std::ranges::adjacent_find

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 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

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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