std::search_n(3)

NAME
std::search_n - std::search_n

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt, class
   Size, class T >
                        (constexpr
   ForwardIt search_n( ForwardIt since C++20)
   first, ForwardIt last, (until C++26)
                       Size count,
   const T& value );
   template< class ForwardIt, class

Size,
             class T = typename
   std::iterator_traits
                        (since C++26)
   <ForwardIt>::value_type >
   constexpr ForwardIt search_n(
   ForwardIt first, ForwardIt last,
   Size count, const T& value );
   template< class ExecutionPolicy,
             class ForwardIt, class
   Size, class T > (since
   ForwardIt search_n( C++17)
   ExecutionPolicy&& policy, (until
                       ForwardIt C++26)
   first, ForwardIt last,
                       Size count,
   const T& value );
   template< class ExecutionPolicy,
             class ForwardIt, class

Size,
             class T = typename
   std::iterator_traits
                        (since
   <ForwardIt>::value_type > C++26)
   ForwardIt search_n(
   ExecutionPolicy&& policy,
                       ForwardIt
   first, ForwardIt last,
                       Size count,
   const T& value );
   template< class ForwardIt, class (1)
   Size, class T, class BinaryPred >
                        (constexpr
   ForwardIt search_n( ForwardIt since C++20)
   first, ForwardIt last, (until
                        C++26)
                       Size count,
   const T& value, BinaryPred p );
   template< class ForwardIt, class

Size,
                        (2)
             class T = typename
   std::iterator_traits
                        (since
   <ForwardIt>::value_type, C++26)
             class BinaryPred >
   constexpr ForwardIt search_n(
   ForwardIt first, ForwardIt last,
                       Size count,
   const T& value, BinaryPred p );
   template< class ExecutionPolicy,
   class ForwardIt, class Size,
                        (3)
             class T, class
   BinaryPred > (since
   ForwardIt search_n( C++17)
   ExecutionPolicy&& policy, (until
                       ForwardIt C++26)
   first, ForwardIt last,
                       Size count,
   const T& value, BinaryPred p );
   template< class ExecutionPolicy,
   class ForwardIt, class Size, (4)
             class T = typename
   std::iterator_traits
   <ForwardIt>::value_type, (since
             class BinaryPred > C++26)
   ForwardIt search_n(
   ExecutionPolicy&& policy,
                       ForwardIt
   first, ForwardIt last,
                       Size count,
   const T& value, BinaryPred p );
   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
   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. See execution policy for details.
                 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);
   p - 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
                       T was required to be EqualityComparable, removed the
   LWG 283 C++98 but requirement
                       the value type of InputIt is not always T
                       the complexity upper limit was N·count, the upper limit is 0
   LWG 426 C++98 it is negative if count is negative if count is
                        non-positive
                       if count > 0, the complexity upper limit changed the upper
   LWG 714 C++98 was N·count, but in limit to N in this
                       the worst case the number of case
                       comparisons/operations is always N
   LWG 2150 C++98 the condition of “sequence occurence” was corrected
                       incorrect

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