[Glass] 10.12345 round: 2 => 10.12 ?

[John M McIntosh]

Buried in the Pharo rounding logic is an intent to round to a representation that is represented in the IEEE float values 

In squeak a decade back you could have float X = float constant Y 

But post the pharo rounding decision that = may fail because the constant is rounded.

The premise is you should not compare float numbers for exact equality



Sent from my iPhone

> On Nov 15, 2013, at 12:04 PM, Martin McClure <martin.mcclure at gemtalksystems.com> wrote:
> 
>> On 11/15/2013 08:44 AM, Martin McClure wrote:
>>> On 11/15/2013 06:20 AM, Mariano Martinez Peck wrote:
>>> Hi guys... mmmm In Pharo we have the message #round:. Here it is :
>>> 
>>> Float >> round: numberOfWishedDecimal
>>>         "only leave a fixed amount of decimal"
>>>         "10.12345 round: 2 => 10.12"
>>>         | v |
>>>         v := 10 raisedTo: numberOfWishedDecimal.
>>>         ^ ((self * v) rounded / v) asFloat
>>> 
>>> 
>>> I see in GemStone Float has #roundTo: but doesn't seem to answer the
>>> same...
>> 
>> From memory, #roundTo: is defined by ANSI to round to the nearest
>> multiple of the argument -- so nnn roundTo: 2 would round to the nearest
>> even number.
>> 
>> To round with two decimal places, you would use roundTo: 1/100.
> 
> Also note that 0.01 is not exactly representable as a float, so if you do this calculation on floats, you'll be getting some error, a larger error the larger your number is, since you'll be rounding to a multiple of a number that is not *quite* 1/100. And the result will probably not, by default, print to a nice abc.de result -- it might be something like 10.1199998724.
> 
> It may be better to convert the float to a ScaledDecimal with two fractional digits -- this will round precisely, and print nicely.
> 
> Or just use Fractions or ScaledDecimals in the first place and avoid floats altogether. Which is better depends on your application.
> 
> Regards,
> 
> -Martin
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