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 >
(constexpr
ForwardIt search_n( ForwardIt since C++20)
first, ForwardIt last, (until
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 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
T was required to be EqualityComparable, removed the
LWG 283 C++98 but requirement
the value type of InputIt is not always T
the complexity upper limit was N·count, the upper 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 N·count, 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 corrected
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 criteria
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 element in a range
(C++20) (niebloid)