Re: How to call 'Clear' from within a function?
- To: mathgroup at smc.vnet.net
- Subject: [mg124874] Re: How to call 'Clear' from within a function?
- From: James Stein <mathgroup at stein.org>
- Date: Fri, 10 Feb 2012 05:54:39 -0500 (EST)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
- References: <201202091038.FAA18317@smc.vnet.net> <CAEtRDSdUiP2DYhCo8DLRn0ANKdbjZMw3VdtXy+6+khaj3=0U2g@mail.gmail.com>
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....
- References:
- How to call 'Clear' from within a function?
- From: James Stein <mathgroup@stein.org>
- How to call 'Clear' from within a function?