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MathGroup Archive 2002

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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


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