I'm teaching some middle and high school students programming right now, and I found that some of them really liked online programming puzzles. So I created http://www.kapparate.com/coder/ , and right now there's 4 categories of puzzles.

All the puzzles are set up right now so that variables are pre-initialized, and the user plugs in some code in the middle.

For example, the problem might say these are pre-initialized:

int x = ????;
int y = ????;
int z;

and then the program might ask the student to write the final line of code: z = x + y;.

Now I know I could go a long way in improving the usability of this site (like having an area that lists the pre-defined variables), but I was wondering if this concept seems sound. I know some sites have kids fill in functions, but not all of my students know what functions are yet, and I'm trying to introduce online programming puzzles before that.