home | help 
std::ranges::generate_n(3)     C++ Standard Libary    std::ranges::generate_n(3)

NAME
     std::ranges::generate_n - std::ranges::generate_n

Synopsis
	Defined in header <algorithm>
	Call signature
	template< std::input_or_output_iterator O, std::copy_constructible F
	>

	requires      std::invocable<F&>      &&     std::indirectly_writable<O,
     (since C++20)
	std::invoke_result_t<F&>>
	constexpr O

	    generate_n( O first, std::iter_difference_t<O> n, F gen );

	Assigns the result of successive invocations of the function object  gen
     to each
	element  in  the range [first, first + n), if 0 < n. Does nothing other-
     wise.

	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 inhib-
     ited.

	In practice, they may be implemented as function objects, or  with  spe-
     cial compiler
	extensions.

Parameters
	first - the beginning of the range of elements to modify
	n     - number of elements to modify
	gen   - the generator function object.

Return value
	Iterator  one  past the last element assigned if 0 < count, first other-
     wise.

Complexity
	Exactly n invocations of gen() and assignments.

Possible implementation
       struct generate_n_fn
       {
	   template<std::input_or_output_iterator O, std::copy_constructible F>
	   requires std::invocable<F&> &&  std::indirectly_writable<O,	std::in-
     voke_result_t<F&>>
	   constexpr  O  operator()(O first, std::iter_difference_t<O> n, F gen)
     const
	   {
	       for (; n-- > 0; *first = std::invoke(gen), ++first)
	       {}
	       return first;
	   }
       };

       inline constexpr generate_n_fn generate_n {};

Example
     // Run this code

      #include <algorithm>
      #include <array>
      #include <iostream>
      #include <random>
      #include <string_view>

      auto dice()
      {
	  static std::uniform_int_distribution<int> distr {1, 6};
	  static std::random_device engine;
	  static std::mt19937 noise {engine()};
	  return distr(noise);
      }

      void print(const auto& v, std::string_view comment)
      {
	  for (int i : v)
	      std::cout << i << ' ';
	  std::cout << '(' << comment << ")\n";
      }

      int main()
      {
	  std::array<int, 8> v;

	  std::ranges::generate_n(v.begin(), v.size(), dice);
	  print(v, "dice");

	  std::ranges::generate_n(v.begin(), v.size(), [n {0}] mutable {  return
     n++; });
	  // same effect as std::iota(v.begin(), v.end(), 0);
	  print(v, "iota");
      }

Possible output:
      5 5 2 2 6 6 3 5 (dice)
      0 1 2 3 4 5 6 7 (iota)

See also
	ranges::generate	saves the result of a function in a range
	(C++20) 		(niebloid)
	ranges::generate_random fills a range with random numbers from a uniform
     random bit
	(C++26) 		generator
				(niebloid)
	ranges::fill		assigns a range of elements a certain value
	(C++20) 		(niebloid)
	ranges::fill_n		assigns a value to a number of elements
	(C++20) 		(niebloid)
	ranges::transform	applies a function to a range of elements
	(C++20) 		(niebloid)
				assigns the results of successive function calls
     to N
	generate_n		elements in a range
				(function template)

http://cppreference.com 	   2024.06.10	      std::ranges::generate_n(3)
home | help