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 may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

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 first;
}

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 <iostream>
#include <vector>
#include <functional>

int main()
{
std::vector<int> v1{0, 1, 2, 3, 40, 40, 41, 41, 5};
// ^^ ^^
namespace ranges = std::ranges;

auto i1 = ranges::adjacent_find(v1.begin(), v1.end());
if (i1 == v1.end()) {
std::cout << "No matching adjacent elements\n";
} else {
std::cout << "The first adjacent pair of equal elements is at ["
<< ranges::distance(v1.begin(), i1) << "] == " << *i1 << '\n';
}

auto i2 = ranges::adjacent_find(v1, ranges::greater());
if (i2 == v1.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(v1.begin(), i2) << "] == " << *i2 << '\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