# std::ranges::search_n

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
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)
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)

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

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.

### Parameters

first, last | - | the range of elements to examine (aka haystack)
r | - | the range of elements to examine (aka haystack)
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 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, returns std::ranges::subrange{last, last}.

If count <= 0, 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.

### Notes

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

### Possible implementation

    struct search_n_fn
    {
        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>
            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 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>
            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

    #include <algorithm>
    #include <iomanip>
    #include <iostream>
    #include <iterator>
    #include <string>

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

Output:

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

### See also

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

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