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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: <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.
> 
Hi,

see http://documents.wolfram.com/mathematica/functions/Attributes

"If" has the attribute HoldAll set, which explains the described behaviour.

Peter