Re: Filtering a list of list for certain elements that are neighbours

*To*: mathgroup at smc.vnet.net*Subject*: [mg88857] Re: [mg88834] Filtering a list of list for certain elements that are neighbours*From*: "W_Craig Carter" <ccarter at mit.edu>*Date*: Mon, 19 May 2008 05:18:19 -0400 (EDT)*References*: <200805181134.HAA23914@smc.vnet.net>

Hello Steven, I have a solution that you can generalize (and generalize to sublists greater than 3). Here is the short answer: neighboring[a_, b_, list_List] := Or @@ Map[MatchQ[#, Sort[{a, b}]] &, Map[Sort, Rest[Transpose[{list, RotateRight[list]}]]]] notNeighboring[a_, b_, list_List] := Not[neighboring[a, b, list]] Select[mylist, notNeighboring[a, b, #] &] (*returns {{a, c, b}, {b, c, a}} *) Select[mylist, neighboring[a, b, #] &] (*returns {{a, b, c}, {b, a, c}, {c, a, b}, {c, b, a}}*) (*===this method works only for symbols, for strings use StringMatchQ==*) The long answer is something like this: Transpose[{mylist, RotateRight[list]}] (*forms neigboring pairs*) result =Rest[Transpose[{mylist, RotateRight[list]}] ] (*discounts the end matching case*) (*Sort[] ensures that each pair is in a cannonical order*) Map[MatchQ[#,Sort[{a,b}]&,result] (*is a list of true or falses*) Thus, test = Table[RandomChoice[{aa, bb, cc, dd, ww, xx, yy, zz}, 5], {4}] test // MatrixForm (* returns for example {{dd, ww, cc, bb, bb}, {dd, xx, dd, cc, dd}, {zz, aa, aa, aa, cc}, {ww, ww, cc, bb, ww}} *) Select[test, neighboring[ww, cc, #] &] (* returns {{dd, ww, cc, bb, bb}, {ww, ww, cc, bb, ww}} *) On Sun, May 18, 2008 at 7:34 AM, Steven Siew <stevensiew2 at gmail.com> wrote: > I have a list of lists > > mylist={{a,b,c},{a,c,b},{b,a,c},{b,c,a},{c,a,b},{c,b,a}} > > I want to filter out all lists that have "a being a neighbor to b". > > I am looking for the following result. > > {{a,c,b},{b,c,a}} > > > For this toy example, I can probably do it by hand but my real problem > has 120 lists in a list. And I need to filter out various combination > of letters like "s cannot be next to d" , "s cannot be next to j" , "c > cannot be next to j" , "h cannot be next to d" and "h cannot be next > to s". > > -- W. Craig Carter

**References**:**Filtering a list of list for certain elements that are neighbours***From:*Steven Siew <stevensiew2@gmail.com>