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;
SieveOfEratosthenes(n);
return 0;
}``````

Code Example 2

``````//sieve of eratosthenes or prime of sieve
#include<iostream>
#include<math.h>
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)
arr[j]=1;
}
for(long long int i=2;i<=n;i++)
{
if(arr[i]==0)
cout<<i<<" ";
}

}
int main()
{

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