std::ranges::adjacent_find
| Defined in header <algorithm>
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| Call signature |
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| template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_binary_predicate< |
(1) | (since C++20) |
| template< ranges::forward_range R, class Proj = std::identity, std::indirect_binary_predicate< |
(2) | (since C++20) |
Searches the range [first, last) for the first two consecutive equal elements.
The function-like entities described on this page are 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.
In practice, they may be implemented as function objects, or with special compiler extensions.
Contents |
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
#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
| (C++20) |
removes consecutive duplicate elements in a range (niebloid) |
| finds the first two adjacent items that are equal (or satisfy a given predicate) (function template) |
