NAME¶

std::ranges::adjacent_find - std::ranges::adjacent_find

Synopsis¶

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>, (1) (since C++20)
std::projected<I, Proj>> Pred = ranges::equal_to >
constexpr I

adjacent_find( I first, S last, Pred pred = {}, Proj proj = {}
);
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 = (2) (since C++20)
ranges::equal_to >
constexpr ranges::borrowed_iterator_t<R>

adjacent_find( R&& r, Pred pred = {}, Proj proj = {} );

Searches the range [first, last) for 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 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.

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