std::adjacent_find

 

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Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

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

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.
- 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) (niebloid)