From: Robert Klemme <shortcutter@...>
Date: 2007-05-24T01:19:52+09:00
Subject: Re: Enumerator for Hash


Wow!  5 identical postings in just two minutes.  You must have been 
hitting "send" really hard...

On 23.05.2007 18:00, Ryan Hinton wrote:
> On May 23, 3:41 am, Robert Klemme <shortcut...@googlemail.com> wrote:
> <snip>
>> Make clear what you actually want.  You could also show some code.
> 
> Sorry for the confusion.  I was trying to be considerate and brief,
> but apparently I was only obscure.
> 
> I want a Hash-like object that instead of calling each object's #hash
> method to get its hash value, calls another to-be-specified method.  I
> probably want to specify an alternative to #eql? as well.  Again, this
> is parallel to the way the standard library Enumerator class creates
> an Enumerable-like object with an alternative #each method.

You could wrap your keys in a custom class.  For added convenience you 
could also wrap Hash with another class which wraps keys on insertion 
and unwraps on reading.

> One use cases is uniqueness.
> 
> # existing array with a bunch of complex objects
> my_array = [...]
> 
> # find unique objects using normal #hash and #eql? methods;
> # yes, I know Array#uniq does this -- it's an example
> normal_uniq_hash = Hash.new
> my_array.each {|obj| normal_uniq_hash[obj] = obj}
> # now normal_uniq_hash can be iterated to get the unique objects from
> my_array
> 
> # find "unique" objects using different #hash and #eql? methods
> alt_uniq_hash = AltHash.new(:alt_hash, :alt_eql?)  # postulating a new
> AltHash
> my_array.each {|obj| alt_uniq_hash[obj] = obj}
> # now alt_uniq_hash can be iterated to get the "unique" objects from
> my_array

In that case I'd probably rather do this:

my_array.inject({}) {|hs, obj| hs[obj.key] ||= obj; hs }.values

For example - using a string's length as uniqueness criterion:

irb(main):001:0> my_array = %w{foo bar dodo dida longword}
=> ["foo", "bar", "dodo", "dida", "longword"]
irb(main):002:0> my_array.inject({}) {|hs, obj| hs[obj.size] ||= obj; hs 
}.values
=> ["longword", "foo", "dodo"]

> Is this more clear?

Yes.

> Thanks for your help!

You're welcome.  And please be easy on the trigger. ;-)

Kind regards

	robert