Forum: Ruby How to modify Hash to track key insertions?

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jgbailey (Guest)
on 2005-11-23 21:24
(Received via mailing list)
I am trying to figure out how I can modify the Hash object so that I am
notified each time a key is inserted in a Hash when it is constructed
using the syntax below:

  a = { "x" => "1", "y" => "2" }

I've tried overriding []=, but that will only get called when the Hash
is constructed explicitly.

Here's a little bit of code showing what I've tried and the output I'd
like to see. Thanks for any help!

class Hash
 alias :old_assignment :[]=
 def []=(key, value)
  puts "#{key} => #{value}"
  old_assignment(key, value)
 end
end

puts "Creating a"
a = { "x" => "1", "y" => "2" }
puts "Creating b"
b = Hash.new
b["1"] = "x"
b["2"] = "y"

# Expected output
#  Creating a
#  x => 1
#  y => 2
#  Creating b
#  1 => x
#  2 => y

# Actual output
#  Creating a
#  Creating b
#  1 => x
#  2 => y

p.s. I'm ultimately doing this to get to an OrderedHash. I looked at
Ara's code but the problem is I need to *replace* the functionality of
Hash - it's not an option for me to explicitly create an OrderedHash
object.
p.p.s. Is there a way to avoid the alias statement for []=? For some
reason, calling super in there would give a NoMethodError.
Daniel S. (Guest)
on 2005-11-23 21:44
(Received via mailing list)
m4dc4p wrote:
> like to see. Thanks for any help!
> a = { "x" => "1", "y" => "2" }
> #  1 => x
> Hash - it's not an option for me to explicitly create an OrderedHash
> object.
> p.p.s. Is there a way to avoid the alias statement for []=? For some
> reason, calling super in there would give a NoMethodError.
>

Personally I'd like to see (private?) methods Hash#get and Hash#set that
should be used internally for getting and setting values. That way,
you'd only have to override one method when wanting to change Hash's
behaviour.

   class StrHash
     def set(key, value)
       raise TypeError unless value.respond_to? :to_str
       super(key, value.to_str)
     end
   end

In your case:

   class Hash
     alias_method :__set__, :set

     def set(key, value)
       puts "#{key} => #{value}"
       __set__(key, value)
     end
   end


Cheers,
Daniel
bob.news (Guest)
on 2005-11-24 10:30
(Received via mailing list)
m4dc4p wrote:
> like to see. Thanks for any help!
<snip/>

> p.s. I'm ultimately doing this to get to an OrderedHash. I looked at
> Ara's code but the problem is I need to *replace* the functionality of
> Hash - it's not an option for me to explicitly create an OrderedHash
> object.

It's usually difficult to fiddle with built in classes as they employ
some
optimizations.  What's the problem with replacing "h = {1=>2, 3=>4}" by
"h
= OrdererdHash.create(1=>2, 3=>4)"?  Note that if you manipulate the
hash
constructor {} you'll affect *all* hashes created in your app and libs -
something which might produce side effects (even if it's "only"
performance) you don't want.

My 0.02 EUR: stick with the explicit approach and drop manipulating
built
in constructs.

Kind regards

    robert
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