std::search_n

Defined in header `<algorithm>`
template< class ForwardIt, class Size, class T > ForwardIt search_n( ForwardIt first, ForwardIt last, Size count, const T& value );	(1)	(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 );	(1)		(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 );	(2)	(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 );	(2)		(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 );	(3)	(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 );	(3)		(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 );	(4)	(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 );	(4)		(since C++26)

Searches the source range [first, last) for the first occurrence of count consecutive elements equal to value.

1) Elements are compared using operator==.

2) Elements are compared using the given binary predicate p.

3,4) Same as (1,2), but executed according to policy.

These overloads participate in overload resolution only if the value of the following expression is true:

`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)

Parameters

first, last	-	the pair of iterators defining the source range
count	-	the number of consecutive elements to search for
value	-	the value 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`.
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 occurrence in the source range.

If count is non-positive or there is no such occurrence in the source range, returns last.

Complexity

Given $\scriptsize N$ $N$ as std::distance(first, last):

1) At most

N

comparisons using operator==.

2) At most

N

applications of the predicate p.

3)

𝓞(N)

comparisons using operator==.

4)

𝓞(N)

applications of the predicate p.

Exceptions

3,4) During the execution process:

If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

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 (2)

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
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`, which is negative if `count` is negative	the upper limit is `0` if `count` is non-positive
LWG 714	C++98	if `count` is positive, 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

Compiler support
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Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
Memory management library
Metaprogramming library (C++11)
General utilities library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
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Numerics library
Date and time library
Input/output library
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Technical specifications
Symbols index
External libraries

ranges::search_n (C++20)	searches for the first occurrence of a number consecutive copies of an element in a range (algorithm function object)[edit]
search	searches for the first occurrence of a range of elements (function template & algorithm function object)[edit]
ranges::search (C++20)
find_end	finds the last sequence of elements in a certain range (function template & algorithm function object)[edit]
ranges::find_end (C++20)
findfind_iffind_if_not (C++11)	finds the first element satisfying specific criteria (function template & algorithm function object)[edit]
ranges::findranges::find_ifranges::find_if_not (C++20)(C++20)(C++20)

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