[Date Index] [Thread Index] [Author Index]

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

• To: mathgroup at smc.vnet.net
• Subject: [mg124869] Re: How to call 'Clear' from within a function?
• From: David Bailey <dave at removedbailey.co.uk>
• Date: Fri, 10 Feb 2012 05:52:55 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <jh07qo$huj$1@smc.vnet.net>

On 09/02/2012 10:40, James Stein 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?

Well Clear has attributes HoldAll, so you would need:

In[6]:= Clear [ x ] ;
x
x = 2
Clear [ Evaluate[ToString [ Unevaluated [ x ] ]] ] ;
x

Out[7]= x

Out[8]= 2

Out[10]= x
>
> 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....

The same comment applies, but why not just put:

Clear[arg]

David Bailey
http://www.dbaileyconsultancy.co.uk