Re: Pattern matching

*To*: mathgroup at smc.vnet.net*Subject*: [mg116245] Re: Pattern matching*From*: DrMajorBob <btreat1 at austin.rr.com>*Date*: Tue, 8 Feb 2011 05:06:07 -0500 (EST)

Leonid, I've been caught in thousands of errors, but as far as I know, this is your first. Don't let it get to be a habit! Bobby On Mon, 07 Feb 2011 13:11:51 -0600, Leonid Shifrin <lshifr at gmail.com> wrote: > 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 >> -- DrMajorBob at yahoo.com