Re: Google's aptitude test equation

*To*: mathgroup at smc.vnet.net*Subject*: [mg55030] Re: [mg55004] Google's aptitude test equation*From*: DrBob <drbob at bigfoot.com>*Date*: Thu, 10 Mar 2005 05:24:30 -0500 (EST)*References*: <200503091134.GAA06997@smc.vnet.net>*Reply-to*: drbob at bigfoot.com*Sender*: owner-wri-mathgroup at wolfram.com

Here's a minor rewrite: prob = "wwwdot" - "google" == "dotcom"; eqn = prob /. s_String :> FromDigits@ToExpression@Characters@s; vars = Variables[Subtract @@ eqn]; ineqs = And @@ Thread[0 <= vars <= 9]; Timing[sol = Reduce[eqn && ineqs, vars, Integers];] Length[sol] Timing[Select[sol, Unequal @@ #[[All,2]] & ]] {8.875 Second,Null} 449 {0.*Second, (c == 4 && d == 5 && e == 3 && g == 1 && l == 0 && m == 6 && o == 8 && t == 9 && w == 7) || (c == 4 && d == 5 && e == 6 && g == 1 && l == 0 && m == 3 && o == 8 && t == 9 && w == 7)} If we impose e < m (since e and m can be interchanged), it goes faster: (sol = Reduce[eqn && ineqs && e < m, vars, Integers];) // Timing Length@sol Select[sol, Unequal @@ #[[All, 2]] &] // Timing {5.3910000000000196*Second, Null} 204 {0.*Second, c == 4 && d == 5 && e == 3 && g == 1 && l == 0 && m == 6 && o == 8 && t == 9 && w == 7} Bobby On Wed, 9 Mar 2005 06:34:29 -0500 (EST), Lorenzo Castelli <gcastelli at NOSPAMracine.ra.it> wrote: > Hi everybody, I have a little question about Mathematica (version 5.1). > Please note that I just began to use this program, so sorry if I'm missing > some trivial points. > > As an exercise I've tried to resolve the Google's equation that appeared on > the web some time ago, using this page as a reference: > http://mathworld.wolfram.com/news/2004-10-13/google/ > > The text of the exercise is: > 1. Solve this cryptic equation, realizing of course that values for M and E > could be interchanged. No leading zeros are allowed. > WWWDOT - GOOGLE = DOTCOM > > I've implemented the solution in Mathematica in this way: > > - Setup the problem: > prob = "wwwdot" - "google" == "dotcom" > > - Setup the equation associated with the problem: > eqn = prob /. s_String :> FromDigits[ToExpression[Characters[s]], 10] > > - Extract the variables of the equation: > vars = Union[Cases[eqn, _Symbol, Infinity]] > > - Setup the constraints: > ineqs = And @@ (0 <= # <= 9 & /@ vars) > > - Resolve the equation: > (sol = Reduce[eqn && ineqs, vars, Integers]) // Timing > > This operation gives 449 solutions, calculated in roughly 13 seconds. > Only two of these solutions are correct, since I have to remove the > solutions which yield more than one variable to have the same value. > > Obviously this task is pretty easy, so I should expect it to take just a > short time to compute. > And that is actually the case if use a code like this: > > Select[sol, Unequal @@ (Part[#, 2] & /@ Cases[#, _Equal, 1]) &] // Timing > > The two solutions are extracted in a fraction of a second. > > On the contrary if I try to do that in the following way, Mathematica starts > a long session of evaluation and eventually crashes after some minutes: > > Reduce[sol && Unequal @@ vars] // Timing > > I understand that Reduce might be too general to perform this operation in > an efficient way, but I haven't been able to find a more suitable function. > > Also I've noticed that if I change the constraints on the single variables > for the first Reduce in order to explicity consider that no leading zeros > are allowed (so just replacing the 0<=x<=9 with 0<x<=9 for the variables w, > g and d), the evaluation time jumps from 13 seconds to 40 seconds. > That's odd, as in this case the new solutions are obviously just an easily > computable subset of the previous ones. > > So I don't really know if Reduce is actually the best function even in the > first usage. > > Can you help me out? > > Thanks, > > Lorenzo Castelli > E-Mail: gcastelli at racine.ra.it > > > > -- DrBob at bigfoot.com

**References**:**Google's aptitude test equation***From:*"Lorenzo Castelli" <gcastelli@NOSPAMracine.ra.it>

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