# Incorrect -0.0.hash - Ruby bug **URL:** \ **Category:** Ruby\ **Created:** [July 22, 2009, 6:24pm UTC](https://www.ruby-forum.com/t/incorrect-0-0-hash-ruby-bug/172514 "2009-07-22T18:24:21Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![tbaustert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ecae2f/32.png) [@tbaustert](https://www.ruby-forum.com/u/tbaustert)\ **Post date:** [July 22, 2009, 6:24pm UTC](https://www.ruby-forum.com/t/incorrect-0-0-hash-ruby-bug/172514/1 "2009-07-22T18:24:21Z")
Hello. I just found a bug in Ruby, version 1.9.1p0, Windows. In the rdoc to Object#hash we read ([class Object - RDoc Documentation](http://ruby-doc.org/core/classes/Object.html#M000337)): “This function must have the property that a.eql?(b) implies a.hash == b.hash.” Now see: irb(main):214:0\> z=0.0 =\> 0.0 irb(main):215:0\> nz=-0.0 =\> -0.0 irb(main):216:0\> z.eql?(nz) =\> true irb(main):217:0\> z.hash =\> 1041500564 irb(main):218:0\> nz.hash =\> -434138511 So the class Float does not fully follow the #hash contract. Well, isn’t that strange? TPR. ---
**Author:** ![tbaustert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ecae2f/32.png) [@tbaustert](https://www.ruby-forum.com/u/tbaustert)\ **Post date:** [July 22, 2009, 6:42pm UTC](https://www.ruby-forum.com/t/incorrect-0-0-hash-ruby-bug/172514/2 "2009-07-22T18:42:58Z")
Morning Thomas, On Wed, Jul 22, 2009 at 9:24 AM, Thomas B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hello. I just found a bug in Ruby, version 1.9.1p0, Windows. > > In the rdoc to Object#hash we read > ([class Object - RDoc Documentation](http://ruby-doc.org/core/classes/Object.html#M000337)): > > “This function must have the property that a.eql?(b) implies a.hash == > b.hash.” Actually Float overrides both #eql? and #hash so the contract is not applicable inside that class. Is that the best option in the world?? - probably not - but that would make it a non bug in theory. John ---
**Author:** ![tbaustert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ecae2f/32.png) [@tbaustert](https://www.ruby-forum.com/u/tbaustert)\ **Post date:** [July 22, 2009, 6:54pm UTC](https://www.ruby-forum.com/t/incorrect-0-0-hash-ruby-bug/172514/3 "2009-07-22T18:54:06Z")
2009/7/22 John W Higgins [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > > b.hash." > > Actually Float overrides both #eql? and #hash so the contract is not > applicable inside that class. Is that the best option in the world?? - > probably not - but that would make it a non bug in theory. Is there a reason for this? Perhaps the inexact nature of Floats means you ought not to use them as hash keys? ---
**Author:** ![tbaustert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ecae2f/32.png) [@tbaustert](https://www.ruby-forum.com/u/tbaustert)\ **Post date:** [July 22, 2009, 6:58pm UTC](https://www.ruby-forum.com/t/incorrect-0-0-hash-ruby-bug/172514/4 "2009-07-22T18:58:13Z")
On 7/22/09, John W Higgins [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > “This function must have the property that a.eql?(b) implies a.hash == > > b.hash.” My understanding is that this relationship shall be kept for the purpose of correct hash key behavior 553/60 \> ruby -v -e ‘p( {0.0 =\> 42}[-0.0] )’ ruby 1.9.1p243 (2009-07-16 revision 24175) [i686-linux] nil which is not nice. That said floats as hash keys are a bad idea of course because of equality is not very well defined anyway. Cheers Robert > Actually Float overrides both #eql? and #hash so the contract is not > applicable inside that class. Is that the best option in the world?? - > probably not - but that would make it a non bug in theory. > > John – Toutes les grandes personnes ont d’abord été des enfants, mais peu d’entre elles s’en souviennent. All adults have been children first, but not many remember. [Antoine de Saint-Exupéry] ---
**Author:** ![tbaustert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ecae2f/32.png) [@tbaustert](https://www.ruby-forum.com/u/tbaustert)\ **Post date:** [July 22, 2009, 7:02pm UTC](https://www.ruby-forum.com/t/incorrect-0-0-hash-ruby-bug/172514/5 "2009-07-22T19:02:57Z")
James C. wrote: > Is there a reason for this? Perhaps the inexact nature of Floats means > you > ought not to use them as hash keys? In my program it’s very convenient to use float vectors as keys because I create a graph where points (vertices) are calculated numerically as cross points of lines and so on (that’s why I get -0.0 from time to time). I round the floats to 8 decimal digits, and added a quick fix for the hash: alias :hash\_orig :hash def hash eql?(0.0) ? 0.0.hash\_orig : hash\_orig end and it works like it should, so I think it was a good idea to use them as keys 🙂 ---
**Author:** ![tbaustert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ecae2f/32.png) [@tbaustert](https://www.ruby-forum.com/u/tbaustert)\ **Post date:** [July 22, 2009, 7:25pm UTC](https://www.ruby-forum.com/t/incorrect-0-0-hash-ruby-bug/172514/6 "2009-07-22T19:25:44Z")
> Actually Float overrides both #eql? and #hash so the contract is not > applicable inside that class. Is that the best option in the world?? - > probably not - but that would make it a non bug in theory. I’d assume it overrides both methods in order to fulfill the contract not to break it. Contract are there so that people can rely on it. Breaking it is an error.