Your isPrime function will return 0 for even-number and return 1 for odd-number because your loop always end at the first loop (because you always return a value at the first loop.)

Try this one instead:
[code]
int i;
for(i=2; i<(int)sqrt(n); i++) {
if(n%i==0) return 0;
}
return 1;
[/c
You can also further optimize your code by:

(Toggle Plain Text)

int i;
int k = (int)sqrt(n);

for(i=2; i<k; i++) {
  if(n%i==0) return 0;
}
return 1;

int i; int k = (int)sqrt(n); for(i=2; i<k; i++) { if(n%i==0) return 0; } return 1;

It should actually be "<= k" rather than less than.

Use 9 as an example and you'll see. The for loop ends and fails to mod 9 against 3, hence returning a value of 1.