Re: How best to implement a hash table in Mathematica

• To: mathgroup at smc.vnet.net
• Subject: [mg125123] Re: How best to implement a hash table in Mathematica
• From: Joseph Gwinn <joegwinn at comcast.net>
• Date: Wed, 22 Feb 2012 05:33:57 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <jho21j\$5sb\$1@smc.vnet.net> <jhqmsq\$fbt\$1@smc.vnet.net> <jhstus\$nku\$1@smc.vnet.net> <jhvu5c\$80d\$1@smc.vnet.net>

```In article <jhvu5c\$80d\$1 at smc.vnet.net>,
Richard Fateman <fateman at cs.berkeley.edu> wrote:

> On 2/19/2012 11:49 PM, Joseph Gwinn wrote:
> > In article<jhqmsq\$fbt\$1 at smc.vnet.net>,
> >   Richard Fateman<fateman at cs.berkeley.edu>  wrote:
>
> >> Initializing a hashtable with 10^4 (identical?) empty keys makes no
> >> sense. Did I misunderstand your message?
> >
> > No, you understood correctly.  Please see my response to David Bailey
> > for the rationale.
> >
> > Joe Gwinn
> >
> Again, I may misunderstand, but I think you can accomplish the same
> thing by typing
>
> f[x_]:= {}
>
> if {} is the default empty value.

This creates the hashtable, and gives it a name.  In a hashtable, it is
not the table that's empty, it's this or that specific cell that's
empty, in an unpredictable and changing pattern.  So one would be
attempting to update and access f[x], where x is a random integer.  So,
I just tried this, and it works just fine, including handling variable
lists.

So, this and related approaches may be workable.  I will have to
implement them in a real application, and check performance and
scalability.

Joe Gwinn

```

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