std::ranges::take_view<V>::take_view
take_view() requires std::default_initializable<V> = default; |
(1) | (since C++20) |
constexpr take_view( V base, ranges::range_difference_t<V> count ); |
(2) | (since C++20) |
Constructs a take_view.
1) Default constructor.
Value-initializes the underlying view and initializes 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 Eratosthenes method.
#include <bit>
#include <bitset>
#include <iomanip>
#include <iostream>
#include <limits>
#include <ranges>
constexpr unsigned clog2(auto x) // ~ ⌈ log₂(x) ⌉
{
return std::numeric_limits<decltype(x)>::digits - std::countl_zero(x);
}
template <unsigned Primes> struct Sieve
{
bool operator()(int n) {
if (n < 2) return false;
if (n == 2) return true;
if (n % 2 == 0 || bits.test(n / 2))
return false;
for (int i{n}, j{3}, k{}; (k = i * j) < size; j += 2)
bits.set(k / 2);
return true;
}
static constexpr int prime_numbers = Primes;
// a(n) ~= n*(log(n) + log(log(n)));
static constexpr int size = Primes*(clog2(Primes) + clog2(clog2(Primes)));
// Keep only odd numbers; 0 means it is a prime
private: std::bitset<size / 2 + 1> bits;
};
int main()
{
Sieve<100> sieve{};
auto primes_gen = std::views::iota(1)
| std::views::filter(sieve);
auto primes = std::ranges::take_view{ primes_gen, sieve.prime_numbers };
for (int n{1}; auto prime : primes) {
std::cout << prime << (n++ % 10 ? ' ' : '\n');
}
std::cout
<< "\n" "sieve.prime_numbers: " << sieve.prime_numbers
<< "\n" "sieve.size: " << sieve.size
<< "\n" "sizeof(sieve): " << sizeof sieve << " bytes\n";
}
Output:
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269 271 277 281 283 293 307 311 313 317 331 337 347 349 353 359 367 373 379 383 389 397 401 409 419 421 431 433 439 443 449 457 461 463 467 479 487 491 499 503 509 521 523 541 sieve.prime_numbers: 100 sieve.size: 1000 sizeof(sieve): 64 bytes
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