combine elements of a large vector in matlab

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Swagatika
Swagatika 2012-11-22
I need to take pairwise combination of a large vector v (770x1). The commands combnk and combntns come with a precaution that they are slow for large vectors.
>> tic, a = combnk(v,2); toc
Elapsed time is 3.081594 seconds.
>> tic, a = combntns(v,2); toc
Elapsed time is 3.147092 seconds.
Is there any other alternative that is more efficient?

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Matt Fig
Matt Fig 2012-11-22
编辑：Matt Fig 2012-11-22
>> tic, A = nchoosek(1:770,2); toc
Elapsed time is 2.286846 seconds.
>> tic, B = combnk(1:770,2); toc
Elapsed time is 2.299141 seconds.
>> tic, C = combinator(770,2,'c','no'); toc
Elapsed time is 0.019995 seconds.
>> isequal(sortrows(A),sortrows(B),sortrows(C))
ans =
1
COMBINATOR is found here: File Exchange File.
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Matt Fig
Matt Fig 2012-11-22
编辑：Matt Fig 2012-11-22
I bet you didn't bother to read the help yet. If you have another vector than 1:770, simply use the output from COMBINATOR as an index into your vector.
>> A = [9 8 7];
>> A(combinator(length(A),2,'c','n'))
ans =
9 8
9 7
8 7
Also, if your question has been answered, please accept the answer. Thanks.

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