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std::adjacent_find(3) C++ Standard Libary std::adjacent_find(3) NAME std::adjacent_find - std::adjacent_find Synopsis Defined in header <algorithm> template< class ForwardIt > ForwardIt adjacent_find( ForwardIt first, ForwardIt last (1) (const- expr since C++20) ); template< class ExecutionPolicy, class ForwardIt > ForwardIt adjacent_find( ExecutionPolicy&& policy, (2) (since C++17) ForwardIt first, ForwardIt last ); template< class ForwardIt, class BinaryPred > ForwardIt adjacent_find( ForwardIt first, ForwardIt (3) (const- expr since C++20) last, BinaryPred p ); template< class ExecutionPolicy, class ForwardIt, class BinaryPred > ForwardIt adjacent_find( ExecutionPolicy&& policy, (4) (since C++17) ForwardIt first, ForwardIt last, BinaryPred p ); 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. 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); p - 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 re- gardless 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 con- verted 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::dis- tance(first, last): 1) Exactly \(\scriptsize \min(M+1,N-1)\)min(M+1,N-1) comparisons us- ing 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::termi- nate 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 or- der\n"; else std::cout << "The last element in the non-decreasing subse- quence 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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior the predicate was applied std::find applied std::min( LWG 240 C++98 (first, last, value) - first times (result - first) + 1, for (1,3), where value was never (last - first) - 1) times defined See also unique removes consecutive duplicate elements in a range (function template) ranges::adjacent_find finds the first two adjacent items that are equal (or satisfy (C++20) a given predicate) (niebloid) http://cppreference.com 2024.06.10 std::adjacent_find(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Exceptions | Possible implementation | Example | Output: | See also
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