Re: Pattern matching
- To: mathgroup at smc.vnet.net
- Subject: [mg116218] Re: Pattern matching
- From: Roland Franzius <roland.franzius at uos.de>
- Date: Mon, 7 Feb 2011 06:05:07 -0500 (EST)
- References: <iilth1$2mk$1@smc.vnet.net>
Am 06.02.2011 11:35, schrieb StatsMath:
> 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?
Using
Trace[fact[4] //. {fact[n_] :> (Print["a:", n]; n fact[n - 1]),
fact[0] :> (Print["b:", n]; 1)}]
you see the second rule is never used. The first rule at
fact[0] -> 0 fact[-1]
evaluates the expression to 0 and there is no further rule for "0".
--
Roland Franzius