# std::adjacent_find

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< cpp‎ | algorithm

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 (1,3) or p(*it, *(it + 1)) != false for (2,4).

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;
    }

### Example

Run this code

    #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

### See also

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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