std::adjacent_find() algorithm

since C++20

// (1)
template< class ForwardIt >
constexpr ForwardIt adjacent_find( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class BinaryPredicate >
constexpr ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );

// (3)
template< class ExecutionPolicy, class ForwardIt >
ForwardIt adjacent_find( ExecutionPolicy&& policy,
                         ForwardIt first, ForwardIt last );

// (4)
template< class ExecutionPolicy, class ForwardIt, class BinaryPredicate >
ForwardIt adjacent_find( ExecutionPolicy&& policy,
                         ForwardIt first, ForwardIt last, BinaryPredicate p );

since C++17

// (1)
template< class ForwardIt >
ForwardIt adjacent_find( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class BinaryPredicate >
ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );

// (3)
template< class ExecutionPolicy, class ForwardIt >
ForwardIt adjacent_find( ExecutionPolicy&& policy,
                         ForwardIt first, ForwardIt last );

// (4)
template< class ExecutionPolicy, class ForwardIt, class BinaryPredicate >
ForwardIt adjacent_find( ExecutionPolicy&& policy,
                         ForwardIt first, ForwardIt last, BinaryPredicate p );

until C++17

// (1)
template< class ForwardIt >
ForwardIt adjacent_find( ForwardIt first, ForwardIt last );

// (2)
template< class ForwardIt, class BinaryPredicate >
ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );

Searches the range [first; last) for two consecutive equal elements.

• (1) Elements are compared using operator==.

• (2) Elements are compared using the given binary predicate p.

• (3 - 4) Same as (1 - 2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (since C++20) 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 function should be equivalent to the following: bool fun(const Type1& a, const Type2& b); The signature does not need to have const&. The function must not modify the objects passed to it. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so 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 them.

Type requirements

ForwardIt

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 (2)

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;
}

Examples

Main.cpp

#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