That above example would state that numbers such as 9, 15 and 27 were prime - they're not. If you want to check primes, try the following:
int prime = 1;
for (int i = 0; i < sqrt(number); i++)
{
if ((number / i) == (int)(number / i))
/* i is a factor - the number divided by i is the same when converted to an integer. */
prime = 0;
}
if (prime == 0)
printf("%i is not a prime number.", number);
else
printf("%i is a prime number.", number);
Not tested, but it should work.