std::search_n

 

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External libraries

 

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
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	search search_n lexicographical_compare lexicographical_compare_three_way (C++20)	findfind_iffind_if_not (C++11) find_end find_first_of
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)
C library
qsort	bsearch

 

Defined in header <algorithm>
	(1)
template< class ForwardIt, class Size, class T > ForwardIt search_n( ForwardIt first, ForwardIt last,                     Size count, const T& value );			(until C++20)
template< class ForwardIt, class Size, class T > constexpr ForwardIt search_n( ForwardIt first, ForwardIt last,                               Size count, const T& value );			(since C++20)
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt search_n( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,                     Size count, const T& value );		(2)	(since C++17)
		(3)
template< class ForwardIt, class Size, class T, class BinaryPredicate > ForwardIt search_n( ForwardIt first, ForwardIt last,                     Size count, const T& value, BinaryPredicate p );				(until C++20)
template< class ForwardIt, class Size, class T, class BinaryPredicate > constexpr ForwardIt search_n( ForwardIt first, ForwardIt last,                               Size count, const T& value, BinaryPredicate p );				(since C++20)
template< class ExecutionPolicy, class ForwardIt,           class Size, class T, class BinaryPredicate > ForwardIt search_n( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,                     Size count, const T& value, BinaryPredicate p );			(4)	(since C++17)

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 do not participate in overload resolution unless

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.
- Size must be convertible to 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 N as std::distance(first, last):

1,2) at most N comparisons with value using operator==

3,4) at most 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> 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, class BinaryPredicate> ForwardIt search_n(ForwardIt first, ForwardIt last, Size count, const T& value, BinaryPredicate 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; }

Example

Run this code

#include <iostream>
#include <algorithm>
#include <iterator>
 
template<class Container, class Size, class T>
[[nodiscard]]
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()
{
    const char sequence[] = "1001010100010101001010101";
 
    std::cout << std::boolalpha
              << "Has 4 consecutive zeros: "
              << consecutive_values(sequence, 4, '0') << '\n'
              << "Has 3 consecutive zeros: "
              << consecutive_values(sequence, 3, '0') << '\n';
}

Output:

Has 4 consecutive zeros: false
Has 3 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

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 a range of elements (function template)
ranges::search_n (C++20)	searches for a number consecutive copies of an element in a range (niebloid)