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## [#14271] BigDecimal(&amp;quot;0.00005&amp;quot;).round == 1

Date:
2007-09-27 11:21
Priority:
3
Submitted By:
Jano Svitok (janek)
Assigned To:
Akinori MUSHA (knu)
Category:
Math
State:
Open
Platform:

Summary:
BigDecimal(&amp;quot;0.00005&amp;quot;).round == 1

 Detailed description ```from ruby-talk:271159 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. ------------------------------ The problem is most probably in ext/bigdecimal/bigdecimal.c, VpMidRound(). 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 ```

### Followup

 Message ```Date: 2008-11-15 19:42 Sender: Roger Pack may want to check that the patch worked: http://redmine.ruby-lang.org/issues/show/740 Thanks!``` ```Date: 2008-11-11 01:28 Sender: Roger Pack created http://redmine.ruby-lang.org/issues/show/740``` ```Date: 2008-11-08 01:08 Sender: Roger Pack appears to work on 1.9, broken on 1.8 Could you submit a new copy to the new tracker: http://redmine.ruby-lang.org/ --this tracker is being shelved. Thanks! -=R ``` ```Date: 2008-03-05 22:17 Sender: Jano Svitok Patch for this issue is in http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/27336 0 I guess it was discussed in http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-dev/32659 -- I don't understand Japanese :(``` ```Date: 2007-09-30 19:53 Sender: Jano Svitok some BigDecimal tests can be found at http://svn.codehaus.org/jruby/trunk/jruby/test/test_big_decimal.r b```