[Glass] 10.12345 round: 2 => 10.12 ?

[Martin McClure]

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