Re: pattern matching: rules that stop other rules?
- To: mathgroup at smc.vnet.net
- Subject: [mg71606] Re: pattern matching: rules that stop other rules?
- From: Peter Pein <petsie at dordos.net>
- Date: Sat, 25 Nov 2006 05:36:41 -0500 (EST)
- References: <email@example.com>
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. > Hi, see http://documents.wolfram.com/mathematica/functions/Attributes "If" has the attribute HoldAll set, which explains the described behaviour. Peter