[Pharo-project] About float: new chapter looking for reader and contributors

Nicolas Cellier nicolas.cellier.aka.nice at gmail.com
Thu Apr 14 11:31:53 CEST 2011


I created a git account last night but must install latex on my mac.
I don't have your rythm :)

Nicolas

2011/4/14 Stéphane Ducasse <stephane.ducasse at inria.fr>:
> Nicolas
>
> why don't you grab the latex file and edit it?
> because I do not know anything about float.
>
> Stef
>
>
>
>
>
> On Apr 13, 2011, at 10:37 PM, Nicolas Cellier wrote:
>
>> Here is a small variation. It's not perfect, and most elements are
>> already in Stephane's text.
>> But maybe we can jump deeper in the first example that claims some explanations.
>>
>> We can first explain that 0.1 will answer the Float nearest to (1/10).
>> Thanks to this important property, if we enter 0.1, it will be printed
>> 0.1 and reinterpreted the same. A nice property to have!
>>
>> However, what is fundamental is that this Float won't be equal to (1/10).
>> How is this so ? Hey we didn't learn that in school !
>> Well, Float have an internal representation in base 2, of the form
>> mantissa * (2 raisedTo: exponent).
>> There is no way to represent (1/10) in base 2 with a finite number of
>> bits...  (like illustrated with 1.3 example, #significandAsInteger,
>> #exponent, #ulp and such kind of messages...).
>> A rounding operation has to occur to make this number fit in Float
>> internal representation ( it might be good to tell that Pharo Float
>> follow the IEEE 754 double precision standard with a link to
>> http://en.wikipedia.org/wiki/IEEE_754-1985 . This standard governs the
>> way most FPU work on this planet).
>> This rounding operation introduce a rounding error, and thus we don't
>> get exactly (1/10).
>>
>> To know the exact internal representation of 0.1 the Smalltalk way is
>> as usual to send a message: 0.1 asTrueFraction.
>> The same goes with 0.2, it's not equal to (2/10), check it with 0.2
>> asTrueFraction.
>>
>> So what goes on with this operation 0.1+0.2 ?
>> In fact you asked the system to perform following operation (0.1
>> asTrueFraction+0.2 asTrueFraction) asFloat.
>> (0.1 asTrueFraction+0.2 asTrueFraction) is an exact operation, but the
>> result again does not fit in Float internal representation:
>> (0.1 asTrueFraction+0.2 asTrueFraction) numerator highBit -> 54
>> (0.1 asTrueFraction+0.2 asTrueFraction) numerator lowBit -> 1
>> This means that 54 bits of mantissa are required to represent this
>> number. Float only have 53
>> Float precision -> 53
>> So converting it to aFloat requires another rounding operation, and
>> due to this rounding error,
>> (0.1 asTrueFraction+0.2 asTrueFraction) ~= (0.1 asTrueFraction+0.2
>> asTrueFraction) asFloat.
>> That's exactly how your IEEE-754 compliant FPU will work.
>>
>> In the whole process, 3 rounding errors have been cumulated:
>> (1/10) asFloat -> 0.1 (the error is not yet visible, but it's here already)
>> (2/10) asFloat -> 0.2 (same)
>> (0.1 asTrueFraction+0.2 asTrueFraction) asFloat -> 0.30000000000000004
>>
>> This is to be compared to 0.3, which has a single rounding error:
>> (3/10) asFloat -> 0.3
>>
>> Due to these rounding errors, and the way they cumulate or annihilate,
>> both result might differ. And in this case, they differ.
>> This is a very important property of Float. Float are fast thanks to
>> hardware acceleration, and thanks to these rounding errors that keep
>> the number of significant bits from growing. But the counterpart is
>> that operations on Float are inexact. Due to this inexactness, it is
>> most often inadequate to rely on strict equality of Float, as this
>> very simple example illustrates. Instead, Float comparisons shall
>> better be tested with fuzzy tolerance.
>>
>> Note that the nice thing with Pharo is that you immediately see that
>> (0.1+0.2) ~= 0.3 thanks to extraneous digits printed.
>> Pharo does not hide the horrible truth by shortening the printed
>> representation to 0.3 like some system do.
>> However, the decimal printed form does not show you the truth about
>> 0.1 ~= (1/10). Inexactness is hidden. This is an implicit knowledge to
>> remember about Float, that you now learned with this lesson.
>>
>> Remember, this is not a deficiency of Pharo. This is the way Float are
>> expected to work whatever the language.
>>
>> Nicolas
>>
>> 2011/4/13 laurent laffont <laurent.laffont at gmail.com>:
>>>
>>>
>>>
>>> On Tue, Apr 12, 2011 at 6:07 PM, stephane ducasse <stephane.ducasse at free.fr>
>>> wrote:
>>>>
>>>> Hi guys
>>>>
>>>> I started to put in shape a chapter on floats based on the numerous
>>>> discussions we got over the years.
>>>> You can find it on github under the pharo by example book/Floats
>>>
>>>
>>> For me it's a good candidate for a ProfStef tutorial (text, evaluations,
>>> text, ...)
>>> Laurent
>>>
>>>>
>>>> Stef
>>>>
>>>
>>>
>>
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>
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