# Bug in BigDecimal#round? **URL:** \ **Category:** Ruby\ **Created:** [September 27, 2007, 12:31am UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627 "2007-09-27T00:31:53Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![parrfolio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/47e85d/32.png) [@parrfolio](https://www.ruby-forum.com/u/parrfolio)\ **Post date:** [September 27, 2007, 12:31am UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627/1 "2007-09-27T00:31:53Z")
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. ---
**Author:** ![parrfolio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/47e85d/32.png) [@parrfolio](https://www.ruby-forum.com/u/parrfolio)\ **Post date:** [September 27, 2007, 3:47am UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627/2 "2007-09-27T03:47:45Z")
On 9/27/07, Ryan P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > 97, 98, 99] > require ‘bigdecimal’ > } > p number\_of\_zeroes\_triggering\_misbehavior > 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&atid=1698](http://rubyforge.org/tracker/?func=add&group_id=426&atid=1698) > ([rubyforge.org](http://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 ---
**Author:** ![parrfolio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/47e85d/32.png) [@parrfolio](https://www.ruby-forum.com/u/parrfolio)\ **Post date:** [September 27, 2007, 1:26pm UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627/3 "2007-09-27T13:26:11Z")
On 9/27/07, Jano S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, > > > > bd = BigDecimal.new(“0.#{‘0’ \* i}7”); [i, bd.round.to\_s] > > BigDecimal#round to return nonzero:" > > We would be grateful for a pointer to a patch that fixes this issue if > > As you have probably noticed, it happens when 1. the number of leading > > BigDecimal.new(“0.000000005”).round(-4) #=\> 0.1E5 == 10000 reported as #14271 ---
**Author:** ![parrfolio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/47e85d/32.png) [@parrfolio](https://www.ruby-forum.com/u/parrfolio)\ **Post date:** [September 27, 2007, 3:16pm UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627/4 "2007-09-27T15:16:21Z")
On Sep 27, 6:25 am, “Jano S.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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. ---
**Author:** ![parrfolio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/47e85d/32.png) [@parrfolio](https://www.ruby-forum.com/u/parrfolio)\ **Post date:** [September 27, 2007, 3:47pm UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627/5 "2007-09-27T15:47:31Z")
On 9/27/07, Ryan P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > On Sep 27, 6:25 am, “Jano S.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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/projects/bfts/](http://rubyforge.org/projects/bfts/) tracker, ruby-core mailing list) Jano ---
**Author:** ![parrfolio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/47e85d/32.png) [@parrfolio](https://www.ruby-forum.com/u/parrfolio)\ **Post date:** [September 27, 2007, 4:12pm UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627/6 "2007-09-27T16:12:11Z")
On 9/27/07, Jano S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > opening the BigDecimal class. Last night I could not come with > 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/projects/bfts/](http://rubyforge.org/projects/bfts/) tracker, ruby-core > mailing list) JRuby has some: [http://svn.codehaus.org/jruby/trunk/jruby/test/test\_big\_decimal.rb](http://svn.codehaus.org/jruby/trunk/jruby/test/test_big_decimal.rb) 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. ---
**Author:** ![parrfolio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/47e85d/32.png) [@parrfolio](https://www.ruby-forum.com/u/parrfolio)\ **Post date:** [October 2, 2007, 5:51am UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627/7 "2007-10-02T05:51:10Z")
From: “Ryan P.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) I haven’t seen the bug yet in detail. But I will fix it anyway. Thank you for reporting. removed\_email\_address@domain.invalid ---
**Author:** ![parrfolio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/47e85d/32.png) [@parrfolio](https://www.ruby-forum.com/u/parrfolio)\ **Post date:** [October 9, 2007, 3:16pm UTC](https://www.ruby-forum.com/t/bug-in-bigdecimal-round/115627/8 "2007-10-09T15:16:43Z")
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 P. fixed(Patch of the patch from “NATORI Shin”). Thank you in advance. Shigeo Kobayashi.