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

*To*: mathgroup at smc.vnet.net*Subject*: [mg124904] Re: How to call 'Clear' from within a function?*From*: Bob Hanlon <hanlonr357 at gmail.com>*Date*: Sat, 11 Feb 2012 06:39:20 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <201202091038.FAA18317@smc.vnet.net>

I stand corrected. David Bailey's response answers your question. Bob Hanlon On Fri, Feb 10, 2012 at 5:54 AM, James Stein <mathgroup at stein.org> wrote: > Thanks for responding, Bob. Just three points: > > (1) According to Mathematica's online documentation: > > Clear [ "form-1", "form-2", ...]", clears values and definitions for > all symbols whose names match any of the string patterns form-i. > > This suggests that Clear will "clear strings", as you put it, and so it does: > > ClearAll [ f , x ]; > f [ name_String ] := Clear [ name ] ; > x = 3 ; > f [ "x" ] ; > x (* now prints as "x, not "3" *) > > (2) There are reasons to clear Strings rather than Symbols. A less > simple example of a function to Clear symbols might, for example, take > one or more string arguments from which it then constructs a list of > Symbols to be Cleared. For example, given "x", it might clear x2, x4, > x6, ... x100. > > (3) I remain in the dark about why this example fails: > > 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' *) ; > > It appears that Clear [ string ] works, > but Clear [ ToString [ Unevaluated [ symbol ] ] ] does not work. > Is a "String" not a "String Pattern" representing itself? > What mistake am I making? > > -- James > > > On Thu, Feb 9, 2012 at 3:17 AM, Bob Hanlon <hanlonr357 at gmail.com> wrote: >> >> 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.... >

**Follow-Ups**:**Re: How to call 'Clear' from within a function?***From:*James Stein <mathgroup@stein.org>

**References**:**How to call 'Clear' from within a function?***From:*James Stein <mathgroup@stein.org>