# std::search_n

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
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)
| (2) |
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)
| (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)

### 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. ​

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
--- | --- | --- | ---
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

### See also

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

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