Re: pattern matching: rules that stop other rules?

*To*: mathgroup at smc.vnet.net*Subject*: [mg71629] Re: pattern matching: rules that stop other rules?*From*: "Steve Luttrell" <steve_usenet at _removemefirst_luttrell.org.uk>*Date*: Sat, 25 Nov 2006 05:37:10 -0500 (EST)*References*: <ek63v6$913$1@smc.vnet.net>

Attributes[If] gives {HoldRest,Protected} It's the HoldRest that is causing the behaviour you observe. You can set the Attributes of any user-defined function, so you can create behaviours such as this yourself. Steve Luttrell West Malvern, UK <croddie at princeton.edu> wrote in message news:ek63v6$913$1 at smc.vnet.net... > 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. >