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MathGroup Archive 2012

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Re: How to call 'Clear' from within a function?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124865] Re: How to call 'Clear' from within a function?
  • From: Bob Hanlon <hanlonr357 at gmail.com>
  • Date: Fri, 10 Feb 2012 05:51:31 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201202091038.FAA18317@smc.vnet.net>

Clear[x];
x
x = 2
Clear[x] (* you clear symbols not strings *)
x


x

2

x

ClearAll[f];
SetAttributes[f, HoldFirst];
f[arg_] := (Print[Unevaluated[arg]]; Clear[arg]);
x = 2
f[x]
x

2

x

x


Bob Hanlon

On Thu, Feb 9, 2012 at 5:38 AM, James Stein <mathgroup at stein.org> wrote:
> Before trying to encapsulate 'Clear' in a function, I try this:
>
> Clear [ x ] ;
> x
> x = 2
> Clear [ ToString [ Unevaluated [ x ] ] ] ;
> x
>
> And I expect to get three  lines of output:   x, 2, and x
> But instead I get these three lines output:  x, 2 and 2.
>
> What do I misunderstand, or need to know?
>
> I observe that a function CAN obtain the name of a symbol passed to it:
>
> ClearAll [ f ] ;
> SetAttributes [ f, HoldFirst ] ;
> f [ arg_ ] := ( Print [ Unevaluated [ arg ] ] ;
>  Clear [ ToString [ Unevaluated [ arg ] ] ] ) ;
> x = 2; (* assign a value to  'x' *)
> f [ x ] ;  (* 'f' can print the symbol 'x' ! *)
> x  (* but 'x' still has value '2' ; the 'Clear' had no effect !? *)
>
> Similar problems with trying to call 'Set' from within a function....



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