Yes, I was working at a large insurance company and some C code that did calculations on lots and lots of entries triggered an audit due to sizeable discrepancies in monetary terms.

I turned out that the code compiled on a x86 PC didn't gave the same results as the same code compiled on the mainframe.

Given enough loops and runs, the skew became too large to ignore. This led to some serious finger pointing and head scratching from de devs. They found the root cause but, like, 2 months after they started looking.

Also there is the banker's rounding.��https://www.eecis.udel.edu/~breech/contest.inet.fall.07/problems/bankers-rounding.html


Phil

On Thu, Oct 27, 2016 at 11:15 AM, Guille Polito <guillermopolito@gmail.com> wrote:

I am also for what Martin says :).


And also: I'd argue that you NEVER EVER want to use simple Floats for financial applications. In financial applications you really need precision, and Floats have hardware limitations that do not provide precision.

I thought ScaledDecimal (which I never used in Pharo) was meant to provide high-precision decimal numbers to the expense of using an internal representation that is not tied to the hardware limitations.

-------- Original Message --------





On 10/27/2016 10:34 AM, Nicolas Cellier
wrote:







2016-10-27 9:51 GMT+02:00 Denis
Kudriashov <dionisiydk@gmail.com>:





2016-10-27 8:31 GMT+02:00
Martin McClure <martin@hand2mouse.com>:


I think roundTo: is OK. #round: is not, and
should be deprecated (at least for Floats).
For Floats, the idea of rounding to a specific
number of decimal digits is a fantasy. Here's
why: Floats cannot exactly represent most
common decimal fractions. For example:


0.1 -- not representable


0.2 -- not representable


0.3 -- not representable


0.4 -- not representable


0.5 -- hey, representable!


0.6 -- not representable


0.7 -- not representable


0.8 -- not representable


0.9 -- not representable


1.0 -- representable.


*Printing* a Float to a specific rounded
decimal format is a sensible idea, and should
be encouraged. But trying for a "rounded
Float" *number* just can't be done exactly
(except by converting to a Fraction).


Fractions can be rounded to exact decimal
fractions, so something like "myFraction
roundTo: 1/100" makes sense. But the current
implementation of round: on Fraction that
converts to a Float just gets you the misery
detailed above.






On
10/26/2016 01:00 PM, Nicolas Cellier wrote:




I've put a single slice in the inbox
for the 3 issues because they all are
related.


See slice comment:



For Float, implement guard to prevent
overflow (15471), and use exact
representation for intermediate results
(asFraction) so as to avoid double
rounding problems (15473)






 The double rounding problem is not the
fundamental problem, the fundamental problem is
that what is desired does not exist, because
Floats cannot exactly represent most decimal
fractions. So this can't really fix it.




Exactly. Thank's for good explanation







Nonetheless, if user asked to round a
Float, we must give him back a Float.

We ain't gonna answer a ScaledDecimal because we think that
it's better for him: we don't know what is better for him.

And we MUST do our best to round correctly to the NEAREST
Float that is 0.1e0, 0.2e0, ... 1.0e0




If user asked to round a Fraction or
ScaledDecimal, then it's different.

We'd better keep the exactness and use ScaledDecimal rather
than convert to a Float.




That's exactly these two things that the SLICE posted in inbox
is doing.




i completely agree (well, i would have preferred a pure Fraction
result in the pure Fraction case instead of a ScaledDecimal, but
so what)


werner


p.s. a pure Fraction result in the pure Fraction case would also
insure that the rest of Number is more or less independent of the
not very useful ScaledDecimal