Re: Match Pairs of Numbers

*To*: mathgroup at smc.vnet.net*Subject*: [mg86921] Re: [mg86879] Match Pairs of Numbers*From*: János <janos.lobb at yale.edu>*Date*: Wed, 26 Mar 2008 04:53:57 -0500 (EST)*References*: <200803250614.BAA10496@smc.vnet.net>

On Mar 25, 2008, at 2:14 AM, Andrew Beveridge wrote: > Any help would be appreciated. > > I am using Mathematica 5.2 & 6.0 > > I am having trouble finding the correct form of Pattern Matching for > pairs of Numbers. I do not know whether to use Select, Cases, Pick, > etc. I am also unsure how to set the pattern matching argument. > > Assume I have the list > > lst = {{1,1}, {1,2}, {1,3}, {1,4}, {2,2}, {3,1}, {3,2}, {3,3}, > {4,1}, {4,4}} > > > I would like to match/select pairs that have the same set of numbers > only reversed, that is, the selection would find > > {{1,3}, {3,1}, {1,4}, {4,1} } > > Thanks for your help > > > Andrew C Beveridge > MS M888 > Los Alamos National Laboratory > Los Alamos, NM 87545 > > 505-665-2092 > e-mail: acbev at lanl.gov > Here is an algo-rhythmic newbie approach :) > In[2]:= > Last[Reap[While[ > Length[lst] >= 1, > i = Random[Integer, > {1, Length[lst]}]; > c = lst[[i]]; > rc = Reverse[c]; > If[Length[Position[lst, > rc]] > 0 && > c != rc, > Sow[{c, rc}]; , Null]; > lst = Delete[lst, > Position[lst, rc]]; > lst = Delete[lst, > Position[lst, c]]; ]]] > Out[2]= > {{{{4, 1}, {1, 4}}, > {{3, 1}, {1, 3}}}} J=E1nos ---------------------------------------------- Trying to argue with a politician is like lifting up the head of a corpse. (S. Lem: His Master Voice)

**References**:**Match Pairs of Numbers***From:*Andrew Beveridge <acbev@lanl.gov>