[Pharo-project] Unexpected behavior of roundTo:

Nicolas Cellier nicolas.cellier.aka.nice at gmail.com
Fri Apr 8 19:14:34 CEST 2011


Once more, Floats are inexact.
    0.01 ~= 0.01s2
The name #absPrintExactlyOn:base: is lying, it does not print exactly,
but it prints the shortest decimal representation than will be rounded
to the same Float when read back.

To print it exactly, you need to use #printShowingDecimalPlaces: indeed.
As every finite Float is a represented internally as a Fraction with a
denominator being a power of 2, every finite Float has a decimal
representation with a finite number of decimals digits (just multiply
numerator and denominator with adequate power of 5, and you'll get the
digits).

So try:
0.01 printShowingDecimalPlaces: 59
-> 0.01000000000000000020816681711721685132943093776702880859375

You see that even if you try to execute the operation without rounding
error, then convert it back to Float, you get the error:
 (2.8011416510246336 asTrueFraction roundTo: 0.01 asTrueFraction) asFloat
->  2.8000000000000003

When you perform the #roundTo: operations in Float inexact arithmetic,
you may accumulate more rounding errors, so the result may vary.

If you want to round to an exact hundredth, then use exact arithmetic and try:
   2.8011416510246336 roundTo: 0.01s2

Hope it helps to consider Float for what they are, inexact but fast.
Don't put too much expectations on them.

Nicolas

2011/4/8 Hilaire Fernandes <hilaire.fernandes at gmail.com>:
> It looks like a known issue:
>
> printShowingDecimalPlaces: placesDesired
>        "This implementation avoids any rounding error caused by rounded or
> roundTo: "
>        ^ self asTrueFraction printShowingDecimalPlaces: placesDesired
>
>
>
> Le 08/04/2011 18:47, Hilaire Fernandes a écrit :
>> 2.8011416510246336 roundTo: 0.01
>>
>> returns
>>
>>  2.8000000000000003
>>
>>
>> Tested with 1.2
>>
>>
>> Should I fill a ticket, I don't see anythings similar?
>>
>> Hilaire
>>
>
>
> --
> Education 0.2 -- http://blog.ofset.org/hilaire
>
>
>



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