std::ranges::for_each_n,std::ranges::for_each_n_result(3)

NAME
std::ranges::for_each_n,std::ranges::for_each_n_result - std::ranges::for_each_n,std::ranges::for_each_n_result

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I, class Proj = std::identity,
             std::indirectly_unary_invocable<std::projected<I, Proj>> Fun > (since
   constexpr for_each_n_result<I, Fun> (1) C++20)
       for_each_n( I first, std::iter_difference_t<I> n, Fun f, Proj proj =
   {});

Helper types
   template< class I, class F > (2) (since
   using for_each_n_result = ranges::in_fun_result<I, F>; C++20)
   1) Applies the given function object f to the projected result by proj of
   dereferencing each iterator in the range [first, first + n), in order.
   If the iterator type is mutable, f may modify the elements of the range through the
   dereferenced iterator. If f returns a result, the result is ignored. If n is less
   than zero, the behavior is undefined.
   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 - iterator denoting the begin of the range to apply the function to
   n - the number of elements to apply the function to
   f - the function to apply to the projected range [first, first + n)
   proj - projection to apply to the elements

Return value
   An object {first + n, std::move(f)}, where first + n may be evaluated as
   std::ranges::next(std::move(first), n) depending on iterator category.

Complexity
   Exactly n applications of f and proj.

Possible implementation
 struct for_each_n_fn
 {
     template<std::input_iterator I, class Proj = std::identity,
              std::indirectly_unary_invocable<std::projected<I, Proj>> Fun>
     constexpr for_each_n_result<I, Fun>
         operator()(I first, std::iter_difference_t<I> n, Fun fun, Proj proj = Proj{}) const
     {
         for (; n-- > 0; ++first)
             std::invoke(fun, std::invoke(proj, *first));
         return {std::move(first), std::move(fun)};
     }
 };
 inline constexpr for_each_n_fn for_each_n {};

Example
// Run this code
 #include <algorithm>
 #include <array>
 #include <iostream>
 #include <ranges>
 #include <string_view>
 struct P
 {
     int first;
     char second;
     friend std::ostream& operator<<(std::ostream& os, const P& p)
     {
         return os << '{' << p.first << ",'" << p.second << "'}";
     }
 };
 auto print = [](std::string_view name, auto const& v)
 {
     std::cout << name << ": ";
     for (auto n = v.size(); const auto& e : v)
         std::cout << e << (--n ? ", " : "\n");
 };
 int main()
 {
     std::array a {1, 2, 3, 4, 5};
     print("a", a);
     // Negate first three numbers:
     std::ranges::for_each_n(a.begin(), 3, [](auto& n) { n *= -1; });
     print("a", a);
     std::array s { P{1,'a'}, P{2, 'b'}, P{3, 'c'}, P{4, 'd'} };
     print("s", s);
     // Negate data members 'P::first' using projection:
     std::ranges::for_each_n(s.begin(), 2, [](auto& x) { x *= -1; }, &P::first);
     print("s", s);
     // Capitalize data members 'P::second' using projection:
     std::ranges::for_each_n(s.begin(), 3, [](auto& c) { c -= 'a'-'A'; }, &P::second);
     print("s", s);
 }

Output:
 a: 1, 2, 3, 4, 5
 a: -1, -2, -3, 4, 5
 s: {1,'a'}, {2,'b'}, {3,'c'}, {4,'d'}
 s: {-1,'a'}, {-2,'b'}, {3,'c'}, {4,'d'}
 s: {-1,'A'}, {-2,'B'}, {3,'C'}, {4,'d'}

See also
   range-for loop(C++11) executes loop over range
   ranges::for_each applies a function to a range of elements
   (C++20) (niebloid)
   for_each_n applies a function object to the first N elements of a
   (C++17) sequence
                        (function template)
   for_each applies a function to a range of elements
                        (function template)