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std::ranges::adjacent_find

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
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:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] 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

[edit] 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.

[edit] Complexity

Exactly min((result-first)+1, (last-first)-1) applications of the predicate and projection where result is the return value.

[edit] 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;

[edit] Example

#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 at: "
                  << ranges::distance(v1.begin(), 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) << '\n';
    }
}

Output:

The first adjacent pair of equal elements at: 4
The last element in the non-decreasing subsequence is at: 7

[edit] See also

removes consecutive duplicate elements in a range
(niebloid) [edit]
finds the first two adjacent items that are equal (or satisfy a given predicate)
(function template) [edit]