Using freopen()

I was going through the facebook puzzles page and 1st puzzle was very simple, just to make you familiarize with the submission process. I made that puzzle and submitted (haven’t got reply yet :(), anyways, after the submission I was going through a page where this problem was solved in variety of languages. I was very proud of my implementation (because I though I have taken care of most of the cases), but there was one very neat and clean implementation available for C. It used freopen(). Damn! I have never used that function, I don’t remember if I have ever heard of it. It is basically a brother of dup() I guess, but closes the first FILE pointer. The usage which I saw was, redirecting a file to stdin. I could never thought of using freopen to redirect any text file to stdin. The nice usage will allow us to use scanf on that file now :) 

So here is the problem definition, and how can one simply implement it using freopen. 

The program will take one input file containing an integer value. It may be preceded with multiple white spaces and it can be ended with multiple white spaces. Your output should iterate from 1 to that number (in file), and print "hop" if the number is multiple of 3 & 5, "hophop" if its multiple of 5, and "hoppit" if multiple of 3. And here is the code using freopen :) 
 

#include <stdio.h>

int main(int argc, char *argv[])
{
if (argc != 2) {
printf("Invalid args\n");
printf("Usage: %s <filename>\n", argv[0]);
return -1;
} else {
if (freopen(argv[1], "r", stdin)) {
unsigned long long ull_val = 0, i;
scanf("%llu", &ull_val);
for (i=1; i<=ull_val;i++) {
if (i%3 ==0 & i%5==0)
printf("Hop\n");
else if (i%3 == 0)
printf("Hoppity\n");
else if (i%5 == 0)
printf("Hophop\n");
}
} else {
printf("Unable to open the file\n");
return -1;
}

}
return 0;
}