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 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.

// Run this code

#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