std::adjacent_find

Defined in header `<algorithm>`
	(1)
`template< class ForwardIt > ForwardIt adjacent_find( ForwardIt first, ForwardIt last );`		(until C++20)
`template< class ForwardIt > constexpr ForwardIt adjacent_find( ForwardIt first, ForwardIt last );`		(since C++20)
`template< class ExecutionPolicy, class ForwardIt > ForwardIt adjacent_find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );`	(2)	(since C++17)
	(3)
`template< class ForwardIt, class BinaryPredicate > ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );`		(until C++20)
`template< class ForwardIt, class BinaryPredicate > constexpr ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );`		(since C++20)
`template< class ExecutionPolicy, class ForwardIt, class BinaryPredicate > ForwardIt adjacent_find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, BinaryPredicate p );`	(4)	(since C++17)

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>>` is `true`.	(until C++20)
`std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `true`.	(since C++20)

Parameters

first, last	-	the range of elements to examine
policy	-	the execution policy to use. See execution policy for details.
p	-	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);` 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 (1,2) or p(*it, *(it + 1)) != false for (3,4).

If no such elements are found, last is returned.

Complexity

1,3) Exactly std::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

adjacent_find (1)
`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; }`
adjacent_find (3)
`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; }`

adjacent_find (1)

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;
}

adjacent_find (3)

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

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 240	C++98	the predicate was applied `std::find` `(first, last, value) - first` times for (1,3), where `value` was never defined	applied `std::min(` `(result - first) + 1,` `(last - first) - 1)` times

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

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