But how do you convert to binary without prepending in the conversion?

Also Weeyum I love your solution but it is still prepending.

I also did solutions without prepending or reversing, if you are interested.

I've done a ruby solution that doesn't use a reverse or prepend.
(I've also done one that does, so make sure you're looking at the right one.)

Thanks CrazyMerlyn, I have approved your fork.

What you are saying is not correct, or you are precluding things that are not precluded by this kata. I can't say more without spoiling the challenge. The test do not preclude you from using reverse, although the description says you should try and avoid it. So I suggest you submit a solution and then look at the solutions submitted and see if you cannot find one that does not use a reverse operation.