Le Mardi 01 Août 2006 06:46, Dave Stevenson a écrit :
>
> AR 51045 FixedPoint should implement #storeOn:
>
Thanks for creating the AR
> Number>>storeOn: simply sends printOn:, which is inappropriate for
> FixedPoint since the number read back is not equivalent to the original.
>
> For example:
> | stream |
>
> stream := String new writeStream.
> 0.5s1 squared storeOn: stream.
> (Object readFromString: stream contents) asRational printString
>
> answer: '(3/10)'
> original: '(1/4)'
>
> They still print the same (0.3s), but they are now different. May not
> seem like much, but after all, 5% is 5%, and it adds up after a while...
>
> Perhaps the example above should store itself as '0.25s1' to preserve
> precision.
>
That would be all right if you can make sure your fraction has only powers of
2 and 5 at denominator, what happens when you create a FixedPoint and deal
only with (* = -). Then there is an exact decimal representation (in a finite
number of digits, possibly many).
But if you mix arithmetic with other numbers, especially Fraction, or use
FixedPoint division, then you can no more have an exact representation in a
finite number of decimals.
try (1/3) * (0.5s1)... or (0.5s1) / (0.3s1)
Either you store with some kind of message send (fraction asFixedPoint:...)
Or you loose exactness at load/store operation (whatever the arbitrary number
of digits over scale you decide).
It depends on application side... Maybe second solution is sufficient for
majority of financial apps, but not for a symbolic algebra framework.
Maybe storeOn: message itself is limited and a little deprecated for a serious
load/store implementation...
Nicolas
Locked
