Re: Pattern matching
- To: mathgroup at smc.vnet.net
- Subject: [mg116228] Re: Pattern matching
- From: Richard Fateman <fateman at cs.berkeley.edu>
- Date: Mon, 7 Feb 2011 06:07:02 -0500 (EST)
- References: <iilth1$2mk$1@smc.vnet.net>
On 2/6/2011 2:35 AM, StatsMath 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!
>
it looks to me like
fact[0] is replaced by 0*fact[-1] which is simplified to 0. the other
rule isn't seen.