From: Charles Hixson <charleshixsn@...>
Date: 2001-03-16T12:51:35+09:00
Subject: [ruby-talk:12710] Re: (long) Re: hash.invert loses data if equal values exist - is this the right behaviour?

On Tuesday 06 March 2001 22:36, you wrote:
> Hi,
>
> In message "[ruby-talk:12179] Re: (long) Re: hash.invert loses data if
> equal values exist - is this the right behaviour?"
>
>     on 01/03/07, Dave Thomas <Dave@PragmaticProgrammer.com> writes:
> >To be fair, the example does show it:
> >
> >  dave[14:58:19] ri invert
> >  ----------------------------------------------------------------------
> >     hsh.invert -> aHash
> >  ----------------------------------------------------------------------
> >     Returns a new hash created by using hsh's values as keys, and the
> >     keys as values.
> >
> >        h = { "n" => 100, "m" => 100, "y" => 300, "d" => 200, "a" => 0 }
> >        h.invert   #=> {200=>"d", 300=>"y", 0=>"a", 100=>"n"}
> >
> >
> >Should we make it more explicit?
>
> I hope so.  Could you add some explicit notices like the below?
>
>   ----------------------------------------------------------------------
>      hsh.invert -> aHash
>   ----------------------------------------------------------------------
>      Returns a new hash created by using hsh's values as keys, and the
>      keys as values.  Note that the result is uncertain if equvalent
>      hsh's values are associated to different keys.
>
>         h = { "n" => 100, "m" => 100, "y" => 300, "d" => 200, "a" => 0 }
>         h.invert   #=> {200=>"d", 300=>"y", 0=>"a", 100=>"n"}
>
> Thanks,
>
> -- Gotoken

What you want to return is a bag, not a set.  So the general inverse of a 
hash would have the second element be a list of values:
         h = { "n" => 100, "m" => 100, "y" => 300, "d" => 200, "e"=>200, "a" 
=> 0, "k"=>100}
         h.invert   #=> {200=>["d", "e"], 300=>["y"], 0=>["a"], 100=>["n", 
"k"]}