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Re: Pattern matching

  • To: mathgroup at smc.vnet.net
  • Subject: [mg116217] Re: Pattern matching
  • From: "Mr. Wizard" <gleam at flashmail.com>
  • Date: Mon, 7 Feb 2011 06:04:56 -0500 (EST)

>Have a question regarding the applicaiton of pattern rules:
>
>fact[4] //. {fact[n_] :> n fact[n-1], fact[0] -> 1}
>
>This is a bug since fact[0] needs to be defined 1st, but I expected
>the above to grind away idefnitely but it returned a a value 0,
>instead of an infininte computation.
>
>Can you help me understand why the above returns 0?
>
>Thanks!


When fact[0] occurs in the expression, it is matched by your first 
rule, and so becomes 0 * fact[0 - 1].

In:= 0 * "some random expression"
Out= 0

Paul 




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