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std::ranges...w::take_view(3)  C++ Standard Libary std::ranges...w::take_view(3)

NAME
     std::ranges::take_view::take_view - std::ranges::take_view::take_view

Synopsis
	take_view()  requires  std::default_initializable<V> = default;      (1)
     (since C++20)
	constexpr explicit take_view( V  base,				     (2)
     (since C++20)
	ranges::range_difference_t<V> count );

	Constructs a take_view.

	1)  Default  constructor. Value-initializes the underlying view and ini-
     tializes the
	count to 0. After construction, base() returns a copy of V() and  size()
     returns 0.
	2)  Initializes  the  underlying view with std::move(base) and the count
     with count.
	After construction, base() returns a copy of base and size() returns the
     smaller of
	count and ranges::size(base).

Parameters
	base  - the underlying view
	count - number of elements to take

Example
	Prints first n prime numbers that are generated using Sieve  of  Eratos-
     thenes method.

     // Run this code

      #include <bit>
      #include <bitset>
      #include <iomanip>
      #include <iostream>
      #include <limits>
      #include <ranges>

      constexpr unsigned clog2(auto x) // ~ a loga(x) a
      {
	  return std::numeric_limits<decltype(x)>::digits - std::countl_zero(x);
      }

      template<unsigned Count>
      struct FirstPrimes
      {
	  static constexpr int count = Count;

	  constexpr bool operator()(int n) // is prime?
	  {
	      return n < 2 ? false :
		     n == 2 ? true :
		     n % 2 == 0 or bits_.test(n / 2) ? false : true;
	  }
      private:
	  consteval static auto init()
	  {
	      std::bitset<size_ / 2 + 1> bits;
	      for (int n{3}; n < size_; n += 2)
		  for (int i{n}, j{3}, k{}; (k = i * j) < size_; j += 2)
		      bits.set(k / 2);
	      return bits;
	  }

	  // Keep only odd numbers; 0 means it is a prime
	  constexpr static auto bits_ { init() };

	  // a(n) <= n * (log(n) + log(log(n)))
	  static    constexpr	 int   size_   =   Count   *   (clog2(Count)   +
     clog2(clog2(Count)));
      };

      int main()
      {
	  constexpr FirstPrimes<42> primes;

	  auto primes_view = std::ranges::take_view{ std::views::iota(1)
						   | std::views::filter(primes)
						   , primes.count };

	  std::cout << "First " << primes.count << " prime numbers are:\n";
	  for (int new_line{1}; const int prime : primes_view)
	      std::cout << std::setw(3) << prime << (new_line++ %  7  ?  '  '  :
     '\n');
      }

Output:
      First 42 prime numbers are:
	2   3	5   7  11  13  17
       19  23  29  31  37  41  43
       47  53  59  61  67  71  73
       79  83  89  97 101 103 107
      109 113 127 131 137 139 149
      151 157 163 167 173 179 181

	Defect reports

	The  following	behavior-changing  defect  reports were applied retroac-
     tively to
	previously published C++ standards.

	   DR	  Applied to		 Behavior as published		    Cor-
     rect behavior
	LWG 3714  C++20      the multi-parameter constructor was not	    made
     explicit
	(P2711R1)	     explicit

http://cppreference.com 	   2024.06.10	   std::ranges...w::take_view(3)
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