Re: pattern matching: rules that stop other rules?

*To*: mathgroup at smc.vnet.net*Subject*: [mg71607] Re: pattern matching: rules that stop other rules?*From*: Peter Pein <petsie at dordos.net>*Date*: Sat, 25 Nov 2006 05:36:44 -0500 (EST)*References*: <ek63v6$913$1@smc.vnet.net>

croddie at princeton.edu schrieb: > Hello. I've been using Mathamatica for quite a while but without ever > finding out how the language works fundamentally, which I'm trying to > do now. I'd be grateful for some help in understanding patterns - it > seems like a powerful idea to me. There is something I can't work out. > > Define a function f [x_]:=0 say > Now If [ p, 1, f [ 2] ] evaluates to itself. So the rule in the > definition is not applied to f [ 2 ]. > > Replace If with some other undefined function, say qwerty, and you get > qwerty [ p, 1, 0] not surprisingly. And If [ p, 1, f [ 2] ] /. f > [x_]->0 returns If [ p, 1, 0 ]. > > Is there a rule associated with If that stops a rule (if that's the > right expression) from being applied inside it? Can users write such > rules? > > Thanks for any help. > Sorry, "If" has "HoldRest" set.