Re: Summary:Ways to get Odd Columns&Rows of Matrix

*To*: mathgroup at christensen.cybernetics.net*Subject*: [mg715] Re: Summary:Ways to get Odd Columns&Rows of Matrix*From*: Count Dracula <lk3a at kelvin.seas.virginia.edu>*Date*: Tue, 11 Apr 1995 13:25:08 GMT*Organization*: University of Virginia

In Message-ID: <3m6mj3$qnu at news0.cybernetics.net> Robby Villegas wrote: >I think this general coding will work for higher tensors. The levels of >the tensor get rotated around a full cycle, and end up where they were >originally: > everyother[tensor_List] := > With[{n = TensorRank[tensor]}, > Nest[Transpose[Part[#, Range[1, Length[#], 2]], RotateRight @ Range[n]]&, > tensor, n] > ] This somewhat baroque function is equivalent to the simpler: odd[tensor_List] := Part[ tensor, Sequence @@ (Range[1, #, 2] &) /@ Dimensions[tensor] ] The simpler function also works for any tensor and it is faster. The built-in optimized functionality of Part used with several indexing lists is used in odd, but not in everyother. As Robby Villegas points out, everyother is not meant as a practical solution. In[3]:= u = Array[a, {5, 11, 20, 27}]; In[4]:= Timing[odd[u]][[1]] Out[4]= 0.06 Second In[5]:= Timing[everyother[u]][[1]] Out[5]= 1.67 Second In[6]:= everyother[u]==odd[u] Out[6]= True -- ___________________________________________________________________________________ Levent Kitis lk3a at cars.mech.virginia.edu lk3a at kelvin.seas.virginia.edu University of Virginia Department of Mechanical, Aerospace and Nuclear Engineering ___________________________________________________________________________________