# std::adjacent_find

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class ForwardIt > ForwardIt adjacent_find( ForwardIt first, ForwardIt last ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt adjacent_find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last ); | (2) | (since C++17)
template< class ForwardIt, class BinaryPred > ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPred p ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class BinaryPred > ForwardIt adjacent_find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, BinaryPred 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 participate in overload resolution only if all following conditions are satisfied:

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

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.
- `BinaryPred` must meet the requirements of BinaryPredicate.

### 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

Given result as the return value of `adjacent_find`, \(\scriptsize M\)M as std::distance (first, result) and \(\scriptsize N\)N as std::distance (first, last):

1) Exactly \(\scriptsize \min(M+1,N-1)\)min(M+1,N-1) comparisons using operator==.
2) \(\scriptsize O(N)\)O(N) comparisons using operator==.
3) Exactly \(\scriptsize \min(M+1,N-1)\)min(M+1,N-1) applications of the predicate p.
4) \(\scriptsize O(N)\)O(N) applications of the predicate p.

### 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 BinaryPred>
    ForwardIt adjacent_find(ForwardIt first, ForwardIt last, BinaryPred 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) (algorithm function object)

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