Honaker primes: Difference between revisions

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===Without external libraries===

<syntaxhighlight lang="c++">

#include <cstdint>

#include <iomanip>

#include <iostream>

#include <string>

#include <vector>

uint32_t honaker_index = 0;

uint32_t prime_index = 0;

std::vector<uint32_t> primes;

struct HonakerPrime {

uint32_t honaker_index, prime_index, prime;

std::string to_string() {

return "(" + std::to_string(honaker_index) + ": "

+ std::to_string(prime_index) + ", "

+ std::to_string(prime) + ")";

}

};

void sieve_primes(const uint32_t& limit) {

primes.emplace_back(2);

const uint32_t half_limit = ( limit + 1 ) / 2;

std::vector<bool> composite(half_limit);

for ( uint32_t i = 1, p = 3; i < half_limit; p += 2, ++i ) {

if ( ! composite[i] ) {

primes.emplace_back(p);

for ( uint32_t a = i + p; a < half_limit; a += p ) {

composite[a] = true;

}

}

}

}

uint32_t digital_sum(uint32_t number) {

uint32_t sum = 0;

while ( number > 0 ) {

sum += number % 10;

number /= 10;

}

return sum;

}

HonakerPrime nextHonakerPrime() {

honaker_index++;

prime_index++;

while ( digital_sum(prime_index) != digital_sum(primes[prime_index - 1]) ) {

prime_index++;

}

return HonakerPrime(honaker_index, prime_index, primes[prime_index - 1]);

}

int main() {

sieve_primes(5'000'000);

std::cout << "The first 50 Honaker primes (honaker index: prime index, prime):" << std::endl;

for ( uint32_t i = 1; i <= 50; ++i ) {

std::cout << std::setw(17) << nextHonakerPrime().to_string() << ( i % 5 == 0 ? "\n" : " " );

}

for ( uint32_t i = 51; i < 10'000; ++i ) {

nextHonakerPrime();

}

std::cout << "\n" << "The 10,000th Honaker prime is: " + nextHonakerPrime().to_string() << std::endl;

}

</syntaxhighlight>

{{ out }}

<pre>

The first 50 Honaker primes (honaker index: prime index, prime):

(1: 32, 131) (2: 56, 263) (3: 88, 457) (4: 175, 1039) (5: 176, 1049)

(6: 182, 1091) (7: 212, 1301) (8: 218, 1361) (9: 227, 1433) (10: 248, 1571)

(11: 293, 1913) (12: 295, 1933) (13: 323, 2141) (14: 331, 2221) (15: 338, 2273)

(16: 362, 2441) (17: 377, 2591) (18: 386, 2663) (19: 394, 2707) (20: 397, 2719)

(21: 398, 2729) (22: 409, 2803) (23: 439, 3067) (24: 446, 3137) (25: 457, 3229)

(26: 481, 3433) (27: 499, 3559) (28: 508, 3631) (29: 563, 4091) (30: 571, 4153)

(31: 595, 4357) (32: 599, 4397) (33: 635, 4703) (34: 637, 4723) (35: 655, 4903)

(36: 671, 5009) (37: 728, 5507) (38: 751, 5701) (39: 752, 5711) (40: 755, 5741)

(41: 761, 5801) (42: 767, 5843) (43: 779, 5927) (44: 820, 6301) (45: 821, 6311)

(46: 826, 6343) (47: 827, 6353) (48: 847, 6553) (49: 848, 6563) (50: 857, 6653)

The 10,000th Honaker prime is: (10000: 286069, 4043749)