Re: named pattern variable scoped as global, should be local

*To*: mathgroup at smc.vnet.net*Subject*: [mg56765] Re: [mg56696] named pattern variable scoped as global, should be local*From*: "David Park" <djmp at earthlink.net>*Date*: Thu, 5 May 2005 06:02:47 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Lee, This is a feature of Mathematica. You get a similar problem with Set and SetDelayed when the right hand side of a definition has values. The problem goes away if you use RuleDelayed. x = 7; Module[{x}, {1, 2, 3} /. x_ :> 2x] {2, 4, 6} If we use a plain Rule and Trace the evaluation, you can see what happens. x = 7; {Module[{x}, {1, 2, 3} /. x_ -> 2 x], {{{{x, 7}, 2 7, 14}, x_ -> 14, x_ -> 14}, {1, 2, 3} /. x_ -> 14, 14}, 14} The rhs of the rule is evaluated to 14. Then the pattern x_ matches the entire list and replaces it by 14. I generally avoid giving values to single letter symbols like x since we so often want to use them symbolically. That way you aren't forced to used SetDelayed and RuleDelayed as often. You can always define Rules to give values to symbols like x when you want to. David Park djmp at earthlink.net http://home.earthlink.net/~djmp/ From: leenewman at gmail.com [mailto:leenewman at gmail.com] To: mathgroup at smc.vnet.net When using a named pattern variable within a module, it should be scoped locally within the pattern. However, this does not seem to work as advertised. For example, shouldn't the pattern variable x in the statements below be local to the pattern? Does anyone know whether this is a bug, or whether I am just missing something about the usage of variables in patterns/condition constructs? (* this returns 14! *) x = 7; Module[{x},{1, 2, 3} /. x_ -> 2x]; (* this returns {2,4,6}, assuming q is not globally defined. *) Remove[q]; Module[{q},{1, 2, 3} /. q_ -> 2q]; Lee