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Re: why does DownValues not return all downvalues for a symbol?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99425] Re: why does DownValues not return all downvalues for a symbol?
  • From: Szabolcs <szhorvat at gmail.com>
  • Date: Tue, 5 May 2009 06:01:19 -0400 (EDT)
  • References: <gtnk2p$3l6$1@smc.vnet.net>

On May 4, 11:44 pm, Michael <michael2... at gmail.com> wrote:
> I'm trying to get the functionality of "associative arrays" or "alist"
> or "hash tables"  to work in Mathematica.  However I'm seeing a quirk
> and I wonder if anybody can explain what is going on or suggest a
> better approach?
>
> I can define a simple associate array like so:
> f["red"]=1;
> f["blue"]=2;
> ...
> If all my parameters are strings, then I can find the "keys" to my
> alist with the following function:
> StringKeys[y_] := DownValues[y] /. {( _[y[str_String]] :> _) -> str};
> SetAttributes[StringKeys, HoldFirst];
>
> In[141]:= StringKeys[f]
>
> Out[141]= {"blue", "red"}
>
> However, if I try to define a more complex alist, I cannot find any
> way to retrieve the definition!
>
> In[142]:= g["x"]["y"] = 1;
>

That's a SubValue, not a DownValue.  But why not use g[{"x","y"}] or
something similar?  You should also consider using a list of Rules
(possibly with Dispatch).


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