C programming, Determine if a number is a prime number or not.

#include <stdio.h>

int main() {

  int n, i, flag = 0;

  printf("Enter a positive integer: ");

  scanf("%d", &n);

  if (n == 0 || n == 1)

    flag = 1;

  for (i = 2; i <= n / 2; ++i) {

    if (n % i == 0) {

      flag = 1;

      break;

    }

  }

  if (flag == 0)

    printf("%d is a prime number.", n);

  else

    printf("%d is not a prime number.", n);

  return 0;

}