# std::adjacent_find

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< cpp‎ | algorithm

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

    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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