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comp.lang.ruby

Bug in BigDecimal#round ?

Ryan Platte

9/26/2007 10:28:00 PM

A coworker discovered some disturbing behavior in BigDecimal#round.
The output I get from the program below is:

===== begin output

These values appended to '0.0000' cause BigDecimal#round to return
nonzero:
[5, 6, 7, 8, 9, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,
63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,
80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
97, 98, 99]

These numbers of zeroes between the decimal point and a 7 cause
BigDecimal#round to return nonzero:
[4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 64, 68, 72,
76, 80, 84, 88, 92, 96, 100]

===== end output

===== begin code

require 'bigdecimal'

vary_number_after_four_zeroes = (1..100).map { |i|
bd = BigDecimal.new("0.0000#{i}"); [i, bd.round.to_s]
}
seed_values_triggering_misbehavior_with_four_zeroes =
vary_number_after_four_zeroes.reject {|i, bds| bds == '0.0'}.map {|
i, bds| i}

vary_number_of_zeroes = (1..100).map { |i|
bd = BigDecimal.new("0.#{'0' * i}7"); [i, bd.round.to_s]
}
number_of_zeroes_triggering_misbehavior =
vary_number_of_zeroes.reject {|i, bds| bds == '0.0'}.map {|i, bds|
i}

puts "These values appended to '0.0000' cause BigDecimal#round to
return nonzero:"
p seed_values_triggering_misbehavior_with_four_zeroes
puts
puts "These numbers of zeroes between the decimal point and a 7 cause
BigDecimal#round to return nonzero:"
p number_of_zeroes_triggering_misbehavior

===== end code

I get identical reports on Linux and Windows versions of Ruby:

ruby 1.8.6 (2007-03-13 patchlevel 0) [i686-linux]

ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]

We would be grateful for a pointer to a patch that fixes this issue if
one is available.
7 Answers

Jano Svitok

9/27/2007 1:47:00 AM

0

On 9/27/07, Ryan Platte <ryan.platte@gmail.com> wrote:
> A coworker discovered some disturbing behavior in BigDecimal#round.
> The output I get from the program below is:
>
> ===== begin output
>
> These values appended to '0.0000' cause BigDecimal#round to return
> nonzero:
> [5, 6, 7, 8, 9, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,
> 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,
> 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
> 97, 98, 99]
>
> These numbers of zeroes between the decimal point and a 7 cause
> BigDecimal#round to return nonzero:
> [4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 64, 68, 72,
> 76, 80, 84, 88, 92, 96, 100]
>
> ===== end output
>
> ===== begin code
>
> require 'bigdecimal'
>
> vary_number_after_four_zeroes = (1..100).map { |i|
> bd = BigDecimal.new("0.0000#{i}"); [i, bd.round.to_s]
> }
> seed_values_triggering_misbehavior_with_four_zeroes =
> vary_number_after_four_zeroes.reject {|i, bds| bds == '0.0'}.map {|
> i, bds| i}
>
> vary_number_of_zeroes = (1..100).map { |i|
> bd = BigDecimal.new("0.#{'0' * i}7"); [i, bd.round.to_s]
> }
> number_of_zeroes_triggering_misbehavior =
> vary_number_of_zeroes.reject {|i, bds| bds == '0.0'}.map {|i, bds|
> i}
>
> puts "These values appended to '0.0000' cause BigDecimal#round to
> return nonzero:"
> p seed_values_triggering_misbehavior_with_four_zeroes
> puts
> puts "These numbers of zeroes between the decimal point and a 7 cause
> BigDecimal#round to return nonzero:"
> p number_of_zeroes_triggering_misbehavior
>
> ===== end code
>
> I get identical reports on Linux and Windows versions of Ruby:
>
> ruby 1.8.6 (2007-03-13 patchlevel 0) [i686-linux]
>
> ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]
>
> We would be grateful for a pointer to a patch that fixes this issue if
> one is available.

It seems to me as a bug. The code in trunk is the same, so I guess
it's not fixed yet.
Please file a bug at
http://rubyforge.org/tracker/?func=add&group_id=426&...
(rubyforge.org, project ruby)

The problem is most probably in ext/bigdecimal/bigdecimal.c, VpMidRound().

As you have probably noticed, it happens when 1. the number of leading
zeros is divisible by 4 (BASE_FIG, the number of decimal digits stored
in one U_LONG), and the next digit is >= 5 (I guess due to selected
rounding mode)

That means, the problematic numbers have format 0.XE-Y where X in
[5..9] and Y%4 == 0

It's even visible in
BigDecimal.new("0.5").round(-4).to_i # => 10000
BigDecimal.new("0.000000005").round(4) #=> 0.1E-3 == 0.0001
BigDecimal.new("0.000000005").round(-4) #=> 0.1E5 == 10000

Jano Svitok

9/27/2007 11:26:00 AM

0

On 9/27/07, Jano Svitok <jan.svitok@gmail.com> wrote:
> On 9/27/07, Ryan Platte <ryan.platte@gmail.com> wrote:
> > A coworker discovered some disturbing behavior in BigDecimal#round.
> > The output I get from the program below is:
> >
> > ===== begin output
> >
> > These values appended to '0.0000' cause BigDecimal#round to return
> > nonzero:
> > [5, 6, 7, 8, 9, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,
> > 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,
> > 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
> > 97, 98, 99]
> >
> > These numbers of zeroes between the decimal point and a 7 cause
> > BigDecimal#round to return nonzero:
> > [4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 64, 68, 72,
> > 76, 80, 84, 88, 92, 96, 100]
> >
> > ===== end output
> >
> > ===== begin code
> >
> > require 'bigdecimal'
> >
> > vary_number_after_four_zeroes = (1..100).map { |i|
> > bd = BigDecimal.new("0.0000#{i}"); [i, bd.round.to_s]
> > }
> > seed_values_triggering_misbehavior_with_four_zeroes =
> > vary_number_after_four_zeroes.reject {|i, bds| bds == '0.0'}.map {|
> > i, bds| i}
> >
> > vary_number_of_zeroes = (1..100).map { |i|
> > bd = BigDecimal.new("0.#{'0' * i}7"); [i, bd.round.to_s]
> > }
> > number_of_zeroes_triggering_misbehavior =
> > vary_number_of_zeroes.reject {|i, bds| bds == '0.0'}.map {|i, bds|
> > i}
> >
> > puts "These values appended to '0.0000' cause BigDecimal#round to
> > return nonzero:"
> > p seed_values_triggering_misbehavior_with_four_zeroes
> > puts
> > puts "These numbers of zeroes between the decimal point and a 7 cause
> > BigDecimal#round to return nonzero:"
> > p number_of_zeroes_triggering_misbehavior
> >
> > ===== end code
> >
> > I get identical reports on Linux and Windows versions of Ruby:
> >
> > ruby 1.8.6 (2007-03-13 patchlevel 0) [i686-linux]
> >
> > ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]
> >
> > We would be grateful for a pointer to a patch that fixes this issue if
> > one is available.
>
> It seems to me as a bug. The code in trunk is the same, so I guess
> it's not fixed yet.
> Please file a bug at
> http://rubyforge.org/tracker/?func=add&group_id=426&...
> (rubyforge.org, project ruby)
>
> The problem is most probably in ext/bigdecimal/bigdecimal.c, VpMidRound().
>
> As you have probably noticed, it happens when 1. the number of leading
> zeros is divisible by 4 (BASE_FIG, the number of decimal digits stored
> in one U_LONG), and the next digit is >= 5 (I guess due to selected
> rounding mode)
>
> That means, the problematic numbers have format 0.XE-Y where X in
> [5..9] and Y%4 == 0
>
> It's even visible in
> BigDecimal.new("0.5").round(-4).to_i # => 10000
> BigDecimal.new("0.000000005").round(4) #=> 0.1E-3 == 0.0001
> BigDecimal.new("0.000000005").round(-4) #=> 0.1E5 == 10000
>

reported as #14271

Ryan Platte

9/27/2007 1:15:00 PM

0

On Sep 27, 6:25 am, "Jano Svitok" <jan.svi...@gmail.com> wrote:
> > It seems to me as a bug. The code in trunk is the same, so I guess
> > it's not fixed yet.
>
> reported as #14271

Thank you very much for reporting the bug.

--
Ryan Platte

Jano Svitok

9/27/2007 1:47:00 PM

0

On 9/27/07, Ryan Platte <ryan.platte@gmail.com> wrote:
> On Sep 27, 6:25 am, "Jano Svitok" <jan.svi...@gmail.com> wrote:
> > > It seems to me as a bug. The code in trunk is the same, so I guess
> > > it's not fixed yet.
> >
> > reported as #14271
>
> Thank you very much for reporting the bug.

I guess you can workaround the bug until it gets properly fixed - by
opening the BigDecimal class. Last night I could not come with
anything reasonable though ;-)

class BigDecimal
alias_method :old_round, :round
def round(*args)
if XXX
self.class.new(YYY)
else
old_round(*args)
end
end
end

the problematic parts are XXX and YYY... ;-)
args[0] and self.exponent might be usable...

I haven't found any unit tests for BigDecimal, so if you create some,
please post them somewhere (I'm not sure where - possible choises are:
ruby tracker, http://rubyforge.org/proj... tracker, ruby-core
mailing list)

Jano

Wilson Bilkovich

9/27/2007 2:11:00 PM

0

On 9/27/07, Jano Svitok <jan.svitok@gmail.com> wrote:
> On 9/27/07, Ryan Platte <ryan.platte@gmail.com> wrote:
> > On Sep 27, 6:25 am, "Jano Svitok" <jan.svi...@gmail.com> wrote:
> > > > It seems to me as a bug. The code in trunk is the same, so I guess
> > > > it's not fixed yet.
> > >
> > > reported as #14271
> >
> > Thank you very much for reporting the bug.
>
> I guess you can workaround the bug until it gets properly fixed - by
> opening the BigDecimal class. Last night I could not come with
> anything reasonable though ;-)
>
> class BigDecimal
> alias_method :old_round, :round
> def round(*args)
> if XXX
> self.class.new(YYY)
> else
> old_round(*args)
> end
> end
> end
>
> the problematic parts are XXX and YYY... ;-)
> args[0] and self.exponent might be usable...
>
> I haven't found any unit tests for BigDecimal, so if you create some,
> please post them somewhere (I'm not sure where - possible choises are:
> ruby tracker, http://rubyforge.org/proj... tracker, ruby-core
> mailing list)
>

JRuby has some:
http://svn.codehaus.org/jruby/trunk/jruby/test/test_big_...

If you come up with some more, you could either submit them as a patch
there, or post them to ruby-core.

When we get around to BigDecimal in Rubinius, we will probably use the
JRuby test case as a starting point, and then fill in any gaps we
find. If you've posted some of them to ruby-core, that makes life
easier.

Shigeo Kobayashi

10/2/2007 3:50:00 AM

0

From: "Ryan Platte" <ryan.platte@gmail.com>

I haven't seen the bug yet in detail.
But I will fix it anyway.
Thank you for reporting.

shigeo@tinyforest.jp

Shigeo Kobayashi

10/9/2007 1:16:00 PM

0

Subject: Re: Bug in BigDecimal#round ?

I attached the fix as a patch file.
Could anyone(perhaps Matz ?) apply the patch ?

With a change log like:
Round method bug pointed by Ryan Platte fixed(Patch of the patch from
"NATORI Shin").

Thank you in advance.

Shigeo Kobayashi.