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