fixed the loop to include index variable

aside: This chapter is by far my favorite.

It:

1) challenged me to go out on my own
2) proved that I understand enough so far to actually accomplish that

I was hesitant to take the challenge, up to this point I was feeling like
I wasn't really getting the language (the lisp part, not the C) because
I could not really write anything coherent using \ and &... but I was
pretty much able to implement the necessary conditionals in a way that
felt "right."