on floating-point numbers

[Chris Angelico]

On Sat, Sep 4, 2021 at 12:08 AM o1bigtenor <o1bigtenor at gmail.com> wrote:
> Hmmmmmmm - - - ZI would suggest that you haven't looked into
> taxation yet!
> In taxation you get a rational number that MUST be multiplied by
> the amount in currency.

(You can, of course, multiply a currency amount by any scalar. Just
not by another currency amount.)

> The error rate here is stupendous.
> Some organizations track each transaction with its taxes rounded.
> Then some track using  use untaxed and then calculate the taxes
> on the whole (when you have 2 or 3 or 4 (dunno about more but
> who knows there are some seriously tax loving jurisdictions out there))
> the differences between adding amounts and then calculating taxes
> and calculating taxes on each amount and then adding all items
> together can have some 'interesting' differences.
>
> So financial data MUST be able to handle rational numbers.
> (I have been bit by the differences enumerated in the previous!)

The worst problem is knowing WHEN to round. Sometimes you have to do
intermediate rounding in order to make something agree with something
else :(

But if you need finer resolution than the cent, I would still
recommend trying to use fixed-point arithmetic. The trouble is
figuring out exactly how much precision you need. Often, 1c precision
is actually sufficient.

ChrisA