gistfile1.c

#include <stdio.h>
#include <math.h>
#define MAX 1000010
int prime[MAX] = {0};
void make_prime();
int reverse(int);
int main()
{
  int n;
  make_prime();
  while (scanf("%d", &n) == 1)
  {
    if (!prime[n])
    {
      if (n != reverse(n) && !prime[reverse(n)])
        printf("%d is emirp.\n", n);
      else
        printf("%d is prime.\n", n);
    }
    else
      printf("%d is not prime.\n", n);
  }
  return 0;
}
void make_prime()
{
  int x, y, sqrtM;
  for (x = 2, sqrtM = sqrt(MAX); x < sqrtM; x++)
    if (!prime[x])
      for (y = x*2; y < MAX; y+=x)
        prime[y] = 1;
}
int reverse(int a)
{
  int x, y, ap;
  for (x = 10; a/x > 9; x*=10);
    for (ap=0, y=1, x=(a<10?1:x); a>0 && x; ap+=(a/x)*y, a%=x, y*=10, x/=10);
  return ap;
}