NAME¶

std::search_n - std::search_n

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt, class Size, class T >

ForwardIt search_n( ForwardIt first, ForwardIt (until C++20)
last, Size count,

const T& value );
template< class ForwardIt, class Size, class T >

constexpr ForwardIt search_n( ForwardIt first, (since C++20)
ForwardIt last, Size count,

const T& value );
template< class ExecutionPolicy, class
ForwardIt, class Size, class T >

ForwardIt search_n( ExecutionPolicy&& policy, (2) (since C++17)
ForwardIt first, ForwardIt last,

Size count, const T& value );
template< class ForwardIt, class Size, class T,
class BinaryPredicate > (1)

ForwardIt search_n( ForwardIt first, ForwardIt (until C++20)
last, Size count,

const T& value, BinaryPredicate p );
template< class ForwardIt, class Size, class T,
class BinaryPredicate >

constexpr ForwardIt search_n( ForwardIt first, (3) (since C++20)
ForwardIt last, Size count,

const T& value, BinaryPredicate p );
template< class ExecutionPolicy, class
ForwardIt, class Size, class T,

class BinaryPredicate > (4) (since C++17)
ForwardIt search_n( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last,

Size count, const T& value, BinaryPredicate 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 do not
participate in overload resolution unless
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.

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.

Return value¶

Iterator to the beginning of the found sequence in the range [first, last). If no
such sequence is found, last is returned.
If count is zero or negative, first is returned.

Complexity¶

At most last - first applications of the predicate.

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¶

First version¶

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;
Size cur_count = 0;

while (true) {
++cur_count;
if (cur_count >= count) {
// success
return candidate;
}
++first;
if (first == last) {
// exhausted the list
return last;
}
if (!(*first == value)) {
// too few in a row
break;
}
}
}
return last;
}

Second version¶

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;
Size cur_count = 0;

while (true) {
++cur_count;
if (cur_count >= count) {
// success
return candidate;
}
++first;
if (first == last) {
// exhausted the list
return last;
}
if (!p(*first, value)) {
// too few in a row
break;
}
}
}
return last;
}

Example¶

// Run this code

#include <iostream>
#include <algorithm>
#include <iterator>

template <class Container, class Size, class T>
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;
std::cout << "Has 4 consecutive zeros: "
<< consecutive_values(sequence,4,'0') << '\n';
std::cout << "Has 3 consecutive zeros: "
<< consecutive_values(sequence,3,'0') << '\n';
}

Output:¶

Has 4 consecutive zeros: false
Has 3 consecutive zeros: true

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)