From: Robert Klemme <shortcutter@...>
Date: 2009-10-26T23:15:12+09:00
Subject: Re: Hash Reverse ?

2009/10/26 Aldric Giacomoni <aldric@trevoke.net>:
> Salai Koko wrote:
>> I am preparing for my orals exam in Ruby (my last chance.. , If I fail
>> the test, no more possible to study Computer Science related :-) )
>>
>> http://codepad.org/foNb9Pd3
>>
>> class Hash
>>   def hash_revert
>>     hash_new = Hash.new
>>     self.each {|key,value|
>>       if not hash_new.has_key?(key) then hash_new[value] = key end
>>     }
>>     return hash_new
>>   end
>> end
>>
>> h = {2=>"a", 1=> "b", 3 =>"a", 4=> "a", 5 => "b", 6=>"c"}
>> p h.hash_revert #--> {"a"=>4, "b"=>1, "c"=>6}
>>
>> or Should I get this ??
>> # => {"a"=>2, "b"=>1, "c"=>6}
>>
>> Because HASH is working like "Set" ??
>>
>> many thanks in advance,
>> salai.
>
> In Ruby 1.9, hashes keep the order of input. Before Ruby 1.9, the order
> is "random" - so it's not guaranteed which will be the first key that
> you will see with 'each'.

As far as I can see salai does not want to reverse the order of
elements but rather the key value relationship.

Salai, it really depends on the requirements.  Here's another valid variant:

class Hash
  def hash_revert
    r = Hash.new {|h,k| h[k] = []}
    each {|k,v| r[v] << k}
    r
  end
end

irb(main):008:0> {2=>"a", 1=> "b", 3 =>"a", 4=> "a", 5 => "b",
6=>"c"}.hash_revert
=> {"a"=>[2, 3, 4], "b"=>[1, 5], "c"=>[6]}

http://codepad.org/XNvwvOxi

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/