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MathGroup Archive 1998

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Re: keys in a hash table (aka, all assigned indices in an array): awful hack

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13339] Re: [mg13281] keys in a hash table (aka, all assigned indices in an array): awful hack
  • From: David Withoff <withoff>
  • Date: Mon, 20 Jul 1998 02:49:57 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

> suppose I make some assignments like this:
>   fruits[apple]= crunchy
>   fruits[lemon]= sour
>
> I'm treating fruits like a hash table and now I would like a list of all
> the keys.  In this case {apple,lemon}.
>
> It seems like there should be an easy way to do this. All I came up with
> was the following hack:
>
> keys[hash_]:=
>   DownValues[hash] /. {RuleDelayed->(#&), hash->(#&)} //ReleaseHold
>
> the hash values will be simple; we can just do fruits/@keys[fruits]
>
> Thanks,
> Daniel
>
> --
> Daniel Reeves  dreeves at umich.edu  http://interlabs.bradley.edu/~daniel
>
> "Computer Science is no more about computers than astronomy is about
> telescopes."  -- E W Dijkstra

I have done this using

keys[hash_]:=Map[#[[1,1,1]]&,DownValues[hash]]

but the basic idea is the same.  This looks ok to me.  I guess whether
or not this is a hack is in the eye of the beholder.

Dave Withoff
Wolfram Research


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