From: Steve Tuckner <SAT@...>
Date: 2001-04-18T01:33:09+09:00
Subject: [ruby-talk:13766] Re: Odd Hash Behavior (long)

I am kind of new to Ruby, could you explain the change to the hash class in
detail (it might be good for other newbies too)?

-----Original Message-----
From: ts [mailto:decoux@moulon.inra.fr]
Sent: Tuesday, April 17, 2001 11:20 AM
To: ruby-talk@ruby-lang.org
Cc: ruby-talk@ruby-lang.org
Subject: [ruby-talk:13765] Re: Odd Hash Behavior (long)


>>>>> "S" == Steve Tuckner <SAT@MULTITECH.com> writes:

S> Could I change the way a hash works so that when a new key is referenced,
a
S> new object is automatically created.

 This is very quickly written (verify it)

pigeon% cat b.rb
#!/usr/bin/ruby
class << Hash
   alias :old_new :new
   def new(default = nil, &block)
      a = old_new(default)
      if block_given?
         raise "Invalid call" if default
         a.instance_eval <<-EOT
            @block = block
            alias :aref :[]
            def [](a)
               if ! has_key?a 
                  self[a] = @block[]
               end
               aref(a)
            end
         EOT
      end
      a
   end
end
class A
  attr_reader :value
  def initialize
    @value = 0
  end
  def add_one
    @value += 1
  end
end
 
aTestHash = Hash.new { A.new }
 
aTestHash['A'] = A.new
aTestHash['A'].add_one
aTestHash['B'].add_one
aTestHash['B'].add_one
print "aTestHash['A'] = #{aTestHash['A'].value}\n"
print "aTestHash['B'] = #{aTestHash['B'].value}\n"
 
print "\nAll Hash values:\n"
aTestHash.each { |aKey, aA| print "#{aKey} = #{aA.value}\n" }
pigeon% 

pigeon% b.rb
aTestHash['A'] = 1
aTestHash['B'] = 2
 
All Hash values:
A = 1
B = 2
pigeon% 


Guy Decoux