From: Hugh Sasse <hgs@...>
Date: 2005-09-14T01:08:47+09:00
Subject: Sets, uniqueness not unique.

I have been splitting a comma separated values file, and putting
some of the values into an Student class, simply a collection of strings,
so that I can build a database table from them:

require 'set'

class Student
   attr_accessor :forename, :surname, :birth_dt,
     :picture, :coll_status
   def initialize(forename0, surname0, birth_dt0,
                  picture0, coll_status0)
     @forename = forename0
     @surname = surname0
     @birth_dt = birth_dt0
     @picture = picture0
     puts "in student.new() picture is #{picture0.inspect}, @picture is #{@picture.inspect} " if $debug
     @coll_status = coll_status0
   end

   def eql?(other)
     # if self.forename == "John" and other.forename == "John"
       debug = true
     # end
     res = [:forename, :surname, :birth_dt, :picture, :coll_status].all? do |msg|
       print "#{self.send(msg)} == #{(other.send(msg))} gives #{self.send(msg) == (other.send(msg))}" if debug
       self.send(msg) == (other.send(msg))
     end
     return res
   end

   def to_s
     "#{@surname}, #{@forename}, #{@birth_dt}, #{@picture}, #{@coll_status}"
   end
end

And in the body of my program I read the records in from the csv and
add the students if they are new. They tend to be clustered in the
input, hence the last_student test.

class TableMaker
   INPUT = "hugh.csv"

   ACCEPTED_MODULES =  /^\"TECH(100[1-7]|200\d|201[01]|300\d|301[0-2])/

   # Read in the database and populate the tables.
   def initialize(input=INPUT)

     @students = Set.new()
     # [...]
     open(input, 'r') do |infp|
       while record = infp.gets
         record.chomp!
         puts "record is #{record}"
         forename, surname, birth_dt, institution_id, aos_code,
           various, other, fields,
           picture, coll_status, full_desc = record.split(/\s*\|\s*/)

         next unless aos_code =~ ACCEPTED_MODULES

         puts "from record, picture is [#{picture.inspect}]." if $debug
         # Structures for student
         student = Student.new(forename, surname, birth_dt, picture, coll_status)
         if student == last_student
           student = last_student
         else
           student.freeze

           # Avoid duplicates
           unless @students.include? student
             @students.add student
           end
           last_student = student
         end
         # [...]
       end
     end
   end

   # [...]

end


This being a Set I don't really need the call to include? now, but
it's there (from when I was using a hash for this).

I find two things that seem odd to me:

1. eql? is never getting called, despite include?.

2. I end up with duplicate students.

Sets *can't* hold duplicates, and include depends on eql? for Sets.
So what's going on?   I have checked, and the duplicate students seem to
have identical strings, so I wrote the eql? to be sure.

I bet this will be a self.kick(self) reason, but I can't see it yet.

         Thank you,
         Hugh