Re: Pattern matching
- To: mathgroup at smc.vnet.net
- Subject: [mg116244] Re: Pattern matching
- From: Leonid Shifrin <lshifr at gmail.com>
- Date: Tue, 8 Feb 2011 05:05:56 -0500 (EST)
Bobby, Yes, indeed, you are right. Thanks for the correction! Regards, Leonid On Mon, Feb 7, 2011 at 9:39 PM, DrMajorBob <btreat1 at austin.rr.com> wrote: > No... the rule is never applied to fact[-1]. > > If it were, there WOULD be an infinite loop, since fact[-1] -> -1 fact[-2] > (not -1 times the previous zero). > > The previous zero, multiplied by fact[-1] (unevaluated) is zero, and > there's no rule for zero, so ReplaceRepeated stops. > > Bobby > > > On Mon, 07 Feb 2011 05:07:14 -0600, Leonid Shifrin <lshifr at gmail.com> > wrote: > > This is not a bug. The second rule indeed has no chance to execute. The >> reason that this does not iterate infinitely is that when the first rule >> applies for n = 0, we get zero as a result. The next time the rule is >> applied for n = -1, we get zero again, since we multiply -1 and the >> previous >> zero. Since the two consecutive results are the same, ReplaceRepeated >> stops. >> >> I used a similar example in my book, where I made the same statement that >> we should expect infinite iteration, which is apparently incorrect for >> this >> particular problem, as you just pointed out. Will add this to a list of >> things I have to correct. >> >> Regards, >> Leonid >> >> >> On Sun, Feb 6, 2011 at 1:35 PM, 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! >>> >>> >>> > > -- > DrMajorBob at yahoo.com >