std::search_n(3)

💖 std::search_n(3) — C++ Standard Libary 💖

okay so bestie, the manual name is std::search_n(3), and this is the C++ Standard Libary page for std::search_n. It’s the “find me count matching things in a row” algorithm, and she is lowkey iconic. ✨

---

🌸 NAME 🌸

std::search_n — std::search_n (yes, the name is literally the name, no cap 💅).

---

✨ Synopsis ✨

Defined in header <algorithm>.

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

Here are all the overloads, bestie. Every template parameter, default, and qualifier is staying exactly where it belongs. 🫶

non-predicate overloads

• Until C++26:

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.

• Since 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.

• With execution policy, since C++17 until 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.

• With execution policy, since 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.

predicate overloads (1)–(4)

• (1):

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.

• (2):

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.

• (3):

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.

• (4):

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.

---

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:

• until C++20: std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.
• since C++20: std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> 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.
• 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 N as std::distance(first, last):

• 1,2) At most N comparisons using operator==.
• 3,4) At most 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:

• __cpp_lib_algorithm_default_value_type — value 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

okay so that's literally the whole vibe: {0,0,0,1,0,0} has no four-in-a-row zeros, but it does have three, and obvi it has two 💅

---

🫶 Defect reports

so these are the glow-ups that got applied retroactively to C++ standards that were already published — like, the committee went back and fixed the old specs after the fact, no cap:

• LWG 283 — applied to C++98. What was published: T was required to be EqualityComparable, but the value type of InputIt is not always T. The fix: that requirement got removed. bye felicia 👋
• LWG 426 — applied to C++98. What was published: the complexity upper limit was N·count, and it goes negative if count is negative 🫠. The fix: the upper limit is 0 if count is non-positive. slay.
• LWG 714 — applied to C++98. What was published: if count > 0, the complexity upper limit was N·count, but in the worst case the number of comparisons/operations is always N. The fix: changed the upper limit to N in this case ✨
• LWG 2150 — applied to C++98. What was published: the condition of "sequence occurence" was incorrect. The fix: corrected 🎀

fr fr, this is your reminder that standards are living documents and the committee does circle back. iconic behaviour honestly.

---

👀 See also

the friend group of related algorithms, all queued up for you bestie:

• find_end — finds the last sequence of elements in a certain range (function template)
• find, find_if, find_if_not (C++11) — finds the first element satisfying specific criteria (function template)
• search — searches for a range of elements (function template)
• ranges::search_n (C++20) — searches for a number consecutive copies of an element in a range (niebloid)

---

and that's the page, bestie! 🦋💖 this lil masterpiece came from http://cppreference.com, version 2024.06.10 — go drink some water and then go search for your consecutive copies, you've earned it 💅✨🎀