# Problem using set with hash objects **URL:** \ **Category:** Ruby\ **Created:** [September 17, 2007, 10:07pm UTC](https://www.ruby-forum.com/t/problem-using-set-with-hash-objects/114831 "2007-09-17T22:07:44Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![stepheneb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/71e660/32.png) [@stepheneb](https://www.ruby-forum.com/u/stepheneb)\ **Post date:** [September 17, 2007, 10:07pm UTC](https://www.ruby-forum.com/t/problem-using-set-with-hash-objects/114831/1 "2007-09-17T22:07:44Z")
I’m using set in the ruby standard library to produce collections of unique objects from enumerable objects with duplicates but it’s doesn’t appear to work with hash objects. $ ruby --version ruby 1.8.5 (2006-12-25 patchlevel 12) [i686-darwin8.9.1] $ irb irb(main):001:0\> require ‘set’ =\> true irb(main):002:0\> a = [1,1,2,3] =\> [1, 1, 2, 3] irb(main):003:0\> b = [{:a1 =\> “123”}, {:a1 =\> “123”}, {:b1 =\> “123”}] =\> [{:a1=\>“123”}, {:a1=\>“123”}, {:b1=\>“123”}] irb(main):004:0\> seta = a.to\_set =\> #\ irb(main):005:0\> setb = b.to\_set =\> #\“123”}, {:a1=\>“123”}, {:b1=\>“123”}}\> irb(main):006:0\> b[0] == b[1] =\> true Am I doing something wrong? ---
**Author:** ![stepheneb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/71e660/32.png) [@stepheneb](https://www.ruby-forum.com/u/stepheneb)\ **Post date:** [September 17, 2007, 10:29pm UTC](https://www.ruby-forum.com/t/problem-using-set-with-hash-objects/114831/2 "2007-09-17T22:29:51Z")
Alle lunedì 17 settembre 2007, Stephen B. ha scritto: > =\> [1, 1, 2, 3] > irb(main):003:0\> b = [{:a1 =\> “123”}, {:a1 =\> “123”}, {:b1 =\> “123”}] > =\> [{:a1=\>“123”}, {:a1=\>“123”}, {:b1=\>“123”}] > irb(main):004:0\> seta = a.to\_set > =\> #\ > irb(main):005:0\> setb = b.to\_set > =\> #\“123”}, {:a1=\>“123”}, {:b1=\>“123”}}\> > irb(main):006:0\> b[0] == b[1] > =\> true > > Am I doing something wrong? According to the ri documentation, Set internally stores items in a hash. Because of this, it uses the eql? and hash methods, and not ==, to test objects for equality. Hash#eql? (actually, Kernel#eql?) only returns true if two objects are the same object. Since b[0] and b[1] are different objects, Set considers them not equal, and thus stores them both. If you put the same hash in two places of the array you convert to a set, only one of them will be kept: irb: 001\> require ‘set’ true irb: 002\> h = {‘a’ =\> 1, ‘b’ =\> 2} {“a”=\>1, “b”=\>2} irb: 003\> a = [h, {‘c’ =\> 3}, h] [{“a”=\>1, “b”=\>2}, {“c”=\>3}, {“a”=\>1, “b”=\>2}] irb: 004\> a.to\_set.size 2 irb: 005\> p a.to\_set #\1, “b”=\>2}, {“c”=\>3}}\> Other classes, instead, provide their own definition of eql?, which leads to different (often less surprising) results. For instance, Array#eql? returns true if the two arrays have the same elements. String do the same. I hope this helps Stefano ---
**Author:** ![stepheneb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/71e660/32.png) [@stepheneb](https://www.ruby-forum.com/u/stepheneb)\ **Post date:** [September 25, 2007, 11:05pm UTC](https://www.ruby-forum.com/t/problem-using-set-with-hash-objects/114831/3 "2007-09-25T23:05:00Z")
On 17.09.2007 22:20, Jano S. wrote: > > irb(main):002:0\> a = [1,1,2,3] > > Am I doing something wrong? > > Your problem is in hash comparison. Set is internally using hash as > implementation (values are keys in the hash). So in order to obtain > uniqueness of the values, you need to define proper Hash#eq (IIRC). > The default one is comparing object ids - i.e. two totally equivalent > hashes are considered different. > > To make things even worse, hash calls dup when creating a new key to > avoid someone else changing the object. So even if you insert the same > object more times, it will be added each time (resp. its copy). This is only true for Strings - and even then only if the key is not frozen. > For more information search the archives for something like “hash key > dup”. There were recent (as in last two months) threads that discussed > this. Yeah, and “Hash as Hash key” is probably another helpful search phrase. This topic comes up from time to time. Kind regards robert ---
**Author:** ![stepheneb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/71e660/32.png) [@stepheneb](https://www.ruby-forum.com/u/stepheneb)\ **Post date:** [September 25, 2007, 11:05pm UTC](https://www.ruby-forum.com/t/problem-using-set-with-hash-objects/114831/4 "2007-09-25T23:05:09Z")
Ribert, Stefano, and Jano, thanks for the pointers. I’m now using “ara.t.howard” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)’s arrayfields gem to get hash like access to my data structures stored in arrays. This combines well with set. Here’s an example: $ sudo gem install arrayfields $ cat a.rb require ‘set’ require ‘arrayfields’ abc = Array.struct :a, :b, :c a = abc.new [1,2,3] # =\> [1, 2, 3] b = abc.new [1,2,3] # =\> [1, 2, 3] c = abc.new [4,5,6] # =\> [4, 5, 6] p a[:a] # =\> 1 p c[:a] # =\> 4 p a1 = [a,b,c] # =\> [[1, 2, 3], [1, 2, 3], [4, 5, 6]] p b1 = a1.to\_set.to\_a # =\> [[1, 2, 3], [4, 5, 6]] p b1[0][:a] # =\> 1 p b1[1][:c] # =\> 6 $ ruby a.rb 1 4 [[1, 2, 3], [1, 2, 3], [4, 5, 6]] [[1, 2, 3], [4, 5, 6]] 1 6 ---
**Author:** ![stepheneb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/71e660/32.png) [@stepheneb](https://www.ruby-forum.com/u/stepheneb)\ **Post date:** [September 25, 2007, 11:05pm UTC](https://www.ruby-forum.com/t/problem-using-set-with-hash-objects/114831/5 "2007-09-25T23:05:50Z")
On 9/17/07, Stephen B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > =\> [1, 1, 2, 3] > irb(main):003:0\> b = [{:a1 =\> “123”}, {:a1 =\> “123”}, {:b1 =\> “123”}] > =\> [{:a1=\>“123”}, {:a1=\>“123”}, {:b1=\>“123”}] > irb(main):004:0\> seta = a.to\_set > =\> #\ > irb(main):005:0\> setb = b.to\_set > =\> #\“123”}, {:a1=\>“123”}, {:b1=\>“123”}}\> > irb(main):006:0\> b[0] == b[1] > =\> true > > Am I doing something wrong? Your problem is in hash comparison. Set is internally using hash as implementation (values are keys in the hash). So in order to obtain uniqueness of the values, you need to define proper Hash#eq (IIRC). The default one is comparing object ids - i.e. two totally equivalent hashes are considered different. To make things even worse, hash calls dup when creating a new key to avoid someone else changing the object. So even if you insert the same object more times, it will be added each time (resp. its copy). For more information search the archives for something like “hash key dup”. There were recent (as in last two months) threads that discussed this.