[Pharo-project] SIXX problem for ScaledDecimal

Dario Trussardi dario.trussardi at tiscali.it
Fri May 20 12:32:00 EDT 2011


Il giorno 20/mag/2011, alle ore 16.57, Ralph Boland ha scritto:

>> From: Chris Cunningham <cunningham.cb at gmail.com>
>> Subject: Re: [Pharo-project] SIXX problem for ScaledDecimal
>> To: Pharo-project at lists.gforge.inria.fr
>> Message-ID: <BANLkTinP7E_oh2Fi2=axmra6JsoXVJYWkg at mail.gmail.com>
>> Content-Type: text/plain; charset=ISO-8859-1
> 
>> For ScaledDecimal, SIXX should definite store it as the underlying
>> fraction.  Storing it in the printed fashion does change the value of
>> the ScaledDecimal.
> 
>> (1/3) asScaledDecimal: 2  gives  0.33s2
>> and
>> (33/100) asScaledDecimal: 2  gives  0.33s2
>> yet
>> 0.33s2 ~= ( (1/3) asScaledDecimal: 2 )
>> and
>> ( (1/3) asScaledDecimal: 2 ) ~= ( (33/100) asScaledDecimal: 2 )
> 
>> In other words, the ScaledDecimal is about the precise internal number
>> and the scale to display it - which is NOT the scale that it is stored
>> as.
> 
>> -Chris
> 
> I don't agree.  ScaledDecimal would be used where consistency is often
> more important than accuracy, e.g. in Financial calculations where being off by
> a few pennies doesn't matter but always getting the same answer matters a LOT.
> Once a number is converted to a ScaledDecimal computations must always
> produce the same result assuming enough digits.  To ensure this asScaledDecimal
> must produce a scaled decimal for use in computations and, as such, storing
> a fraction internally is unacceptable.
> From a financial point of view  1/3  * 3  is not 1!
> From a programmers point of view asScaledDecimal changes the value
> in  much the same way asInteger does for a float.
> 
> Having said all of this allow me to contradict myself.  It seems to me that
> a scaled decimal should in general store more digits than are displayed.
> For example money might be stored with a large (possibly arbitrary) number
> of digits after the decimal point but only display two digits beyond the decimal
> point.
> I don't know how ScaledDecimal is implemented but I would be inclined to
> use arbitrary or large size integers and store the location of the decimal point
> internally recomputing the position of the decimal point after each calculation.
> This is an expensive way of doing computations but I expect users of
> ScaledDecimal can live with that.  When converting non decimal results to
> ScaledDecimal then how many digits of precision to maintain must be specified
> somehow.
> Dealing with ScaledDecimal is a complicated matter and I have probably failed
> to appreciate many of the complications but whoever is implementing
> ScaledDecimal
> (and Smalltalk needs ScaledDecimal) needs to appreciate those complications.
> Anybody here a financial expert?
> 
> Ralph Boland
> 

I think ScaledDecimal for financial data.

Where rounded is do only for the last of the operations. ( the total of the document for example or when i command it  )

I do : 

( ((1/3 asScaledDecimal:5 ) + (1/3 asScaledDecimal:5 )) * (6 asScaledDecimal:5) ) 


The result 	4 	is not right .


2/3 * 6   when  management 5 decimal are :

	0,66666 * 6,00000 =  3,99996

and not  fraction	 	2/3 * 6/1   -> 	 12 / 3 	- >  4 


I'm surprised about it.

Any idea ?

Dario





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