Re: combinations of n objects 2 at a time

*To*: mathgroup at smc.vnet.net*Subject*: [mg15215] Re: [mg15188] combinations of n objects 2 at a time*From*: Des Penny <penny at suu.edu>*Date*: Tue, 22 Dec 1998 04:01:45 -0500*Organization*: Southern Utah University*References*: <199812180711.CAA03934@smc.vnet.net.>*Sender*: owner-wri-mathgroup at wolfram.com

Dear Albert: The following can easily be developed as a function that handles n objects taken k at a time. Here is the code for n objects, taken 2 at a time: In[1]:= n=6; t=Range[n]; t1=Flatten[Outer[List,t,t],1]; t2=Map[Sort,t1]; t3=Union[t2]; t4=Cases[t3,{x_,y_}/;(x=!=y)] Out[1]= {{1,2},{1,3},{1,4},{1,5},{1,6},{2,3},{2,4},{2,5},{2,6},{3,4},{3,5},{3,6},{4, 5},{4,6},{5,6}} In[2]:= Length[t4] Out[2]= 15 By the way, you have an error in your Diag[] function in your other posting: Diag[matrix_List]:= Table[If[i==j,matrix\[LeftDoubleBracket]i,j\[RightDoubleBracket],0],{i, Length[matrix]},{j,Length[matrix]}] Cheers, Des Penny Physical Science Dept Southern Utah University Cedar City, Utah 84720 Voice: (435) 586-7708 FAX: (435) 865-8051 Email: penny at suu.edu Albert Maydeu-Olivares wrote: > Hello, > > In order to generate all combinations of n objects 2 at a time I load > the `DiscreteMath` add-on. > > <<DiscreteMath`Combinatorica` > > l[n_]:=rt[Ksubsets[Reverse[Table[i,{i,n}]],2]] > > So that with n=objects I get > > l[6] > > {{2,1},{3,1},...,{6,4},{6,5}} > > Any suggestions on how to do this without loading the add-on? > > -- > > Albert Maydeu-Olivares Tel. +34 3 4021079 ext. 3099 Faculty of > Psychology Fax. +34 3 4021362 University of Barcelona E-Mail: > amaydeu at tinet.fut.es Passeig de la Vall d'Hebron, 171. > 08035 - Barcelona (Spain)

**References**:**combinations of n objects 2 at a time***From:*"Albert Maydeu-Olivares" <amaydeu@nil.fut.es>