After a while trying to understand if either ksh or zsh provided a way to prevent taking strings and undefined variables as 0 when doing arithmetic evaluation, there seems to be no feature specifically for it, sadly.
Closest is using set -o nounset (ksh) and setopt no_unset (zsh) to prevent undefined variables from evaluating to zero. If a "string" contains only numbers, a dot and whitespace, it will be treated as a number. Also, if it only contains the name of any other variable and whitespace, it evaluates to that.
Not that I expected shell languages to provide accurate arithmetic.
As a bonus though, it was cool learning about ksh's compound variables, force_float option and especially discipline functions.
