std::ranges:: adjacent_find

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

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

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

#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';
}

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