From: James Edward Gray II <james@...>
Date: 2006-08-11T22:47:32+09:00
Subject: Re: Question on setting up a class and searching on objects

On Aug 11, 2006, at 7:47 AM, Dark Ambient wrote:

> I'm working from Ruby for Rails book but want to try and just
> replicate some of the examples in Ruby alone.
> One method in particular is called whole_name and would give the user
> the ability to search a list by first_name, middle_name, or last_name.

If you want to make a method to search the the individual composers,  
you will need to get them into some data structure (like an Array)  
where you can scan through them.

> class Composer
>
> def listcomp (first_name, middle_name, last_name)
> f = first_name
> m = middle_name
> l = last_name
> end

What are you intending this method to do?  Currently, it doesn't do  
anything meaningful:

1.  first_name, middle_name, and last_name are accepted as arguments
2.  Each of those is assigned to another local variable
3.  All six local variables are discarded as the method exits

> def whole_name
>    first_name + " " +
>    (if middle_name then middle_name + " " else "" end) +
>    last_name
> end
> end

This method doesn't work because it uses local variables that don't  
exist in this method.  I believe this is your hang-up.

Inside a class, you keep data around by putting it into instance  
variables (those start with an @ symbol).  All methods of the class  
can access the instance variables for the current object.

See if this helps explain things:

 >> class Name
 >>   def initialize(first, last, middle = nil)
 >>     # assign these local variables to instance variables...
?>     @first = first
 >>     @last = last
 >>     @middle = middle
 >>   end
 >>
?>   # create methods to read those instance variables...
?>   attr_reader :first, :last, :middle
 >>
?>   def full
 >>     [@first, @middle, @last].join(" ").squeeze(" ")
 >>   end
 >> end
=> nil
 >> james = Name.new("James", "Gray", "Edward")
=> #<Name:0x31a5e0 @last="Gray", @first="James", @middle="Edward">
 >> james.last
=> "Gray"
 >> james.full
=> "James Edward Gray"
 >> dana = Name.new("Dana", "Gray")
=> #<Name:0x314c44 @last="Gray", @first="Dana", @middle=nil>
 >> dana.middle
=> nil
 >> dana.full
=> "Dana Gray"

> comp1 = Composer.new
> comp2 = Composer.new
> comp3 = Composer.new
> comp4 = Composer.new
> comp5 = Composer.new
>
> comp1.listcomp("ludwig","van","bethoven")
> comp2.listcomp("wolfgang","amadeus","mozart")
> comp3.listcomp("franz", "joseph", "hayden")
> comp4.listcomp("George", "frederic", "Handel")
> comp5.listcomp("Johann", "sebastian", "bach")

Here's another idea:

 >> composers = [ %w[ludwig van beethoven],
?>               %w[wolfgan amadeus mozart],
?>               %w[franz joseph hayden],
?>               %w[george frederic handel],
?>               %w[johann sebastian bach] ].map do |f, m, l|
?>   Name.new(f, m, l)
 >> end
=> [#<Name:0x356040 @last="van", @first="ludwig",  
@middle="beethoven">, #<Name:0x35602c @last="amadeus",  
@first="wolfgan", @middle="mozart">, #<Name:0x356018 @last="joseph",  
@first="franz", @middle="hayden">, #<Name:0x356004 @last="frederic",  
@first="george", @middle="handel">, #<Name:0x355ff0  
@last="sebastian", @first="johann", @middle="bach">]

> Question - I'm having a problem calling to the class method
> whole_name. The method is defined without any arguments so how could
> it be called ?

whole_name() is not a class method, it is an instance method.  You  
know this because it is called on instances of the class, not the  
class itself (new() is a class method).

I hope my examples above cleared up calling the methods.

> Question - Did I set up the class correctly , defining the new object
> outside of the class definition ? And would it be better to create an
> array (multidimensional) for the composers instead of initializing
> each one seperately ? (If this question makes sense)

See above.

Hope that helps.

James Edward Gray II