Re: lists of pairs

*To*: mathgroup at smc.vnet.net*Subject*: [mg8525] Re: [mg8486] lists of pairs*From*: "C. Woll" <carlw at u.washington.edu>*Date*: Thu, 4 Sep 1997 02:20:25 -0400*Sender*: owner-wri-mathgroup at wolfram.com

Hi Russell, How about Union[Sort/@edges] Carl Woll Dept of Physics U of Washington On Tue, 2 Sep 1997, Russell Towle wrote: > 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. > > First, here is a sample list of integer pairs: > > edges = {{16, 10}, {10, 4}, {4, 9}, {9, 15}, {15, 16}, {16, 15}, {15, 18}, > {18, 20}, {20, 19}, {19, 16}, {16, 19}, {19, 14}, {14, 8}, {8, 10}, {10, > 16}, {10, 8}, {8, 3}, {3, 1}, {1, 4}, {4, 10}, {4, 1}, {1, 2}, {2, 7}, {7, > 9}, {9, 4}, {9, 15}, {15, 18}, {18, 13}, {13, 7}, {7, 9}, {19, 20}, {20, > 17}, {17, 12}, {12, 14}, {14, 19}, {8, 14}, {14, 12}, {12, 6}, {6, 3}, {3, > 8}, {1, 3}, {3, 6}, {6, 5}, {5, 2}, {2, 1}, {7, 2}, {2, 5}, {5, 11}, {11, > 13}, {13, 7}, {18, 13}, {13, 11}, {11, 17}, {17, 20}, {20, 18}, {12, 17}, > {17, 11}, {11, 5}, {5, 6}, {6, 12}} > > It is this long: > In[35]:= > Length [ edges ] > Out[35]= > 60 > > However, this is twice as long as I want, for every pair {a, b} has an > unwanted opposite {b, a}. > > This is my present method: > > (*find duplicates (indices in reverse order)*) > (*7.60 seconds*) > c=Table[ > Position[edges, x_ /; x == edges[[i]] || x == Reverse[edges[[i]]] ], > {i,Length[edges]}]; > > Now I have the positions of all duplicates, twice over, so I use Union and > Take the First elements. > f=Map[Take[#,1]&, Union[c]] > > And, this is the final list I wanted from the beginning: > f=Flatten[f,2] > > 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 > ------------------------------ > > > >