From: "David A. Black" <dblack@...>
Date: 2005-10-24T11:27:35+09:00
Subject: Re: [SORT?] by x,y allowing x,y to be passed as parms

Hi --

On Mon, 24 Oct 2005, New 2 Ruby wrote:

> Question:  Are there better ways to do this
> sort within the container class?

I see class variables and eval, so I will go out on a limb and say:
yes :-)

> Have  class  CD with several fields defined,
> @list_of_CDs array in class  CD_List holding CD pointers,
> and a couple of methods to sort array of pointers by any two fields.
>
> Below is relevant class entries and code.
>
> class CD
>  attr_reader :size, :cusip, :issuer,  :matures, ...
> ....
> class CD_List
>  @@sortkey1= @@sortkey2 = ""
>  def initialize  @list_of_CDs = Array.new  end
>  ...
>  def new_ord(key1='size',key2='matures')       #Provide defaults
>    @@sortkey1, @@sortkey2 = key1, key2	# Set  class variables
>    @list_of_CDs.sort! { |a,b| cmp_item(a,b) }   # Sort in place
>  end
>  def cmp_item (a,b)	# Adapted from Ryan Pavlik
>    # Allow different sort fields via Class variables
>    f1 = eval('a.'+@@sortkey1) <=> eval('b.'+@@sortkey1)
>    f2 = eval('a.'+@@sortkey2) <=> eval('b.'+@@sortkey2)
>    (f1 == 0 ? f2 : f1)
>  end

One possibility would be to have CD objects themselves know how to
compare themselves with each other via a <=> method, though then you'd
have to have them know how to change that which might be slow and
cumbersome.

If you want to keep the sorting intelligence in the CD_List object, I
would do it more simply.  You don't actually need class variables
anywhere you've used them.  Possibly instance variables... but
possibly not anything, unless you really need to save the sort keys
beyond the sort operation for some reason.

Here's a little mini-version that doesn't do much but might give you
some ideas.  (You could also have CD_List inherit from Array, though
inheriting from base classes is often fraught with unexpected
problems.)

   class CD
     attr_reader :size, :cusip
     def initialize(size,cusip)
       @size,@cusip = size,cusip
     end
   end

   class CD_List
     def initialize
       @cds = []
     end

     def add(cd)
       @cds << cd
     end

     def new_ord(key1='size',key2='cusip')
       @cds.replace(@cds.sort_by {|cd| [cd.send(key1), cd.send(key2)] })
     end
   end

   c = CD.new(10,"abc")
   d = CD.new(5,"def")
   e = CD.new(5,"bcd")

   cdl = CD_List.new
   cdl.add(c)
   cdl.add(d)
   cdl.add(e)

   p cdl.new_ord


David

-- 
David A. Black
dblack@wobblini.net