From: Hugh Sasse <hgs@...>
Date: 2005-09-14T23:12:16+09:00
Subject: Re: Sets, uniqueness not unique.

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On Wed, 14 Sep 2005, Mauricio Fern=E1ndez wrote:

> The defaults are actually those of Hash. You can follow the call chain
> starting from
>
> static struct st_hash_type objhash =3D {
>    rb_any_cmp,
>    rb_any_hash,
> };
>
> in hash.c. For user-defined classes, it will end up using #hash and #eql?
> defined in Kernel. [rb_any_cmp and rb_any_hash have some extra logic for
> Symbol, Fixnum and String values, and some core classes redefine the
> associated methods].
>
OK, it seems I'm thinking along the right lines now.  Here is what I
did in the end:

--- /tmp/T0oTa4V2       Wed Sep 14 15:05:35 2005
+++ populate_tables.rb  Wed Sep 14 15:01:24 2005
@@ -9,10 +9,31 @@

  $debug =3D true

+module StringCollection
+
+  def hash
+    (self.class)::FIELDS.inject(MD5.new()) do |d,m|
+      d << send(m)
+    end.hexdigest.hex
+  end
+
+  def eql?(other)
+    (self.class)::FIELDS.inject(true) do |b,v|
+      begin
+        b && (self.send(v) =3D=3D other.send(v))
+      rescue
+        b =3D false
+      end
+    end
+  end
+
+end
+
  class Student
-  attr_accessor :forename, :surname, :birth_dt,
-    :picture, :coll_status
+  include StringCollection
+
    FIELDS =3D [:forename, :surname, :birth_dt, :picture, :coll_status]
+  FIELDS.each{|f| attr_accessor f }

    def initialize(forename0, surname0, birth_dt0,
                   picture0, coll_status0)
@@ -22,28 +43,22 @@
      @picture =3D picture0
      puts "in student.new() picture is #{picture0.inspect}, @picture is #{=
@picture.inspect} " if $debug
      @coll_status =3D coll_status0
-    @hash =3D FIELDS.inject(MD5.new()) do |d,m|
-      d << send(m)
-    end.hexdigest.hex
    end

-  def hash
-    @hash
-  end

-  def eql?(other)
-    self.hash =3D=3D other.hash
-  end
-
    def to_s
-    "#{@surname}, #{@forename}, #{@birth_dt}, #{@picture}, #{@coll_status}=
, #{@hash}"
+    "#{@surname}, #{@forename}, #{@birth_dt}, #{@picture}, #{@coll_status}=
, #{hash}"
    end

  end

  class CourseModule
-  attr_accessor :aos_code, :dept_code, :aos_type, :full_desc

+  include StringCollection
+
+  FIELDS =3D [:aos_code, :dept_code, :aos_type, :full_desc]
+  FIELDS.each{|f| attr_accessor f }
+
    def initialize( aos_code, dept_code, aos_type, full_desc)
      @aos_code =3D aos_code
      @dept_code =3D dept_code



I was particularly pleased to be able not to repeat the FIELDS, by
means of attr_accessor, and that the idea of doing
(self.class)::FIELDS
actually worked.

In the hope that this helps someone else, and thank you,
         Hugh

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