From: Guillaume Cottenceau <gc@...>
Date: 2001-06-23T04:38:51+09:00
Subject: [ruby-talk:16778] Re: uniq and "equivalence"

Hugh Sasse Staff Elec Eng <hgs@dmu.ac.uk> writes:


[...]

> >  Define a #hash method for your class Trapezoidal_fitness, actually because
> >  it's not defined it use the default #hash function which is the id of the
> >  object.
> 
> That's what I don't want, but how can one map a 4-D set
> of floats on to Fixnums?  Maybe I should redefine uniq:

Of course you can't :-).

Hash functions are used to speed things up, not to do the whole of the
comparisons:

irb> class T
irb> def hash
irb> puts "in hash"
irb> 3
irb> end
irb> end
nil
irb> a = T.new
#<T:0x401b71e8>
irb> b = T.new
#<T:0x402c74b0>
irb> [a,b].uniq
in hash
in hash
[#<T:0x401b71e8>, #<T:0x402c74b0>]

irb> class T
irb> def eql?(other)
irb> puts "in eql?"
irb> true
irb> end
irb> end
nil
irb> a.eql?(b)
in eql?
true
irb> [a,b].uniq
in hash
in hash
in eql?
in hash
in hash
[#<T:0x401b71e8>]


I suppose #hash is used first, #eql? next.


(while I can't guess why #hash seems to be called twice)



-- 
Guillaume Cottenceau - http://mandrakesoft.com/~gc/