std::search_n

 

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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>
	(1)
template< class ForwardIt, class Size, class T > ForwardIt search_n( ForwardIt first, ForwardIt last,                     Size count, const T& value );			(constexpr since C++20) (until C++26)
template< class ForwardIt, class Size,           class T = typename std::iterator_traits                         <ForwardIt>::value_type > constexpr ForwardIt search_n( ForwardIt first, ForwardIt last,                               Size count, const T& value );			(since C++26)
		(2)
template< class ExecutionPolicy,           class ForwardIt, class Size, class T > ForwardIt search_n( ExecutionPolicy&& policy,                     ForwardIt first, ForwardIt last,                     Size count, const T& value );				(since C++17) (until C++26)
template< class ExecutionPolicy,           class ForwardIt, class Size,           class T = typename std::iterator_traits                         <ForwardIt>::value_type > ForwardIt search_n( ExecutionPolicy&& policy,                     ForwardIt first, ForwardIt last,                     Size count, const T& value );				(since C++26)
			(3)
template< class ForwardIt, class Size, class T, class BinaryPred > ForwardIt search_n( ForwardIt first, ForwardIt last,                     Size count, const T& value, BinaryPred p );					(constexpr since C++20) (until C++26)
template< class ForwardIt, class Size,           class T = typename std::iterator_traits                         <ForwardIt>::value_type,           class BinaryPred > constexpr ForwardIt search_n( ForwardIt first, ForwardIt last,                     Size count, const T& value, BinaryPred p );					(since C++26)
				(4)
template< class ExecutionPolicy, class ForwardIt, class Size,           class T, class BinaryPred > ForwardIt search_n( ExecutionPolicy&& policy,                     ForwardIt first, ForwardIt last,                     Size count, const T& value, BinaryPred p );						(since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class Size,           class T = typename std::iterator_traits                         <ForwardIt>::value_type,           class BinaryPred > ForwardIt search_n( ExecutionPolicy&& policy,                     ForwardIt first, ForwardIt last,                     Size count, const T& value, BinaryPred p );						(since C++26)

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.
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 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 implicitly 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::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

search_n (1)
template<class ForwardIt, class Size, class T = typename std::iterator_traits<ForwardIt>::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<ForwardIt>::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-initialization 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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	T was required to be EqualityComparable, but the value type of InputIt is not always T	removed the requirement
LWG 426	C++98	the complexity upper limit was N·count, it is negative if count is negative	the upper limit is ​0​ if count is non-positive
LWG 714	C++98	if count > 0, the complexity upper limit was N·count, but in the worst case the number of comparisons/operations is always N	changed the upper limit to N in this case
LWG 2150	C++98	the condition of “sequence occurence” was incorrect	corrected

See also

find_end	finds the last sequence of elements in a certain range (function template)
findfind_iffind_if_not (C++11)	finds the first element satisfying specific criteria (function template)
search	searches for the first occurrence of a range of elements (function template)
ranges::search_n (C++20)	searches for the first occurrence of a number consecutive copies of an element in a range (niebloid)