# 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 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);

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 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

### See also

unique | removes consecutive duplicate elements in a range (function template)

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