std::search_n

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 all following conditions are satisfied:

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)

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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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;
}

[edit] Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	`202403`	(C++26)	List-initialization for algorithms (1-4)

[edit] 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

[edit] 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