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Re: Pattern matching

  • To: mathgroup at smc.vnet.net
  • Subject: [mg116222] Re: Pattern matching
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Mon, 7 Feb 2011 06:05:52 -0500 (EST)

Because 

fact[0] -> 0 * fact[-1] -> 0 

causing the product to be zero and terminating the recursion.

In slow motion

rules = {fact[n_] :> n fact[n - 1], fact[0] -> 1};

fact[4] /. rules

4 fact[3]

% /. rules

12 fact[2]

% /. rules

24 fact[1]

% /. rules

24 fact[0]

% /. rules

0

Bob Hanlon

---- StatsMath <stats.math8 at gmail.com> wrote: 

=============
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!



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