c 2b 2b find prime numbers

1#include <iostream>
2using namespace std;
3
4int main()
5{
6  int n, i;
7  bool isPrime = true;
8
9  cout << "Enter a positive integer: ";
10  cin >> n;
11
12  for(i = 2; i <= n / 2; ++i)
13  {
14      if(n % i == 0)
15      {
16          isPrime = false;
17          break;
18      }
19  }
20  if (isPrime)
21      cout << "This is a prime number";
22  else
23      cout << "This is not a prime number";
24
25  return 0;
26}

1#include <iostream>  
2using namespace std;  
3int main()  
4{  
5  int n, i, m=0, flag=0;  
6  cout << "Enter the Number to check Prime: ";  
7  cin >> n;  
8  m=n/2;  
9  for(i = 2; i <= m; i++)  
10  {  
11      if(n % i == 0)  
12      {  
13          cout<<"Number is not Prime."<<endl;  
14          flag=1;  
15          break;  
16      }  
17  }  
18  if (flag==0)  
19      cout << "Number is Prime."<<endl;  
20  return 0;  
21}  

1#include <iostream>
2using namespace std;
3
4int main() {
5    int i, n;
6    bool isPrime = true;
7
8    cout << "Enter a positive integer: ";
9    cin >> n;
10
11    // 0 and 1 are not prime numbers
12    if (n == 0 || n == 1) {
13        isPrime = false;
14    }
15    else {
16        for (i = 2; i <= n / 2; ++i) {
17            if (n % i == 0) {
18                isPrime = false;
19                break;
20            }
21        }
22    }
23    if (isPrime)
24        cout << n << " is a prime number";
25    else
26        cout << n << " is not a prime number";
27
28    return 0;
29}
30

1// 6k+-1 optimisation
2bool is_prime6(int num) {
3  int i;
4  if (num == 1)
5    return false;
6  if (num <= 3)
7    return true;
8  if (num % 2 == 0 || num % 3 == 0)
9    return false;
10  if (num < 25)
11    return true;
12  for (i = 5; i * i <= num; i += 6)
13    if (num % i == 0 || num % (i + 2) == 0)
14      return false;
15  return true;
16}