std::ranges::search_n(3)

NAME
std::ranges::search_n - std::ranges::search_n

Synopsis
   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 > (since
   requires std::indirectly_comparable<I, const T*, Pred, Proj> C++20)
   constexpr ranges::subrange<I> (until
       search_n( I first, S last, std::iter_difference_t<I> C++26)
   count,
                 const T& value, Pred pred = {}, Proj proj = {}
   );
   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> (since
   constexpr ranges::subrange<I> C++26)
       search_n( I first, S last, std::iter_difference_t<I>
   count,
                 const T& value, Pred pred = {}, Proj proj = {}
   ); (1)
   template< ranges::forward_range R, class T,
             class Pred = ranges::equal_to, class Proj =
   std::identity > (since
   requires std::indirectly_comparable C++20)
       <ranges::iterator_t<R>, const T*, Pred, Proj> (until
   constexpr ranges::borrowed_subrange_t<R> C++26)
       search_n( R&& r, ranges::range_difference_t<R> count,
                 const T& value, Pred pred = {}, Proj proj = {}
   );
   template< ranges::forward_range R, (2)
             class Pred = ranges::equal_to, class Proj =
   std::identity,
             class T =
   std::projected_value_t<ranges::iterator_t<R>, Proj> > (since
   requires std::indirectly_comparable C++26)
       <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 = {}
   );
   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.
             Feature-test macro Value Std Feature
   __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
                        algorithms

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