Hi @rcb, good job, I see you’re catching up fast

A few points:

• You should try mix format If you can change your editor to do it automatically on save, it gives you a nice feedback loop. On top of that, credo can also bring you some nice tips on style and code organization.

• Cardinal.Checksum.double_every_other/2 could be like this instead, what do you think?

  defp double_every_other([], _pos), do: []
  
  defp double_every_other([head|tail], pos) when rem(pos, 2) == 0 do
    [head | double_every_other(tail, pos + 1)]
  end
  
  defp double_every_other([head|tail], pos) when rem(pos, 2) == 1 do
    [head * 2 | double_every_other(tail, pos + 1)]
  end

Note that rem/2 can only be at the function head because it’s also a guard.

(EDIT: maybe just using Enum.with_index/1 and Enum.map/2 could be better than the above)

• Prefer is_binary/1 than is_bitstring/1 if you want a guard to guarantee it’s a String. Every binary is a bitstring, but not every bitstring is a binary.

Cheers!

Those look a lot cleaner than mine. The double_every_other() function was one I wrote early on when the program worked a bit differently and I don’t think I really revisited it much except for minor cleanup when the program changed. When I get back on my main machine, I’ll try this implementation out.

I didn’t realize that distinction was there between those two. I’ll change those out too.

Thanks for looking things over and taking the time to provide some guidance – I really appreciate it!

Thanks for the advice regarding passing IO to the private function. I’ve removed that and replaced it with regular IO.gets/puts calls. You’re right about the invalid data example too — I overlooked a pretty big error there. Thanks for the heads up on that, along with the link to the Luhn algorithm example and for taking the time to respond. I appreciate the help!

Also, thanks to both @srowley and @axelson for the excellent advice regarding the reduce_double_digits() function. I’ll work on implementing your suggestions too.

Thanks to everyone who took the time to review my code and provide such helpful advice. I really appreciate the help and guidance!