From: Tom Cloyd <tomcloyd@...>
Date: 2009-01-24T06:57:16+09:00
Subject: Re: class 'initialize' method not working ???

Florian Gilcher wrote:
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>
> On Jan 23, 2009, at 9:09 PM, Tom Cloyd wrote:
>
>> I'm baffled by this error, after a hour+ of experimentation with 
>> solutions. I assume it's a compiler error (right term in Ruby?), as 
>> I'm not even getting access to ruby-debug.
>
>
> Its not a ruby error.
>
>> Code snippet:
>> =======
>> def main
>>
>> opdb = Open_db.new( [1,2,3] )
>>
>> end
>>
>> class Open_db
>>   def initialize( dblist )
>>     @dbs = dblist
>>   end
>>
>>   @dbs.each do |cnt|  # <= line producing the error
>>      db_lbl = cnt[0]
>>      db_nm = cnt[1]
>>   end
>> end
>>
>
> That might be irritating at first, but you actually refer to two 
> different variables hier.
>
> First, in #initialize, you refer to an instance variable called @dbs 
> in the context of an object instance (the instance of Open_db).
>
> The second part with @dbs, which is evaluated when loading the class 
> definition is refering to an instance variable in the context of a 
> class definition (as classes in ruby are objects by themselves, this 
> is perfectly valid). The problem is, that in class context, @dbs has 
> not been initialized. Unitialized instance variables are nil by 
> definition => calling each on it fails.
>
> Feel free to ask further questions.
>
> Regards,
> Florian
>
> - --
> Florian Gilcher
>
> smtp:   flo@andersground.net
> jabber: Skade@jabber.ccc.de
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>
Florian, thanks for your response. It's a bit of a mind blower, although 
I think I understand it. I continue to discover that classes have a 
number of subtleties which my reliable old workhorse procedural Ruby 
(all I've done until this week) simply doesn't have. It's fun learning 
about them, though...mostly!

Here's a small class from the same project I'm working on  - and it 
works fine. The only structural difference I can see is that ALL code in 
this second class is wrapped in method definitions. So, if I understand 
you correctly, the code in the second method below is NOT part of the 
class definition evaluation, but is subsumed under the definition of a 
method, thus preventing the problem I was having with the other code. My 
question: Do I have this right?

# dump setnet data to files
class Dump_db
  def initialize( logging_now, log, db_array )
    @logging_now = logging_now
    @log = log
    @db_array = db_array
  end 
  def dump
    @db_array.each do |cnt|
      begin #start error trapping
        dbnm = cnt[ 0 ]
        dbobj = cnt[ 1 ]
        if @logging_now: @log.info( dbnm + ' output to file started' ) end
        fn = dbnm + '.yml'
        open( fn, 'w' ) {|i| YAML.dump( dbobj, i )}
        status = ( ( dbobj.length - 1 ).to_s + dbnm +' written to file' )
        puts "> "+status
        if @logging_now: @log.info status end
      rescue Exception => e
        if @logging_now: @log.error( "#{e}" ) end
        if @logging_now: @log.error e.backtrace end #pinpoints error 
location end
        puts "> ERROR logged to log file..."
        result = nil
      end
    end
  end
end

This is all very interesting, and if I understand it right, it's 
something that I have not seen pointed out in any of the references I've 
been studying. Kind of a major point.

Thanks again for you help (and you also, Lucas).

Tom

-- 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tom Cloyd, MS MA, LMHC - Private practice Psychotherapist
Bellingham, Washington, U.S.A: (360) 920-1226
<< tc@tomcloyd.com >> (email)
<< TomCloyd.com >> (website) 
<< sleightmind.wordpress.com >> (mental health weblog)
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