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std::for_each_n(3)	       C++ Standard Libary	      std::for_each_n(3)

NAME
     std::for_each_n - std::for_each_n

Synopsis
	Defined in header <algorithm>
	template<  class  InputIt,  class  Size,  class UnaryFunc >	  (since
     C++17)
	InputIt for_each_n( InputIt first, Size n, UnaryFunc f	 (1)  (constexpr
     since C++20)
	);
	template< class ExecutionPolicy,

		  class ForwardIt, class Size, class UnaryFunc >
	ForwardIt  for_each_n(	ExecutionPolicy&&  policy,	     (2)  (since
     C++17)

			      ForwardIt first, Size n, UnaryFunc
	f );

	Applies the given function object f to the result of dereferencing every
     iterator in
	the range [first, first + n). If f returns a result, the result  is  ig-
     nored.

	1) f is applied in order starting from first.
	If UnaryFunc is not MoveConstructible, the behavior is undefined.
	2)  f might not be applied in order. The algorithm is executed according
     to policy.
	This overload participates 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)

	If UnaryFunc is not CopyConstructible, the behavior is undefined.

	If n >= 0 is not true, the behavior is undefined.

	If the iterator type (InputIt/ForwardIt) is mutable, f	may  modify  the
     elements of
	the range through the dereferenced iterator.

	Unlike the rest of the parallel algorithms, for_each_n is not allowed to
     make copies
	of the elements in the sequence even if they are TriviallyCopyable.

Parameters
	first  - the beginning of the range to apply the function to
	n      - the number of elements to apply the function to
	policy - the execution policy to use. See execution policy for details.
		 function  object,  to be applied to the result of dereferencing
     every
		 iterator in the range [first, first + n)

		 The signature of the function should be equivalent to the  fol-
     lowing:

	f      -  void fun(const Type &a);

		 The signature does not need to have const &.
		 The  type  Type must be such that an object of type InputIt can
     be
		 dereferenced and then implicitly converted to	Type.

Type requirements
	-
	InputIt must meet the requirements of LegacyInputIterator.
	-
	ForwardIt must meet the requirements of LegacyForwardIterator.
	-
	Size must be convertible to an integral type.

Return value
	An iterator equal to first + n, or more formally, to std::advance(first,
     n).

Complexity
	Exactly n applications of f.

Exceptions
	The overload with a template parameter named ExecutionPolicy reports er-
     rors 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-de-
     fined.
	  * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
	See also the implementation in libstdc++, libc++ and MSVC stdlib.

	template<class InputIt, class Size, class UnaryFunc>
	InputIt for_each_n(InputIt first, Size n, UnaryFunc f)
	{
	    for (Size i = 0; i < n; ++first, (void) ++i)
		f(*first);

	    return first;
	}

Example
     // Run this code

      #include <algorithm>
      #include <iostream>
      #include <vector>

      void println(auto const& v)
      {
	  for (auto count{v.size()}; const auto& e : v)
	      std::cout << e << (--count ? ", " : "\n");
      }

      int main()
      {
	  std::vector<int> vi{1, 2, 3, 4, 5};
	  println(vi);

	  std::for_each_n(vi.begin(), 3, [](auto& n) { n *= 2; });
	  println(vi);
      }

Output:
      1, 2, 3, 4, 5
      2, 4, 6, 4, 5

See also
			      applies a function to a range of elements, storing
     results in
	transform	      a destination range
			      (function template)
	range-for loop(C++11) executes loop over range
	for_each	      applies a function to a range of elements
			      (function template)
	ranges::for_each_n     applies a function object to the first N elements
     of a
	(C++20) 	      sequence
			      (niebloid)

http://cppreference.com 	   2024.06.10		      std::for_each_n(3)
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