Truncatable primes: Difference between revisions

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=={{header|C++}}==

<lang cpp>#include <iostream>

#include "~~sieve_of_eratosthenes~~.h"

#include "prime_sieve.hpp"

bool is_left_truncatable(const ~~sieve_of_eratosthenes~~& sieve, int p) {

bool is_left_truncatable(const prime_sieve& sieve, int p) {

for (int n = 10, q = p; p > n; n *= 10) {

if (!sieve.is_prime(p % n) || q == p % n)

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}

bool is_right_truncatable(const ~~sieve_of_eratosthenes~~& sieve, int p) {

bool is_right_truncatable(const prime_sieve& sieve, int p) {

for (int q = p/10; q > 0; q /= 10) {

if (!sieve.is_prime(q))

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// find the prime numbers up to the limit

~~sieve_of_eratosthenes~~ sieve(limit + 1);

prime_sieve sieve(limit + 1);

int largest_left = 0;

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}</lang>

Contents of ~~sieve_of_eratosthenes~~.h:

Contents of prime_sieve.hpp:

<lang cpp>#ifndef ~~SIEVE_OF_ERATOSTHENES_H~~

<lang cpp>#ifndef PRIME_SIEVE_HPP

#define ~~SIEVE_OF_ERATOSTHENES_H~~

#define PRIME_SIEVE_HPP

#include <algorithm>

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* See https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes.

*/

class ~~sieve_of_eratosthenes~~ {

class prime_sieve {

public:

explicit ~~sieve_of_eratosthenes~~(size_t);

explicit prime_sieve(size_t);

bool is_prime(size_t) const;

private:

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* @param limit the maximum integer that can be tested for primality

*/

inline ~~sieve_of_eratosthenes~~::~~sieve_of_eratosthenes~~(size_t limit) {

inline prime_sieve::prime_sieve(size_t limit) {

limit = std::max(size_t(3), limit);

is_prime_.resize(limit/2, true);

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* @return true if the integer is prime

*/

inline bool ~~sieve_of_eratosthenes~~::is_prime(size_t n) const {

inline bool prime_sieve::is_prime(size_t n) const {

if (n == 2)

return true;