calculating all combinations using nchoosek. A faster way?
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Dear all, I have a cell matrix A which is of dimension 13048 by 4 I need to calculate all the combinations using the following
J2=nchoosek(1:size(refin_total_input2,1),2);
but it takes a lot of time
I tried to preallocate
J2=zeros(85118628,2);
J2=nchoosek(1:size(refin_total_input2,1),2);
Is there a faster way of obtaining J2
I use matlaab 2009 64 bit
thanks
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Answers (1)
Jan
on 24 Aug 2012
Edited: Jan
on 24 Aug 2012
The pre-allocation is useless here, because the new values are not inserted, but the original array is overwritten.
Searching the FEX is always a good idea. There you find FEX: VChooseK.mex. I think, this is the fastest implementation, but it needs a compilation. Or you can download the pre-compiled files and install the required runtime libraries of MSVC 2008. An efficient and smart non-Mex version: FEX: combinator.
Beside the speed the C-MEX has the advantage, that it can create the combinations as UINT8/16/32, which occupy less memory than the 8 bytes per double.
2 Comments
Jan
on 24 Aug 2012
Edited: Jan
on 24 Aug 2012
The help text of the file contains the corresponding explanations: You have to install a compiler, run "mex -setup" and "mex -O VChooseK.c" afterwards.
I will include pre-compiled files on my home page as soon as possible (today if I find the time, but at least this year is guaranteed).
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