# Hash & keys **URL:** \ **Category:** Ruby\ **Created:** [October 13, 2006, 2:05am UTC](https://www.ruby-forum.com/t/hash-keys/71982 "2006-10-13T02:05:57Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![sebw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e56c9b/32.png) [@sebw](https://www.ruby-forum.com/u/sebw)\ **Post date:** [October 13, 2006, 2:05am UTC](https://www.ruby-forum.com/t/hash-keys/71982/1 "2006-10-13T02:05:57Z")
I’m having some trouble understanding how hashes work in Ruby. Consider the following test : irb(main):002:0\> v=Vector[4,5] =\> Vector[4, 5] irb(main):003:0\> v2=Vector[4,5] =\> Vector[4, 5] irb(main):004:0\> v==v2 =\> true irb(main):005:0\> v.hash =\> 17 irb(main):006:0\> v2.hash =\> 17 irb(main):007:0\> h=Hash.new =\> {} irb(main):008:0\> h[v]=“whatever is in (4,5)” =\> “whatever is in (4,5)” irb(main):009:0\> h[v2] =\> nil I’d have expected h[v2] =\> “whatever is in (4,5)”, since v==v2 and both have the same hash value… what am I doing wrong? ---
**Author:** ![sebw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e56c9b/32.png) [@sebw](https://www.ruby-forum.com/u/sebw)\ **Post date:** [October 13, 2006, 2:28am UTC](https://www.ruby-forum.com/t/hash-keys/71982/2 "2006-10-13T02:28:20Z")
Sébastien Wautelet wrote: > I’d have expected h[v2] =\> “whatever is in (4,5)”, since v==v2 and both > have the same hash value… what am I doing wrong? Hash keys can be any object. So your hash, h, has one key, the object v. Take another hash real quick, we’ll call it hash. hash = Hash.new hash[‘rabbit’] = ‘cute’ # ‘cute’ hash[‘blue’] # nil The value for any hash key not defined is nil (unless you explicitly tell the hash during instantiation to use another value). So just because you’re using the value of v as a key in your hash, does not mean that v2 is automatically another key in your hash. ---
**Author:** ![sebw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e56c9b/32.png) [@sebw](https://www.ruby-forum.com/u/sebw)\ **Post date:** [October 13, 2006, 2:36am UTC](https://www.ruby-forum.com/t/hash-keys/71982/3 "2006-10-13T02:36:02Z")
I think you’re getting hung up on this part: Sébastien Wautelet wrote: > irb(main):004:0\> v==v2 > =\> true Yes, v and v2 are equal. But that is irrelevant with regards to your hash. Now, if you had to objects that were identical (not equal, but identical) then it would work: s1 = ‘string’ s2 = s1 h = Hash.new h[s1] # ‘string’ h[s2] # ‘string’ s1.upcase! h[s2] # ‘STRING’ h[s1] # ‘STRING’ Hope that helps. ---
**Author:** ![sebw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e56c9b/32.png) [@sebw](https://www.ruby-forum.com/u/sebw)\ **Post date:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/hash-keys/71982/4 "2006-10-18T10:39:55Z")
Timothy H. wrote: > > irb(main):005:0\> v.hash > > I’d have expected h[v2] =\> “whatever is in (4,5)”, since v==v2 and > > both have the same hash value… what am I doing wrong? > > It’s not enough that they have the same hash value or that obj1 == > obj2. For the purposes of Hash, two objects are the same if > obj1.eql?(obj2). Does v1.eql?(v2)? Testing reveals that they don’t. but that strikes me as a bug in the Vector class. It violates the guidelines given in the PickAxe book (bottom of page 568), and more importantly, it violates common intuition. If two vectors that are created in exactly the same manner aren’t eql?, they’re useless for use as hash keys. I’d say file a bug report, or at least a problem, on this one. Cheers, Ken ---
**Author:** ![sebw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e56c9b/32.png) [@sebw](https://www.ruby-forum.com/u/sebw)\ **Post date:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/hash-keys/71982/5 "2006-10-18T10:39:36Z")
Sébastien Wautelet wrote: > =\> 17 > have the same hash value… what am I doing wrong? It’s not enough that they have the same hash value or that obj1 == obj2. For the purposes of Hash, two objects are the same if obj1.eql?(obj2). Does v1.eql?(v2)? ---
**Author:** ![sebw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e56c9b/32.png) [@sebw](https://www.ruby-forum.com/u/sebw)\ **Post date:** [October 18, 2006, 10:39am UTC](https://www.ruby-forum.com/t/hash-keys/71982/6 "2006-10-18T10:39:59Z")
Daniel W. wrote: > identical) then it would work: > I think it is relevant, given that the Vector class is designed to be > immutable. I would agree that the Vector class should probably define > #eql? as an alias for #==. ---
**Author:** ![sebw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e56c9b/32.png) [@sebw](https://www.ruby-forum.com/u/sebw)\ **Post date:** [October 18, 2006, 10:44am UTC](https://www.ruby-forum.com/t/hash-keys/71982/7 "2006-10-18T10:44:00Z")
On 10/13/06, Phrogz [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I think it is relevant, given that the Vector class is designed to be > immutable. I would agree that the Vector class should probably define > #eql? as an alias for #==. It appears to be a typo in Matrix.rb - This is in ruby1.8.4: class Vector … # # Returns +true+ iff the two vectors have the same elements in the same order. # def ==(other) return false unless Vector === other ``` other.compare_by(@elements) ``` end alias eqn? == ~~~ And Vector seems to be the only implementor of eqn? ## rick@frodo:~/ruby-1.8.4$ ri eqn ------------------------------------------------------------ Vector#eqn? eqn?(other) ``` Alias for #== ``` And it’s not yet fixed in 1.9 rick@frodo:~/ruby-1.8.4$ grep eqn /public/rubysource/ruby1.9/ruby/lib/matrix.rb alias eqn? == – Rick DeNatale My blog on Ruby [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)