std::search_n

Since C++26

Header: <algorithm>

Searches the range [first,last) for the first sequence of count identical elements, each equal to the given value.

# Declarations

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)

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)

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)

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)

# 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
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.

# 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:

# Notes

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

# Example

#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());
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	T was required to be EqualityComparable, butthe 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 0if count is non-positive
LWG 714	C++98	if count > 0, the complexity upper limit was N·count, but inthe worst case the number of comparisons/operations is always N	changed the upperlimit to N in this case
LWG 2150	C++98	the condition of “sequence occurence” was incorrect	corrected

std::search_n

# Declarations

# Parameters

# Return value

# Notes

# Example

# Defect reports

# See also