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 identical 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

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); The signature does not need to have const &, but the function must not modify the objects passed to it. 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 ForwardIterator.

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

Output:

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

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