Check out example codes for "sieve of eratosthenes c++". It will help you in understanding the concepts better.

Code Example 1

// C++ program to print all primes smaller than or equal to 
// n using Sieve of Eratosthenes 
#include <bits/stdc++.h> 
using namespace std; 

void SieveOfEratosthenes(int n) 
	// Create a boolean array "prime[0..n]" and initialize 
	// all entries it as true. A value in prime[i] will 
	// finally be false if i is Not a prime, else true. 
	bool prime[n+1]; 
	memset(prime, true, sizeof(prime)); 

	for (int p=2; p*p<=n; p++) 
		// If prime[p] is not changed, then it is a prime 
		if (prime[p] == true) 
			// Update all multiples of p greater than or 
			// equal to the square of it 
			// numbers which are multiple of p and are 
			// less than p^2 are already been marked. 
			for (int i=p*p; i<=n; i += p) 
				prime[i] = false; 

	// Print all prime numbers 
	for (int p=2; p<=n; p++) 
	if (prime[p]) 
		cout << p << " "; 

// Driver Program to test above function 
int main() 
	int n = 30; 
	cout << "Following are the prime numbers smaller "
		<< " than or equal to " << n << endl; 
	return 0; 

Code Example 2

//sieve of eratosthenes or prime of sieve
using namespace std;
void primeofsieve(long long int n)
	long long int arr[n]={};
	for(int i=2;i<=sqrt(n);i++)
		for(long long int j=i*i;j<=n;j+=i)
	for(long long int i=2;i<=n;i++)
	    	cout<<i<<" ";

int main()

	#ifdef _DEBUG
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
	long long int n;
	cout<<"PRIME NUMBERs ARE : ";
	return 0;

Code Example 3

void eratosthenes_sieve(int N){
	bool primes[N+1];
  	memset(primes, true, sizeof(primes));
  	primes[0] = false;
  	primes[1] = false;
  	for (int p=2; p*p<=N; p++){
      	if (primes[p]){
          	for (int i=p*2; i<=N; i+=p)
              	primes[p] = false;

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