From: Robert Klemme <shortcutter@...>
Date: 2009-08-28T19:57:43+09:00
Subject: Re: data sctructure

2009/8/28 David A. Black <dblack@rubypal.com>:
>  This message is in MIME format.  The first part should be readable text,
>  while the remaining parts are likely unreadable without MIME-aware tools.
>
> Hi --
>
> On Fri, 28 Aug 2009, Juliano ÁØÈ£ wrote:
>
>> Hello guys.
>>
>> I've just started using ruby and I come from Python... I'm still
>> learning to think the Ruby way...
>> Of course, any beginning is hard, but even so I've been having a lot
>> of fun learning Ruby.
>
> Welcome! I'm glad you're enjoying it.
>
>> The problem is that I still don't know much about the tons of methods
>> and I'm maybe letting something pass through my fingers.
>>
>> My problem is this:
>> I have to read a file and set up a data structure of embedded
>> dictionaries. In Python I can easily do that by means of
>> "dict.defaultdict()", but I didn't understand how to do that in
>> Ruby... I finally came up with the following:
>>
>>  def preprocess(lines)
>>   d = {}
>>   lines.each do |line|
>>     c, s, fc, fl = line.split("\t")
>>       if !d.has_key? c
>>         d[c] = {}
>>       end
>>       if !d[c].has_key? s
>>         d[c][s] = {}
>>       end
>>       if !d[c][s].has_key? fc
>>         d[c][s][fc] = []
>>       end
>>     d[c][s][fc].push fl
>>   end
>>   return d
>>  end
>>
>> My question is: Is there an easier way to do this in Ruby as it is in
>> Python?
>
> The idiom for conditional assignment in Ruby is ||= (or-equals). You
> could do this:
>
>  def preprocess(lines)
>    d = {}
>    lines.each do |line|
>      c, s, fc, fl = line.split("\t")
>      d[c]        ||= {}
>      d[c][s]     ||= {}
>      d[c][s][fc] ||= []
>      d[c][s][fc].push fl
>    end
>    return d
>  end
>
> The only thing is, in cases where you want false or nil to be a hash
> value, ||= will over-eagerly do the assignment. But that's not the
> case here.
>
> There are also ways to set the default behavior of hashes, but I think
> the above is probably the most direct way to do what you want.

You mean something like the more involved

 def preprocess(lines)
   d = Hash.new {|h,k| h[k] = Hash.new(&h.default_proc)}

   lines.each do |line|
     c, s, fc, fl = line.split("\t")
     (d[c][s][fc] ||= []) << fl
   end

   d
 end

Kind regards

robert

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