From: ara.t.howard@...
Date: 2006-11-30T00:25:22+09:00
Subject: Re: Array#group_by

On Thu, 30 Nov 2006, Bruno Michel wrote:

> Gareth Adams a �crit :
>> I posted this on the Rails list, but then I realised it would make more 
>> sense
>> over here:
>> 
>> -------
>> 
>> Just thought I'd share a function which has been really useful to me, for 2 
>> reasons
>> 
>> 1) It could easily help someone else
>> 2) People might be able to make it work better or point out where it's not 
>> optimal
>> 
>> class Array
>>   # Groups elements of an array based on a user-defined condition
>>   #   # Each element from this array is passed to the block, and the 
>> returned value
>>   # determines the key under which it will be stored in the output hash
>>   #
>>   # If no block is given then the array indices will be used as the hash 
>> keys
>>   def group_by
>>     hsh = {}
>>     self.dup.each_with_index do |element, i|
>>       if block_given?
>>         key = yield element
>>       else
>>         key = i
>>       end
>>       hsh[key] ||= []
>>       hsh[key] << element
>>     end
>>     hsh
>>   end
>> end
>
>
> Hi,
>
> thanks for the tip. I have q question with do you use a ".dup" on the line 
> self.dup.each_with_index do |element, i| ? Facets has a similar function 
> (partition_by), but don't use a ".dup" before iterating:
>
> http://facets.rubyforge.org/api/core/classes/Enumerable.html#M000526

i'm unlear how this adds to inject:

   harp:~ > cat a.rb
   words = %w( foo bar foobar )

   grouped = words.inject Hash.new{|h,k| h[k] = []} do |h,k|
     h[k[0,1]] << k; h
   end

   p grouped

   harp:~ > ruby a.rb
   {"b"=>["bar"], "f"=>["foo", "foobar"]}


which you can one-line if you wish:

   harp:~ > cat a.rb
   words = %w( foo bar foobar )
   p words.inject({}){|h,k| (h[k[0,1]] ||= []) << k; h}

   harp:~ > ruby a.rb
   {"b"=>["bar"], "f"=>["foo", "foobar"]}


regards.

-a
-- 
if you want others to be happy, practice compassion.
if you want to be happy, practice compassion.  -- the dalai lama