Is there a good way to write a shell script compatible with both bash and zsh that addresses array elements?

Question

The fact that bash is 0-indexed and zsh is 1-indexed can cause problems. For example the following will work as intended with bash, but not zsh (PS I am aware that I don't NEED to use an array for this, it's just an example):

DISK1=Samsung_SSD_850_EVO_120GB_S21SNX0H915161E
DISK2=Samsung_SSD_850_EVO_120GB_S21SNX0H915160K

DISKS=( $DISK1 $DISK2 )

mdadm --create /dev/md0 --level=mirror --raid-devices=2 "${DISKS[0]}" "${DISKS[1]}"

I was thinking I could make this shell-agnostic by writing everything relative to the index of the current shell, if there were a way to get the index of the current shell as a variable, something like this:

INDEX=$(some_function_that_returns_index)
DISK1=Samsung_SSD_850_EVO_120GB_S21SNX0H915161E
DISK2=Samsung_SSD_850_EVO_120GB_S21SNX0H915160K

DISKS=( $DISK1 $DISK2 )

echo "${DISKS[$INDEX]}" "${DISKS[$(expr $INDEX + 1)]}"



mdadm --create /dev/md0 --level=mirror --raid-devices=2 "${DISKS[$INDEX]}" "${DISKS[$(expr $INDEX + 1)]}"

Is there a way to get the index of the current shell, or alternatively get the nth array element? Ideally using just single shell commands and not a custom little script.