Are You A Prime Number

Question

I\'ve been interested in the problem of finding a better prime number recognizer for years. I realize this is a huge area of academic research and study - my interest in this i

Answer 1

#include 
#include 

int IsPrime (int n) {
  int i, sqrtN;
  if (n < 2) { return 0; } /* 1, 0, and negatives are nonprime */
  if (n == 2) { return 2; }
  if ((n % 2) == 0) { return 0; } /* Check for even numbers */
  sqrtN = sqrt((double)n)+1; /* We don't need to search all the way up to n */
  for (i = 3; i < sqrtN; i += 2) {
    if (n % i == 0) { return 0; } /* Stop, because we found a factor! */
  }
  return n;
}

int main()
{
  int n;
  printf("Enter a positive integer: ");
  scanf("%d",&n);
  if(IsPrime(n))
     printf("%d is a prime number.",n);
  else
     printf("%d is not a prime number.",n);
  return 0;
}