoke (proj1, *it), std::invoke (proj, *(it + 1)))) is true.

If no such elements are found, an iterator equal to last is returned.

### Complexity

Exactly min((result - first) + 1, (last - first) - 1) applications of the predicate and projection where `result` is the return value.

### Possible implementation

    struct adjacent_find_fn
    {
        template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
                 std::indirect_binary_predicate<
                     std::projected<I, Proj>,
                     std::projected<I, Proj>> Pred = ranges::equal_to>
        constexpr I operator()(I first, S last, Pred pred = {}, Proj proj = {}) const
        {
            if (first == last)
                return first;
            auto next = ranges::next(first);
            for (; next != last; ++next, ++first)
                if (std::invoke(pred, std::invoke(proj, *first), std::invoke(proj, *next)))
                    return first;
            return next;
        }

        template<ranges::forward_range R, class Proj = std::identity,
                 std::indirect_binary_predicate<
                     std::projected<ranges::iterator_t<R>, Proj>,
                     std::projected<ranges::iterator_t<R>, Proj>> Pred = ranges::equal_to>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, Pred pred = {}, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
        }
    };

    inline constexpr adjacent_find_fn adjacent_find;

### Example

Run this code

    #include <algorithm>
    #include <functional>
    #include <iostream>
    #include <ranges>

    constexpr bool some_of(auto&& r, auto&& pred) // some but not all
    {
        return std::ranges::cend(r) != std::ranges::adjacent_find(r,
            [&pred](auto const& x, auto const& y)
            {
                return pred(x) != pred(y);
            });
    }

    // test some_of
    constexpr auto a = {0, 0, 0, 0}, b = {1, 1, 1, 0}, c = {1, 1, 1, 1};
    auto is_one = [](auto x){ return x == 1; };
    static_assert(!some_of(a, is_one) && some_of(b, is_one) && !some_of(c, is_one));

    int main()
    {
        const auto v = {0, 1, 2, 3, 40, 40, 41, 41, 5}; /*
                                    ^^          ^^       */
        namespace ranges = std::ranges;

        if (auto it = ranges::adjacent_find(v.begin(), v.end()); it == v.end())
            std::cout << "No matching adjacent elements\n";
        else
            std::cout << "The first adjacent pair of equal elements is at ["
                      << ranges::distance(v.begin(), it) << "] == " << *it << '\n';

        if (auto it = ranges::adjacent_find(v, ranges::greater()); it == v.end())
            std::cout << "The entire vector is sorted in ascending order\n";
        else
            std::cout << "The last element in the non-decreasing subsequence is at ["
                      << ranges::distance(v.begin(), it) << "] == " << *it << '\n';
    }

Output:

    The first adjacent pair of equal elements is at [4] == 40
    The last element in the non-decreasing subsequence is at [7] == 41

### See also

ranges::unique (C++20) | removes consecutive duplicate elements in a range (algorithm function object)
adjacent_find | finds the first two adjacent items that are equal (or satisfy a given predicate) (function template)

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