From: Tom Cloyd <tomcloyd@...>
Date: 2009-02-11T19:50:03+09:00
Subject: Re: simple class question

7stud -- wrote:
> Tom Cloyd wrote:
>   
>> class Test
>>
>>   def t1=(val)
>>   @val = val
>>   return 99
>>   end
>>
>>   def t2=(val)
>>   @val = val * 2
>>   return 99
>>   end
>>
>> end
>>     
>
>   
>> I enter at the command line:
>>
>> (rdb:1) p test.t2 = 2
>> 2
>>
>> I should have gotten 4. 
>>     
>
> I think you should have got 99.  You called the method t2=(), and t2=() 
> returns 99.
>
>   
>> I only ever get back the same number I put in.
>> Can someone tell me what the problem is, and what I have to do to get
>> test.t2 to do this simple thing I'm asking of it?
>>
>>     
>
> The problem appears to be that ruby does not respect a return statement 
> in a setter method.  Instead, ruby appears to return the value of the 
> argument to the setter method--which is not necessarily even the value 
> assigned to the instance variable:
>
> class Test
>
>   def t1=(val)
>     @val = val * 2
>     return 99
>   end
>
>   def t1
>     @val
>   end
>
> end
>
> puts t.t1 = 2    #2  (not 99, and not 4!)
> puts t.t1        #4  (as expected)
>
> Normally, a return statement at the end of a method would be the value 
> returned to the method call:
>
> class Test
>
>   def t1=(val)
>     @val = val * 2
>   end
>   def t1
>     @val
>   end
>
>   def a
>     @val = "goodbye"
>     return 99
>   end
>
> end
>
> t = Test.new
> puts t.t1 = "hello"  #hello  (the arg to the setter method)
> puts t.t1            #hellohello  (the actual value of @val)
> puts t.a             #99 (the return value of the method)
> puts t.t1            #goodbye (as expected)
>
> You can test on your own that ruby does respect a return statement in a 
> getter method.
>
>   
>> My other question: the "return 99" business is from Thomas. He doesn't
>> explain it. I've been reading about "return" and I cannot account for
>> why it's there. It doesn't seem to do any thing. Can someone explain 
>> THAT.
>>
>>     
>
> In ruby, a method returns the last expression that was executed in the 
> method or what is explicitly specified in a return statement.
>
>
>   
>> Finally, is this general approach I'm taking the best or usual way to
>> send data to a class method and get some output? Until now, I thought I
>> had to read an accessor variable to get output back, but I'm thinking
>> now that that is probably NOT the best way.
>>
>>     
>
> It depends on what you want to do.  If you simply want to set and get an 
> instance variable's value, using attr_accesor saves you from having to 
> manually type out the setter and getter methods.  However, if you want 
> your  setter or getter method to transform the value of the argument in 
> some way, then you have to define your own getter and setter methods.
>
>
>   
>> What I'm wanting to do, say, create a file object which gives me more
>> data when I call upon it (it'll read a record and do some processing,
>> then pass back the results. Normally I'd use a method, but I'm trying to
>> become more "classy" in my programming, hence this effort. I'm basically
>> still struggling to find a good reason to use a class at all. So far,
>> it's proven far too difficult to get something OUT of a class instance.
>> I don't see the point of the bother, yet. I'm hoping some enlightenment
>> will arrive soon.
>>     
>
> Here is an example of what you can do:
>
> class MyDataProcessor
>   attr_reader :file
>
>   def initialize(fname)
>     @file = File.open(fname)
>   end
>
>   def file=(fname)
>     @file = File.open(fname)
>   end
>
>   def get_data
>     data = @file.gets
>
>     #process data:
>     data.chomp.split if data #data=nil at EOF
>   end
>
> end
>
>
> dp = MyDataProcessor.new("data.txt")
>
> while data = dp.get_data
>   p data  #do something with processed data
> end
>
> dp.file.close
>
>   
Thanks much for your thoughts. You added some material I didn't see (or 
haven't yet understood!) in Robert's response. I can see you were as 
surprised (more or less) as I was by the failure of the "return n" 
statement to do anything. I never could get it to respond as I had expected.

I will study your example. I'm sure it will teach me a few things. Thanks!

~t.

-- 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~