combine elements of a large vector in matlab

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?

Answer 1

[Matt Fig]

>> 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.

[Swagatika]

thanks. That is really fast. But my problem is a bit different from this...the vector is not from 1 to 770, it has different elements which I need to combine, not exactly 1:770.

[Matt Fig]

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.

[Swagatika]

thanks. Just figured that out :)