Re: Clearing a symbol known only through a definition

• To: mathgroup at smc.vnet.net
• Subject: [mg66710] Re: Clearing a symbol known only through a definition
• From: Peter Pein <petsie at dordos.net>
• Date: Sat, 27 May 2006 03:53:07 -0400 (EDT)
• References: <e56e69\$1v3\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Andrew Moylan schrieb:
> Suppose I have a symbol "a" that is defined to be equal to another
> symbol. (E.g. It might be that a := b.) How can I clear the value of the
> symbol 'pointed to' by "a", without knowing explicity what symbol that is?
>
> Clear[a] won't do, of course.
> Clear[Evaluate[a]] won't do, because that will evaluate to
> Clear[Evaluate[b]], which will in turn (in general) evalulate to
> Clear["whatever b evalulates to"].
>
> ----
> Alternatively, here is the particular problem I want to solve:
>
> For any expression e, either e is a symbol, or Head[e] is, or
> Head[Head[e]] is, etc. Call this the "topmost symbol" in the expression.
> Thus, the topmost symbol in f[x][y][z] is f.
>
> I want to write a function that takes an expression and calls Clear[] on
> its topmost symbol. Can anyone think of a simple way to write such a
> function?
>

Hi Andrew,

OwnValue[] is what you're looking for:

b=5;a:=b; a
--> 5

b=3;a
--> 3

OwnValues[a]
--> {HoldPattern[a] :> b}

ReleaseHold[Clear/@(Hold[a]/.OwnValues[a])]; a
--> b

hth,
Peter

```

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