From: "David A. Black" <dblack@...>
Date: 2009-08-28T20:12:11+09:00
Subject: Re: data sctructure

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Hi --

On Fri, 28 Aug 2009, Robert Klemme wrote:

> 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 層硲 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

Yes, that's what I meant.


David

-- 
David A. Black / Ruby Power and Light, LLC / http://www.rubypal.com
Q: What's the best way to get a really solid knowledge of Ruby?
A: Come to our Ruby training in Edison, New Jersey, September 14-17!
    Instructors: David A. Black and Erik Kastner
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