Bitwise Sieve Algorithm

Bitwise Sieve.cpp

/* 
   Author : Remon Hasan
   Algorithm : Bitwise Sieve
*/ 
#include <bits/stdc++.h> 
using namespace std; 

// Checks whether x is prime or composite 
bool ifnotPrime(int prime[], int x) 
{ 
	// checking whether the value of element 
	// is set or not. Using prime[x/64], we find 
	// the slot in prime array. To find the bit 
	// number, we divide x by 2 and take its mod 
	// with 32. 
	return (prime[x/64] & (1 << ((x >> 1) & 31))); 
} 

// Marks x composite in prime[] 
bool makeComposite(int prime[], int x) 
{ 
	// Set a bit corresponding to given element. 
	// Using prime[x/64], we find the slot in prime 
	// array. To find the bit number, we divide x 
	// by 2 and take its mod with 32. 
	prime[x/64] |= (1 << ((x >> 1) & 31)); 
} 

// Prints all prime numbers smaller than n. 
void bitWiseSieve(int n) 
{ 
	// Assuming that n takes 32 bits, we reduce 
	// size to n/64 from n/2. 
	int prime[n/64]; 

	// Initializing values to 0 . 
	memset(prime, 0, sizeof(prime)); 

	// 2 is the only even prime so we can ignore that 
	// loop starts from 3 as we have used in sieve of 
	// Eratosthenes . 
	for (int i = 3; i * i <= n; i += 2) { 

		// If i is prime, mark all its multiples as 
		// composite 
		if (!ifnotPrime(prime, i)) 
			for (int j = i * i, k = i << 1; j < n; j += k) 
				makeComposite(prime, j); 
	} 

	// writing 2 separately 
	printf("2 "); 

	// Printing other primes 
	for (int i = 3; i <= n; i += 2) 
		if (!ifnotPrime(prime, i)) 
			printf("%d ", i); 
} 

// Driver code 
int main() 
{ 
	int n = 30; 
	bitWiseSieve(n); 
	return 0; 
} 

this algo got a trouble , i dont know what is the reason but when i tried to change the value of N variable to 50 , it missed 13

you should try again and you will find the solution. Then Inform me on the comment section.