Sieve_Of_Eratosthenes.cpp

// Problem: Given a number n, print all primes smaller than or equal to n. It is also given that n is a small number. 


// 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 Code 
int main() 
{ 
	int n = 30; 
	cout << "Following are the prime numbers smaller "
		<< " than or equal to " << n << endl; 
	SieveOfEratosthenes(n); 
	return 0; 
}