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