Loading...
Searching...
No Matches
prev_prime.c

Iterate backwards over primes using primesieve_iterator.

Iterate backwards over primes using primesieve_iterator.Note that primesieve_next_prime() runs up to 2x faster and uses only half as much memory as primesieve_prev_prime(). Hence if it is possible to write the same algorithm using either primesieve_prev_prime() or primesieve_next_prime() then it is preferable to use primesieve_next_prime().

#include <primesieve.h>
#include <inttypes.h>
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char** argv)
{
uint64_t limit = 10000000000ull;
if (argc > 1)
limit = atol(argv[1]);
primesieve_iterator it;
primesieve_init(&it);
/* primesieve_jump_to(&it, start_number, stop_hint) */
primesieve_jump_to(&it, limit, 0);
uint64_t prime;
uint64_t sum = 0;
/* Backwards iterate over the primes <= limit */
while ((prime = primesieve_prev_prime(&it)) > 0)
sum += prime;
primesieve_free_iterator(&it);
printf("Sum of the primes: %" PRIu64 "\n", sum);
/* Note that since sum is a 64-bit variable the result
* will be incorrect (due to integer overflow) if
* limit > 10^10. However we do allow limits > 10^10
* since this is useful for benchmarking. */
if (limit > 10000000000ull)
printf("Warning: sum is likely incorrect due to 64-bit integer overflow!");
return 0;
}
primesieve_jump_to
void primesieve_jump_to(primesieve_iterator *it, uint64_t start, uint64_t stop_hint)
Reset the primesieve iterator to start.
primesieve_prev_prime
static uint64_t primesieve_prev_prime(primesieve_iterator *it)
Get the previous prime.
Definition iterator.h:168
primesieve_free_iterator
void primesieve_free_iterator(primesieve_iterator *it)
Free all memory.
primesieve_init
void primesieve_init(primesieve_iterator *it)
Initialize the primesieve iterator before first using it.
primesieve.h
primesieve C API.
primesieve_iterator
C prime iterator, please refer to iterator.h for more information.
Definition iterator.h:52