Re: Deleting a DownValue, Evaluate[f@@argList]=. does not do it

• To: mathgroup at smc.vnet.net
• Subject: [mg20765] Re: [mg20744] Deleting a DownValue, Evaluate[f@@argList]=. does not do it
• From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
• Date: Thu, 11 Nov 1999 00:22:42 -0500
• Sender: owner-wri-mathgroup at wolfram.com

```Here is one possible way :

In[1]:=
Clear[f];
Evaluate[f @@ {1}] = 1;
Evaluate[f @@ {2}] = 2;

In[4]:=
?f

Global`f

f[1] = 1
f[2] = 2

In[5]:=
Apply[Unset, ReplacePart[Hold[{1}], f, {1, 0}]]

In[6]:=
?f
Global`f
f[2] = 2

> From: hanssen at zeiss.de
> Date: Wed, 10 Nov 1999 00:17:53 -0500
> To: mathgroup at smc.vnet.net
> Subject: [mg20765] [mg20744] Deleting a DownValue, Evaluate[f@@argList]=. does not do it
>
> Hi, MathGroup,
>
> f[1]=1;
> f[2]=2;
> ?f
>
> yields
>
> "Global`f"
> f[1] = 1
> f[2] = 2
>
> now get rid of definition for 1
>
> f[1]=.;
> ?f
>
> yields
>
> "Global`f"
> f[2] = 2
>
>
> So far, it works as expected. I want to memorize results for combinations of
> arguments
> like f[a,b,c]=results and so on. To do this, I write f@@argList. Now I
> demonstrate
> this on lists with length 1, to make it simple. When making the assignment, it
> has to be
> wrapped in Evaluate:
>
> Clear[f];
> Evaluate[f@@{1}]=1;
> Evaluate[f@@{2}]=2;
> ?f
>
> yields
>
> "Global`f"
> f[1] = 1
> f[2] = 2
>
> as before. Now I want to get rid of the definition for 1 in the analogous way:
>
> Evaluate[f@@{1}]=.
>
> yields
>
> Unset::usraw: Cannot unset raw object !
> \$Failed
>
> How to proceed without searching through DownValues[f] and deleting
> the one meant - very awkward?
>
> kind regards
>
> Dipl.-Math. Adalbert Hanszen
>

```

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