Re: lists of pairs

*To*: mathgroup at smc.vnet.net*Subject*: [mg8511] Re: [mg8486] lists of pairs*From*: jpk at max.mpae.gwdg.de*Date*: Thu, 4 Sep 1997 02:20:00 -0400*Sender*: owner-wri-mathgroup at wolfram.com

> Hi all, I have a problem in discarding duplicates from a list of integer > pairs, where if pairs of the form {a, b} and {b,a} exist in the list, I > wish to discard (or select) only one, it doesn't matter which. The method > I use below works, but is slow. > -- shnipp shnipp --- > Suggestions? It is desired to minimize the time it takes to do this. > > Russell Towle > Giant Gap Press: books on California history, digital topographic maps > P.O. Box 141 > Dutch Flat, California 95714 > ------------------------------ > Voice: (916) 389-2872 > e-mail: rustybel at foothill.net > ------------------------------ > > > Hi Russel, here is the timing of Your commands, on my machine: In[15]:= Timing[ c=Table[ Position[edges, x_ /; x == edges[[i]] || x == Reverse[edges[[i]]] ], {i,Length[edges]}]; f=Flatten[Map[Take[#,1]&, Union[c]],2] ] Out[15]= {1.69 Second,{1,2,3,4,5,7,8,9,10,12,13,14,17,18,19,22,23,24,28,29,32,33,34,38, 39,43,44,48,49,53}} And here the timing of my version: In[14]:= Timing[ sedges=Sort /@ edges; (* make a sorted copy of edges *) dbls={}; (* here we will collect the double pairs *) sedges //. {a___,i_,b___} /; MemberQ[{a,b},i] :> ( AppendTo[dbls,Position[sedges,i]]; {a,b} ); Flatten[First/@ dbls,2] ] Out[14]= {0.09 Second,{1,2,3,4,5,7,8,9,10,12,13,14,17,18,19,22,23,24,28,29,32,33,34,38, 39,43,44,48,49,53}} Hope that speed increasment is ok for You. Jens