Hi Nicolas,
yes, i know. suppose you calc some financial thing and one intermediate
result will be in $. you have to round this Fraction-result to 2
decimals. now you use this to further calc basepoints as the final
result. you have to convert this to 3 decimals. looking at this final
result with 2 decimals can be be irritating for a moment, if you debug
the calcs. and <g> btw i never use scaledDecimals.
werner
On 10/26/2016 02:06 PM, Nicolas Cellier wrote:
2016-10-26 13:11 GMT+02:00 test <[email protected]
<mailto:[email protected]>>:
Hi Nicolas,
regarding rounding Fractions:
i use fractions if i want to get an exact result (eg for comparing
the result with Float calculations). if #round: returns a Float
all further calcs (with Fractions) will get contaminated, since
the rest will become Floats too. Hence the "asScaledDecimal:
numberOfWishedDecimal" seems better to me, but i wonder why these
transformations at the end are necessary at all? just for the
looks? i'd suppose every person, who knows how to use Fractions,
also knows how to append a #asScaledDecimal: to a result by
himself, should he want that.
taking as an example financial calcs that first use 2 decimals.
occasionaly the final result will be basepoints, often small ones
like 0.003. with scaledDecimals the result would be (ok, look
like) 0 since scaledDecimals also contaminate the calc. of course
one could correct this simply with an #asScaledDecimal:3 at the
end. nevertheless a first look at the zero result would surprise
me for a tenth of a second.
werner
Hi Werner,
I don't know the purpose of round: at all.
Most often this kind of message was used before printing probably
because lack of versatile formatted print messages.
In Squeak I replaced most usages of roundTo: by
printShowing(Max)DecimalPlaces:.
Now if it has been added in Pharo and other languages, there must be
some use cases I presume.
Maybe the analysis could be carried on these use cases?
Beware, converting a Fraction asScaledDecimal will NOT round.
Only the printString is rounded, but the number keeps its whole precision.
Example (1/3 asScaledDecimal: 1)*3 = 1.0s, not 0.9s.
ScaledDecimals as they are now are just Fraction with a different
printString...
Not very much added value.
Nicolas
On 10/26/2016 09:58 AM, Nicolas Cellier wrote:
2016-10-26 9:14 GMT+02:00 stepharo <[email protected]
<mailto:[email protected]>>:
Hi nicolas
So what is the solution? We can integrate fast a solution.
I would really like to see them fix in Pharo 60.
I'm writing a book for newbie and this is the third time I
change one chapter
so may be I should stop and throw away this chapter.
1) for Fraction:
round: numberOfWishedDecimal
v := 10 raisedTo: numberOfWishedDecimal.
^ ((self * v) rounded / v) asFloat
or just replace asFloat if you wish to remain exact:
round: numberOfWishedDecimal
v := 10 raisedTo: numberOfWishedDecimal.
^ ((self * v) rounded / v) asScaledDecimal: numberOfWishedDecimal
2) for Float, it is in 15471:
round: numberOfWishedDecimal
| v maxNumberOfDecimals |
maxNumberOfDecimals := self class precision - 1 - (self
exponent max: self class emin).
maxNumberOfDecimals < numberOfWishedDecimal ifTrue: [^self].
v := 10 raisedTo: numberOfWishedDecimal.
^ ((self asFraction * v) rounded / v) asFloat
or if Fraction already answers a Float:
round: numberOfWishedDecimal
| maxNumberOfDecimals |
maxNumberOfDecimals := self class precision - 1 - (self
exponent max: self class emin).
maxNumberOfDecimals < numberOfWishedDecimal ifTrue: [^self].
^ self asFraction round: numberOfWishedDecimal
It's slower than current implementation, but will round exactly
to the nearest Float.
It's possible to have faster implementation up to 22 decimals if
you provide a fused-multiply-accumulate primitive...
