From: Joel VanderWerf <vjoel@...>
Date: 2005-03-07T05:05:54+09:00
Subject: Re: Noob:Objects as key in hash

James Britt wrote:
> Jim Weirich wrote:
> 
>>  ...
>> The hash function should return an integer to be used for hashing.  
>> Your version returns a string.  One correction might be ...
>>
>>   def hash
>>     [@x.object_id, @y.object_id].to_s.hash
>>   end
>>
>> That will now work.  But you probably really don't want to bother 
>> converting the above to a string.  Why not just do:
>>
>>   def hash
>>     @x.object_id + @y.object_id
>>   end
> 
> 
> Isn't this the same value as
> 
>   @y.object_id + @x.object_id
> 
> giving, for example,
> 
> PointID.new( 1, 2).hash ==  PointID.new( 2, 1).hash
> 
> 
> But they are different coordinates.

That's ok, as long as #eql? says they are different. In fact, many
objects must give the same value in response to #hash, since there are
far more possible objects than Fixnums. The #hash method is only used to
find the hash bin, and then #eql? is used to test equality within the bin.