std::adjacent_find(3)

NAME
std::adjacent_find - std::adjacent_find

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt >
   ForwardIt adjacent_find( ForwardIt first, (until C++20)
   ForwardIt last );
   template< class ForwardIt >
   constexpr ForwardIt adjacent_find( ForwardIt (since C++20)
   first, ForwardIt last );
   template< class ExecutionPolicy, class ForwardIt
   >
   ForwardIt adjacent_find( ExecutionPolicy&& (2) (since C++17)
   policy,
   ForwardIt first, ForwardIt last );
   template< class ForwardIt, class
   BinaryPredicate> (1) (until C++20)
   ForwardIt adjacent_find( ForwardIt first,
   ForwardIt last, BinaryPredicate p );
   template< class ForwardIt, class
   BinaryPredicate> (since C++20)
   constexpr ForwardIt adjacent_find( ForwardIt
   first, ForwardIt last, BinaryPredicate p ); (3)
   template< class ExecutionPolicy, class
   ForwardIt, class BinaryPredicate>
   ForwardIt adjacent_find( ExecutionPolicy&& (4) (since C++17)
   policy,
   ForwardIt first, ForwardIt last, BinaryPredicate
   p );
   Searches the range [first, last) for two consecutive equal elements.
   1) Elements are compared using operator==.
   3) Elements are compared using the given binary predicate p.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

Parameters
   first, last - the range of elements to examine
   policy - the execution policy to use. See execution policy for details.
                 binary predicate which returns true if the elements should be
                 treated as equal.
                 The signature of the predicate function should be equivalent to the
                 following:
                 bool pred(const Type1 &a, const Type2 &b);
   p - While the signature does not need to have const &, the function must
                 not modify the objects passed to it and must be able to accept all
                 values of type (possibly const) Type1 and Type2 regardless of value
                 category (thus, Type1 & is not allowed
                 , nor is Type1 unless for Type1 a move is equivalent to a copy
                 (since C++11)).
                 The types Type1 and Type2 must be such that an object of type
                 ForwardIt can be dereferenced and then implicitly converted to both of
                 them.

Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

Return value
   an iterator to the first of the first pair of identical elements, that is, the first
   iterator it such that *it == *(it+1) for the first version or p(*it, *(it + 1)) !=
   false for the second version.
   If no such elements are found, last is returned

Complexity
   1,3) Exactly min((result-first)+1, (last-first)-1) applications of the predicate
   where result is the return value.
   2,4) O(last-first) applications of the corresponding predicate.

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

First version
   template<class ForwardIt>
   ForwardIt adjacent_find(ForwardIt first, ForwardIt last)
   {
       if (first == last) {
           return last;
       }
       ForwardIt next = first;
       ++next;
       for (; next != last; ++next, ++first) {
           if (*first == *next) {
               return first;
           }
       }
       return last;
   }

Second version
   template<class ForwardIt, class BinaryPredicate>
   ForwardIt adjacent_find(ForwardIt first, ForwardIt last,
                        BinaryPredicate p)
   {
       if (first == last) {
           return last;
       }
       ForwardIt next = first;
       ++next;
       for (; next != last; ++next, ++first) {
           if (p(*first, *next)) {
               return first;
           }
       }
       return last;
   }

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};
     auto i1 = std::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 "
                   << std::distance(v1.begin(), i1) << ", *i1 = "
                   << *i1 << '\n';
     }
     auto i2 = std::adjacent_find(v1.begin(), v1.end(), std::greater<int>());
     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 "
                   << std::distance(v1.begin(), i2) << ", *i2 = " << *i2 << '\n';
     }
 }

Output:
 The first adjacent pair of equal elements is at 4, *i1 = 40
 The last element in the non-decreasing subsequence is at 7, *i2 = 41

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