RE: Pattern Matching in Lists

*To*: mathgroup at smc.vnet.net*Subject*: [mg35560] RE: [mg35547] Pattern Matching in Lists*From*: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>*Date*: Fri, 19 Jul 2002 06:08:03 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

> -----Original Message----- > From: Anthony Mendes [mailto:amendes at zeno.ucsd.edu] To: mathgroup at smc.vnet.net > Sent: Thursday, July 18, 2002 9:07 AM > Subject: [mg35560] [mg35547] Pattern Matching in Lists > > > Hello, > > Suppose w={1,1,1,0,0,1,0,1,0,0,1,0,0}. > > How can I count the number of occurrences of a 1 in w immediately > followed by a 0 in w? > > I have tried every incarnation of Count[] I can think of; for example, > > Count[w,{___,1,0,___}] > > does not seem to work. In general, how can I count the number of > occurrences of a 1 followed by a 0 in a list of 1's and 0's? > Thank you! > > > -- > Tony > _____________________ > amendes at math.ucsd.edu > > Tony, there are certainly many solutions (and many will appear here in this newsgroup). Just to pass some ideas Count[w /. {a___, 1, 0} :> {a, {1, 0}} //. {a___, 1, 0, b__} :> {a, {1, 0}, b}, {1, 0}] Count[w //. {a___, 1, 0, b___} :> {a, \[HappySmiley], b}, \[HappySmiley]] Length[ReplaceList[w, {a___, 1, 0, b___} :> {a, {}, b}]] specialCount[w_] := Module[{ww = w //. {a___, x_, x_, b___} :> {a, x, b}}, If[ww[[1]] === 0, Quotient[Length[ww] - 1, 2], Quotient[Length[ww], 2]]] this will lead to another idea: specialCount2[w_] := With[{l = Length[Split[w]]}, If[w[[1]] === 1, Quotient[l, 2], Quotient[l - 1, 2]]] The last two methods wouldn't work if elements other than 0 or 1 were present, so best use a pattern test for w w_?VectorQ[#, MatchQ[#, 0 | 1] &] & w_?VectorQ[#, IntegerQ[#] && 0 <= # <= 1 & ] & We may also convert the problem to the numeric domain specialCount3[w_] := Block[{t = 1, c = 0}, NestWhile[(If[Mod[#, 2] == 0, t = 0, If[t === 0, ++c; t = 1]]; Quotient[#, 2]) &, FromDigits[w, 2], Positive]; c] -- Hartmut