From: Jason Creighton <androflux@...>
Date: 2003-06-28T05:32:28+09:00
Subject: Re: Using & for arrays of objects

On Fri, 27 Jun 2003 09:48:56 +0900
"Krishna Dole" <kpdole@ucdavis.edu> wrote:

> Hi,
> I'm having trouble using the & (intersection) method on two arrays of
> objects. I know that if the objects don't have comparison methods, & won't
> work because it is looking at the object IDs rather than values stored in
> the objects, but I can't seem to get & to use the comparison methods I want.

& uses hashes under the hood. You need to define #hash and #eql? for your
object so that any object you want to be considered equal have equal hash
values and eql?(other) returns true. The easiest way would be like this:

class N
  def initialize(n)
    @n = n
  end
  def hash()
    @n.hash()
  end
  def eql?(other)
    @n.hash() == other.hash()
  end
  attr_reader :n
end

In use:
>> [ N.new(1), N.new(2) ] & [ N.new(2), N.new(3) ]
=> [#<N:0x402a83a4 @n=2>]

~/prog/ruby$ ri Object#eql?
This is a test 'ri'. Please report errors and omissions
on http://www.rubygarden.org/ruby?RIOnePointEight

------------------------------------------------------------ Object#eql?
     obj.eql?( anObject ) -> true or false
------------------------------------------------------------------------
     Returns true if obj and anObject have the same value. Used by Hash
     to test members for equality. For objects of class Object, eql? is
     synonymous with ==. Subclasses normally continue this tradition,
     but there are exceptions. Numeric types, for example, perform type
     conversion across ==, but not across eql?, so:
        1 == 1.0     #=> true
        1.eql? 1.0   #=> false

~/prog/ruby$ ri Object#hash
This is a test 'ri'. Please report errors and omissions
on http://www.rubygarden.org/ruby?RIOnePointEight

------------------------------------------------------------ Object#hash
     obj.hash -> aFixnum
------------------------------------------------------------------------
     Generates a Fixnum hash value for this object. This function must
     have the property that a.eql?(b) implies a.hash == b.hash. The hash
     value is used by class Hash. Any hash value that exceeds the
     capacity of a Fixnum will be truncated before being used.

Jason Creighton