Forum: Ruby 2000-1992.2 - is this a known bug?

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S2 (Guest)
on 2009-03-07 01:05
(Received via mailing list)
ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]

>> 2000-1992.2
=> 7.79999999999995
Gregory B. (Guest)
on 2009-03-07 01:11
(Received via mailing list)
2009/3/6 S2 <removed_email_address@domain.invalid>:
> ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]
>
>>> 2000-1992.2
> => 7.79999999999995

http://docs.sun.com/source/806-3568/ncg_goldberg.html
Rimantas L. (Guest)
on 2009-03-07 01:12
(Received via mailing list)
>>> 2000-1992.2
> => 7.79999999999995

It is known non-bug:
http://docs.sun.com/source/806-3568/ncg_goldberg.html

Regards,
Rimantas
Christopher D. (Guest)
on 2009-03-07 05:44
(Received via mailing list)
2009/3/6 S2 <removed_email_address@domain.invalid>:
> ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]
>
>>> 2000-1992.2
> => 7.79999999999995
>
>

The use of inexact floating point numbers by default in Ruby (which
follows C and most, but not all, other programming languages) is a
design choice, not a bug. One might dispute whether or not its a good
design choice (personally, I prefer Scheme's numeric tower and exact
numbers by default with conversion to inexact numbers only when an
inexact operation is applied to an exact number or an explicit
designation of an inexact number is given), but its a point about
which people could debate endlessly, and since BigDecimal is at least
in the standard library, its not a matter of whether Ruby can do exact
math, but just how verbose the code is to do it.

ex:

require 'bigdecimal'
require 'bigdecimal/math'

class BigDecimal
  # BigDecimal.to_s is ugly
  def pretty
    digits, magnitude = to_s.split('.')[1].split('E')
    magnitude=magnitude.to_i
    digits.insert(0,'0'*[(1-magnitude),0].max)
    digits.insert([magnitude,1].min,'.')
  end
end

include BigMath

(2000-BigDecimal('1992.2')).pretty
=> "7.8"
Christopher D. (Guest)
on 2009-03-07 05:56
(Received via mailing list)
On Fri, Mar 6, 2009 at 7:42 PM, Christopher D. 
<removed_email_address@domain.invalid>
wrote:
> design choice, not a bug. One might dispute whether or not its a good
> require 'bigdecimal'
> end
>
> include BigMath
>
> (2000-BigDecimal('1992.2')).pretty
> => "7.8"
>
>

Or, while that works for the case presented, if you want it to work
right in general:

class BigDecimal
  def pretty
    return to_i.to_s if self==to_i
    digits, magnitude = to_s.split('.')[1].split('E')
    magnitude = magnitude.to_i
    digits.insert(0,'0'*[(1-magnitude),0].max)
    digits.insert([magnitude, 1].max,'.')
   end
end
Heesob P. (Guest)
on 2009-03-07 06:17
(Received via mailing list)
Hi,

2009/3/7 Christopher D. <removed_email_address@domain.invalid>:
> design choice, not a bug. One might dispute whether or not its a good
> require 'bigdecimal'
> end
>
> include BigMath
>
> (2000-BigDecimal('1992.2')).pretty
> => "7.8"
>
Why not use BigDecimal.to_s('F') ?

Refer to
http://www.ruby-doc.org/stdlib/libdoc/bigdecimal/r...


Regards,

Park H.
Christopher D. (Guest)
on 2009-03-07 06:28
(Received via mailing list)
On Fri, Mar 6, 2009 at 8:15 PM, Heesob P. <removed_email_address@domain.invalid> 
wrote:
>>
>>
>>    digits.insert(0,'0'*[(1-magnitude),0].max)
>
> Refer to 
http://www.ruby-doc.org/stdlib/libdoc/bigdecimal/r...

Because that would be sane?

No, seriouly, I knew there was a BigDecimal formatter somewhere that
I'd seen (and even used, though not recently), but I thought it was
external. Clearly, it's better to use that than to try to reinvent the
wheel.
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