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std::search_n(3) C++ Standard Libary std::search_n(3) NAME std::search_n - std::search_n Synopsis Defined in header <algorithm> template< class ForwardIt, class Size, class T > (constexpr ForwardIt search_n( ForwardIt since C++20) first, ForwardIt last, (until C++26) Size count, const T& value ); template< class ForwardIt, class Size, class T = typename std::iterator_traits (since C++26) <ForwardIt>::value_type > constexpr ForwardIt search_n( ForwardIt first, ForwardIt last, Size count, const T& value ); template< class ExecutionPolicy, class ForwardIt, class Size, class T > (since ForwardIt search_n( C++17) ExecutionPolicy&& policy, (until ForwardIt C++26) first, ForwardIt last, Size count, const T& value ); template< class ExecutionPolicy, class ForwardIt, class Size, class T = typename std::iterator_traits (since <ForwardIt>::value_type > C++26) ForwardIt search_n( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Size count, const T& value ); template< class ForwardIt, class (1) Size, class T, class BinaryPred > (con- stexpr ForwardIt search_n( ForwardIt since C++20) first, ForwardIt last, (un- til C++26) Size count, const T& value, BinaryPred p ); template< class ForwardIt, class Size, (2) class T = typename std::iterator_traits (since <ForwardIt>::value_type, C++26) class BinaryPred > constexpr ForwardIt search_n( ForwardIt first, ForwardIt last, Size count, const T& value, BinaryPred p ); template< class ExecutionPolicy, class ForwardIt, class Size, (3) class T, class BinaryPred > (since ForwardIt search_n( C++17) ExecutionPolicy&& policy, (until ForwardIt C++26) first, ForwardIt last, Size count, const T& value, BinaryPred p ); template< class ExecutionPolicy, class ForwardIt, class Size, (4) class T = typename std::iterator_traits <ForwardIt>::value_type, (since class BinaryPred > C++26) ForwardIt search_n( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Size count, const T& value, BinaryPred p ); Searches the range [first, last) for the first sequence of count identical elements, each equal to the given value. 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 count - the length of the sequence to search for value - the value of the elements to search for 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 type Type1 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type1. The type Type2 must be such that an object of type T can be implic- itly converted to Type2. Type requirements - ForwardIt must meet the requirements of LegacyForwardIterator. - BinaryPred must meet the requirements of BinaryPredicate. - Size must be convertible to an integral type. Return value If count is positive, returns an iterator to the beginning of the first sequence found in the range [first, last). Each iterator it in the sequence should satisfy the following condition: 1,2) *it == value is true. 3,4) p(*it, value) != false is true. If no such sequence is found, last is returned. If count is zero or negative, first is returned. Complexity Given \(\scriptsize N\)N as std::distance(first, last): 1,2) At most \(\scriptsize N\)N comparisons using operator==. 3,4) At most \(\scriptsize N\)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 search_n (1) template<class ForwardIt, class Size, class T = typename std::iterator_traits<For- wardIt>::value_type> ForwardIt search_n(ForwardIt first, ForwardIt last, Size count, const T& value) { if (count <= 0) return first; for (; first != last; ++first) { if (!(*first == value)) continue; ForwardIt candidate = first; for (Size cur_count = 1; true; ++cur_count) { if (cur_count >= count) return candidate; // success ++first; if (first == last) return last; // exhausted the list if (!(*first == value)) break; // too few in a row } } return last; } search_n (3) template<class ForwardIt, class Size, class T = typename std::iterator_traits<For- wardIt>::value_type, class BinaryPred> ForwardIt search_n(ForwardIt first, ForwardIt last, Size count, const T& value, BinaryPred p) { if (count <= 0) return first; for (; first != last; ++first) { if (!p(*first, value)) continue; ForwardIt candidate = first; for (Size cur_count = 1; true; ++cur_count) { if (cur_count >= count) return candidate; // success ++first; if (first == last) return last; // exhausted the list if (!p(*first, value)) break; // too few in a row } } return last; } Notes Feature-test macro Value Std Feature __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initial- ization for algorithms (1-4) Example // Run this code #include <algorithm> #include <cassert> #include <complex> #include <iostream> #include <iterator> #include <vector> template<class Container, class Size, class T> constexpr bool consecutive_values(const Container& c, Size count, const T& v) { return std::search_n(std::begin(c), std::end(c), count, v) != std::end(c); } int main() { constexpr char sequence[] = ".0_0.000.0_0."; static_assert(consecutive_values(sequence, 3, '0')); for (int n : {4, 3, 2}) std::cout << std::boolalpha << "Has " << n << " consecutive zeros: " << consecutive_values(sequence, n, '0') << '\n'; std::vector<std::complex<double>> nums{{4, 2}, {4, 2}, {1, 3}}; #ifdef __cpp_lib_algorithm_default_value_type auto it = std::search_n(nums.cbegin(), nums.cend(), 2, {4, 2}); #else auto it = std::search_n(nums.cbegin(), nums.cend(), 2, std::complex<double>{4, 2}); #endif assert(it == nums.begin()); } Output: Has 4 consecutive zeros: false Has 3 consecutive zeros: true Has 2 consecutive zeros: true Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior T was required to be EqualityComparable, re- moved the LWG 283 C++98 but re- quirement the value type of InputIt is not always T the complexity upper limit was Ncount, the up- per limit is 0 LWG 426 C++98 it is negative if count is negative if count is non- positive if count > 0, the complexity upper limit changed the upper LWG 714 C++98 was Ncount, but in limit to N in this the worst case the number of case comparisons/operations is always N LWG 2150 C++98 the condition of sequence occurence was cor- rected incorrect See also find_end finds the last sequence of elements in a certain range (function template) find find_if finds the first element satisfying specific crite- ria find_if_not (function template) (C++11) search searches for a range of elements (function template) ranges::search_n searches for a number consecutive copies of an ele- ment in a range (C++20) (niebloid) http://cppreference.com 2024.06.10 std::search_n(3)
NAME | Synopsis | Size, | Size, | Size, | Parameters | Type requirements | Return value | Complexity | Exceptions | Possible implementation | Notes | Example | Output: | See also
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