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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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