From: "Ara.T.Howard" <ahoward@...>
Date: 2004-04-24T14:14:06+09:00
Subject: Re: Great answers!  A few more questions

On Sat, 24 Apr 2004, Richard Lionheart wrote:

> Hi Ara,
> 
> Great answers.  BTW, I had another typo in my original post, which I
> corrected below.  I've got a couple more questions,   but if you don't feel
> like spending any more time on this question,  please ignore this post.
> 
> >       join ';'
> 
> That's a lot better than my procedural code.  Works great
> 
> >   myA1 = MyArrayType['x1', 'y1']
> 
> (Q1)  That worked,  but where the heck did you come up with that?  Is that
> documented somewhere?  I've got The Pragmatic Programmer/Thomas and The Ruby
> Way/Fulton and didn't notice that in either work (though I certainly have
> not read every word of either one.)

frankly, i find that book amazing.  it got me into ruby and yet i still find
things in it almost weekly.  to answer your question, it's in 

  'Built-in Classes and Methods'->'Array'->'[]'

the html version of the book is very useful.

note that '[]' is simply an alias for Array#new, which in turn will construct
an Array object and call #initialize on it.


> > Object#puts behaves different if passed an Array, or sub-class of Array
> 
> In my case,  I think I was passing puts the the last expression executed in
> the to_s method of an Array-subclass object.  So I really wasn't passing an
> Array nor a sub-class of Array.
> (Q2)  Do you agree?

yes.  just a warning.


> Below are several variants of code for this question.
> (Q3)  My last question is how can I make the MyArrayType3 invocation work?

  myA3 = MyArrayType(a3) #    undefined method `MyArrayType' for main:Object
                    ^
                    ^
                    ^
                    ^
                    3.new
                    ^^^^^
                    ^^^^^
  myA3 = MyArrayType3.new(a3) #    undefined method `MyArrayType' for main:Object
                    ^^^^^

typo?  ;-)

perhaps something like this (un-tested)?

  class MyArrayType < Array
    def initialize(*args, &block)
      if Array === args.first
        ary = args.shift
        super
        update ary
      else
        super
      end
    end
    def to_s;join '; ';end
  end

again, be very careful extending/inheriting builtin classes - it's powerful
because you get so much for free, but it can come back to bite you when you
expect it to be just __like__ to built-ins, eg:

  ma = MyArrayType['42']
  a = ['forty-two']

  ma + a # => this will be an Array!

  ma << 42.0   # => MyArrayType
  a << 42.0    # => Array

eg.  alot of the methods you inherit will not return objects of your
specialized class, but of the parent class (Array in this case).  this isn't
too much of a problem unless you expect these object to have you new methods
(to_s for instance)...

an aggregate class is often safer and less frustrating to debug...

> MyArrayType1 uses my original code (absent the typo) and is invoked with an
> array of explicitly quoted strings as you suggest.  Works great.
> 
> MyArrayType2 uses your join expresson and is invoked with an array of
> explicitly quoted strings as you suggest.  Works great.
> 
> MyArrayType3 uses your join expresson but adds an initialize method so it
> can be invoked with an array object argument.  Fails.
> (Q4)  Why?

see above.  code looks correct, just a typo.

> Again,  thanks for your great response.

no worries, it's just karma - i've gotten plenty of great responses myself...

> Regards,
> Richard
> 
> 
> class MyArrayType1 < Array
>     def to_s
>         s = ""
>         self.each { |x|
>             s += ";  " if s.length>0
>             s += x.to_s
>         }
>         s
>     end
> end
> 
> class MyArrayType2 < Array
>     def to_s
>         join ';  '
>     end
> end
> 
> class MyArrayType3 < Array
>     def to_s
>         sOut = @s.join ';  '
>     end
>     def intialize(aIn)
>         @s = aIn
>     end
> end
> 
> myA1 = MyArrayType1['x1', 'y1']
> puts myA1.to_s  #    x1;  y1
> 
> myA2 = MyArrayType2['x1', 'y1']
> puts myA2.to_s  #    x1;  y1
> 
> a3 = %w[x1, y1]
> myA3 = MyArrayType(a3) #    undefined method `MyArrayType' for main:Object
> (NoMethodError)
> puts myA3.to_s

cheers.

-a
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