Asp Forum - Hashes in Sets

Paul Mucur

3/13/2008 1:51:00 PM

In Ruby 1.8.6-p111 (the latest version bundled with Mac OS X) and Ruby
1.8.6-p114, the Set class will duplicate any Hash elements it contains:

> require 'set'
> s = Set.new
> s << {}
=> #<Set: {{}}>
> s << {}
=> #<Set: {{}, {}}>
> s << {:a => 1}
=> #<Set: {{:a=>1}, {}, {}}>
> s << {:a => 1}
=> #<Set: {{:a=>1}, {}, {:a=>1}, {}}>

This seems to be due to the fact that the Set class uses a Hash
internally to store its elements as keys and the Hash class itself
does not equate two hashes with the same keys and values:

> {} == {}
=> true
> {}.eql?({})
=> false
> {}.hash == {}.hash
=> false

Defining hash and eql? methods for Hash seems to correct this:

class Hash
# Create a hash based on the keys and values.
def hash
"#{keys.join}#{values.join}".hash
end

# To determine whether two hashes are the same, compare
# their hashes.
def eql?(other)
hash == other.hash
end
end

> {}.eql?({})
=> true
> {:a => 1}.eql?({:a => 1})
=> true
> {:b => 1, :a => 2}.eql?({:a => 2, :b => 1})
=> true
> s = Set.new
=> #<Set: {}>
> s << {}
=> #<Set: {{}}>
> s << {}
=> #<Set: {{}}>

This issue of Hash's eql? method has been brought up before in http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/3ecd14d6e8bbc9cf/88d51d...
and, more recently, http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/572e8d8b01a7d24e/c5f532...

I see that it has been fixed in Ruby 1.9.0 and was wondering if that
change will also be implemented in the 1.8 series?

Thanks in advance,

-- Paul

comp.lang.ruby

Hashes in Sets

Paul Mucur

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