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MathGroup Archive 2009

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why does DownValues not return all downvalues for a symbol?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99369] why does DownValues not return all downvalues for a symbol?
  • From: Michael <michael2718 at gmail.com>
  • Date: Mon, 4 May 2009 05:59:34 -0400 (EDT)

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;

In[143]:=  DownValues[g]

Out[143]= {}

In[144]:= UpValues[g]

Out[144]= {}


Any ideas?

Thanks,

Michael


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