# How to implement a hash whose key is another hash? **URL:** \ **Category:** Ruby\ **Created:** [August 20, 2007, 5:35pm UTC](https://www.ruby-forum.com/t/how-to-implement-a-hash-whose-key-is-another-hash/112527 "2007-08-20T17:35:39Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [August 20, 2007, 5:35pm UTC](https://www.ruby-forum.com/t/how-to-implement-a-hash-whose-key-is-another-hash/112527/1 "2007-08-20T17:35:39Z")
I understand that the key of a hash can be any object. Therefore, I would think that the key of a hash can be another hash. If I have the following code sequence. (I using ruby 1.8.5 (2006-08-25) [i486- linux]) require ‘pp’ test ={} test[“new”=\>“girl”]= 100 test[“new”=\>“girl”]= 200 pp test I am expecting the output to be {{“new”=\>“girl”}=\>200} However the output I am getting is {{“new”=\>“girl”}=\>100, {“new”=\>“girl”}=\>200} Could some explain how to use hash key which is of type hash ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [August 20, 2007, 6:24pm UTC](https://www.ruby-forum.com/t/how-to-implement-a-hash-whose-key-is-another-hash/112527/2 "2007-08-20T18:24:49Z")
unknown wrote: > Could some explain how to use hash key which is of type hash Perhaps studying this will explain your confusion. require ‘pp’ test ={} k1 = {“new”=\>“girl”} k2 = {“new”=\>“girl”} test[k1]= 100 test[k2]= 200 pp k1.eql?(k2) pp test test ={} test[k1]= 100 test[k1]= 200 pp test Note that each instance of “new”=\>“girl” is a different hash object. Basing the key on a hash does not look like a good idea in general. Consider what happens when you extend k1. k1 = {“new”=\>“girl”} test ={} k1[“bad”]=“idea” test[k1]= 100 k1[“bad”]=“idea” test[k1]= 200 pp test Keys should be stable objects. Ian W. ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [August 21, 2007, 2:29am UTC](https://www.ruby-forum.com/t/how-to-implement-a-hash-whose-key-is-another-hash/112527/3 "2007-08-21T02:29:27Z")
unknown wrote: > I understand that the key of a hash can be any object. Therefore, > I would think that the key of a hash can be another hash. If I have > the following code sequence. (I using ruby 1.8.5 (2006-08-25) [i486- linux]) > > require ‘pp’ > test ={} > test[“new”=\>“girl”]= 100 > test[“new”=\>“girl”]= 200 > pp test > > I am expecting the output to be > > {{“new”=\>“girl”}=\>200} > > However the output I am getting is > > {{“new”=\>“girl”}=\>100, > {“new”=\>“girl”}=\>200} > > Could some explain how to use hash key which is of type hash It’s a bad idea to do what you are doing. However, if you really want to, you could try installing CICPHash (sudo gem install cicphash). It’s designed to be a case insensitive hash, but it can easily be generalized to consider arbitrary sets of keys as equal. The default implementation will do what you want for this simple case: irb(main):001:0\> require ‘pp’ =\> true irb(main):002:0\> require ‘cicphash’ =\> true irb(main):003:0\> test = CICPHash.new =\> {} irb(main):004:0\> test[“new”=\>“girl”]= 100 =\> 100 irb(main):005:0\> test[“new”=\>“girl”]= 200 =\> 200 irb(main):006:0\> pp test {{“new”=\>“girl”}=\>200} =\> nil However, the default implementation won’t necessarily work for hashes with multiple keys, as the order of the keys in the hash is undefined. You can change that by redefining the CICPHash#convert\_key method. However, as previously mentioned, you probably shouldn’t use hashes as hash keys. Jeremy ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [September 25, 2007, 11:10pm UTC](https://www.ruby-forum.com/t/how-to-implement-a-hash-whose-key-is-another-hash/112527/4 "2007-09-25T23:10:27Z")
On 8/20/07, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > pp test > > I am expecting the output to be > > {{“new”=\>“girl”}=\>200} > > However the output I am getting is > > {{“new”=\>“girl”}=\>100, > {“new”=\>“girl”}=\>200} The problem is that you are not using the same key … k = {‘new’ =\> ‘girl’} k.hash #=\> 1072992710 h = {} h[k] = 100 h[k] = 200 p h #=\> {{“new” =\> “girl”} =\> 200} k = {‘new’ =\> ‘girl’} k.hash #=\> 1072934850 h[k] = 300 p h #=\> {{“new”=\>“girl”}=\>300, {“new”=\>“girl”}=\>200} To use an object as a hash key, you need to implement a “hash” method that returns the same value when the contents of the hash are the same. But this gets tricky. Blessings, TwP ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [September 25, 2007, 11:02pm UTC](https://www.ruby-forum.com/t/how-to-implement-a-hash-whose-key-is-another-hash/112527/5 "2007-09-25T23:02:35Z")
On Aug 20, 8:58 am, removed\_email\_address@domain.invalid wrote: > I am expecting the output to be > > {{“new”=\>“girl”}=\>200} > > However the output I am getting is > > {{“new”=\>“girl”}=\>100, > {“new”=\>“girl”}=\>200} > > Could some explain how to use hash key which is of type hash irb(main):001:0\> h1 = {} =\> {} irb(main):002:0\> h2 = { :foo=\>‘bar’ } =\> {:foo=\>“bar”} irb(main):003:0\> h1[h2] = ‘hello’ =\> “hello” irb(main):004:0\> h1[h2] = ‘world’ =\> “world” irb(main):005:0\> h1 =\> {{:foo=\>“bar”}=\>“world”} irb(main):006:0\> h1[:a=\>1] = 42 =\> 42 irb(main):007:0\> h1[:a=\>1] = 2000 =\> 2000 irb(main):008:0\> h1 =\> {{:a=\>1}=\>2000, {:foo=\>“bar”}=\>“world”, {:a=\>1}=\>42} If it’s TRULY the same hash, you get what you expected: the new value overwrites the old. What you did, and what you see on lines 6 and 7 above, is to create a new hash on the fly that happens to have the same values as another hash. To Ruby, those are treated as two different keys.