std::ranges::search_n(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💅 std::ranges::search_n — C++ Standard Library vibes

okay bestie, welcome to the std::ranges::search_n(3) page, aka the C++ Standard Library's certified "find me a run of N identical things" queen 👑 she's giving haystack-needle energy and honestly? iconic.

✨ NAME ✨

std::ranges::search_n — literally just std::ranges::search_n, no notes 💅

✨ Synopsis ✨

Defined in header <algorithm> 🎀

Call signature

bestie there are FOUR of these bad boys and they're all doing the same job, just with different template drip. read carefully ok 😭

template< std::forward_iterator I, std::sentinel_for<I> S,
          class T,
          class Pred = ranges::equal_to, class Proj = std::identity >
    requires std::indirectly_comparable<I, const T*, Pred, Proj>
constexpr ranges::subrange<I>
    search_n( I first, S last, std::iter_difference_t<I> count,
              const T& value, Pred pred = {}, Proj proj = {} );

(1) — since C++20, until C++26

template< std::forward_iterator I, std::sentinel_for<I> S,
          class Pred = ranges::equal_to, class Proj = std::identity,
          class T = std::projected_value_t<I, Proj> >
    requires std::indirectly_comparable<I, const T*, Pred, Proj>
constexpr ranges::subrange<I>
    search_n( I first, S last, std::iter_difference_t<I> count,
              const T& value, Pred pred = {}, Proj proj = {} );

(1) — since C++26

template< ranges::forward_range R, class T,
          class Pred = ranges::equal_to, class Proj = std::identity >
    requires std::indirectly_comparable
             <ranges::iterator_t<R>, const T*, Pred, Proj>
constexpr ranges::borrowed_subrange_t<R>
    search_n( R&& r, ranges::range_difference_t<R> count,
              const T& value, Pred pred = {}, Proj proj = {} );

(2) — since C++20, until C++26

template< ranges::forward_range R,
          class Pred = ranges::equal_to, class Proj = std::identity,
          class T = std::projected_value_t<ranges::iterator_t<R>, Proj> >
    requires std::indirectly_comparable
             <ranges::iterator_t<R>, const T*, Pred, Proj>
constexpr ranges::borrowed_subrange_t<R>
    search_n( R&& r, ranges::range_difference_t<R> count,
              const T& value, Pred pred = {}, Proj proj = {} );

(2) — since C++26

1) so basically it searches the range [first, last) for the first sequence of count elements whose projected values are each equal to the given value, according to the binary predicate pred. ✨

2) same as (1) but it uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last. 💫

🦋 the niebloid moment 🦋

The function-like entities described on this page are niebloids, that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup.
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions. no cap, that's just how it is 🤷‍♀️

✨ Parameters ✨

first, last — the range of elements to examine (aka haystack 🐴)
r — the range of elements to examine (aka haystack, again 🐴)
count — the length of the sequence to search for
value — the value to search for (aka needle 🪡)
pred — the binary predicate that compares the projected elements with value
proj — the projection to apply to the elements of the range to examine

✨ Return value ✨

1) returns a std::ranges::subrange object that contains a pair of iterators in the range [first, last) that designate the found subsequence. 🎯

If no such subsequence is found, it returns std::ranges::subrange{last, last}. 😭

If count <= 0, it returns std::ranges::subrange{first, first}. 🫶

2) same as (1) but the return type is ranges::borrowed_subrange_t<R>.

✨ Complexity ✨

Linear: at most ranges::distance(first, last) applications of the predicate and the projection. honestly not bad, slay 💅

✨ Notes ✨

An implementation can improve efficiency of the search in average if the iterators model std::random_access_iterator. 👑

feature-test macro table but make it cute

__cpp_lib_algorithm_default_value_type — value 202403 — std (C++26) — feature: List-initialization for algorithms

✨ Possible implementation ✨

like, here's literally how someone could cook this up 🍓

struct search_n_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Pred = ranges::equal_to, class Proj = std::identity,
             class T = std::projected_value_t<I, Proj>>
    requires std::indirectly_comparable<I, const T*, Pred, Proj>
    constexpr ranges::subrange<I>
        operator()(I first, S last, std::iter_difference_t<I> count,
                   const T& value, Pred pred = {}, Proj proj = {}) const
    {
        if (count <= 0)
            return {first, first};
        for (; first != last; ++first)
            if (std::invoke(pred, std::invoke(proj, *first), value))
            {
                I start = first;
                std::iter_difference_t<I> n{1};
                for (;;)
                {
                    if (n++ == count)
                        return {start, std::next(first)}; // found
                    if (++first == last)
                        return {first, first}; // not found
                    if (!std::invoke(pred, std::invoke(proj, *first), value))
                        break; // not equ to value
                }
            }
        return {first, first};
    }

    template<ranges::forward_range R,
             class Pred = ranges::equal_to, class Proj = std::identity,
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
    requires std::indirectly_comparable<ranges::iterator_t<R>, const T*, Pred, Proj>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, ranges::range_difference_t<R> count,
                   const T& value, Pred pred = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
                       std::move(count), value,
                       std::move(pred), std::move(proj));
    }
};

inline constexpr search_n_fn search_n {};

✨ Example ✨

// Run this code — go off bestie 🏃‍♀️💨

#include <algorithm>
#include <cassert>
#include <complex>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

int main()
{
    namespace ranges = std::ranges;

    static constexpr auto nums = {1, 2, 2, 3, 4, 1, 2, 2, 2, 1};
    constexpr int count{3};
    constexpr int value{2};
    typedef int count_t, value_t;

    constexpr auto result1 = ranges::search_n
    (
        nums.begin(), nums.end(), count, value
    );
    static_assert // found
    (
        result1.size() == count &&
        std::distance(nums.begin(), result1.begin()) == 6 &&
        std::distance(nums.begin(), result1.end()) == 9
    );

    constexpr auto result2 = ranges::search_n(nums, count, value);
    static_assert // found
    (
        result2.size() == count &&
        std::distance(nums.begin(), result2.begin()) == 6 &&
        std::distance(nums.begin(), result2.end()) == 9
    );

    constexpr auto result3 = ranges::search_n(nums, count, value_t{5});
    static_assert // not found
    (
        result3.size() == 0 &&
        result3.begin() == result3.end() &&
        result3.end() == nums.end()
    );

    constexpr auto result4 = ranges::search_n(nums, count_t{0}, value_t{1});
    static_assert // not found
    (
        result4.size() == 0 &&
        result4.begin() == result4.end() &&
        result4.end() == nums.begin()
    );

    constexpr char symbol{'B'};
    auto to_ascii = [](const int z) -> char { return 'A' + z - 1; };
    auto is_equ = [](const char x, const char y) { return x == y; };

    std::cout << "Find a sub-sequence " << std::string(count, symbol) << " in the ";
    std::ranges::transform(nums, std::ostream_iterator<char>(std::cout, ""), to_ascii);
    std::cout << '\n';

    auto result5 = ranges::search_n(nums, count, symbol, is_equ, to_ascii);
    if (not result5.empty())
        std::cout << "Found at position "
                  << ranges::distance(nums.begin(), result5.begin()) << '\n';

    std::vector<std::complex<double>> nums2{{4, 2}, {4, 2}, {1, 3}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto it = ranges::search_n(nums2, 2, {4, 2});
    #else
        auto it = ranges::search_n(nums2, 2, std::complex<double>{4, 2});
    #endif
    assert(it.size() == 2);
}

Output

Find a sub-sequence BBB in the ABBCDABBBA
Found at position 6

✨ See also ✨

ranges::adjacent_find (C++20) (niebloid) — finds the first two adjacent items that are equal (or satisfy a given predicate)
ranges::find, ranges::find_if, ranges::find_if_not (C++20) (niebloid) — finds the first element satisfying specific criteria
ranges::find_end (C++20) (niebloid) — finds the last sequence of elements in a certain range
ranges::find_first_of (C++20) (niebloid) — searches for any one of a set of elements
ranges::includes (C++20) (niebloid) — returns true if one sequence is a subsequence of another
ranges::mismatch (C++20) (niebloid) — finds the first position where two ranges differ
ranges::search (C++20) (niebloid) — searches for a range of elements
search_n (function template) — searches a range for a number of consecutive copies of an element

🌈 and that's the whole page bestie, you ate that. go find your subsequences. 💅✨

sourced from http://cppreference.com and dated 2024.06.10 — this has been your std::ranges::search_n(3) moment, mwah 🫶🎀

📦 where this page comes from

nixpkgs unstable: stdman 2024.07.05
opensuse-leap 16.0: stdman 2024.07.05-bp160.1.9 · MIT
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms