is there an alternative to nchoosek which is too slow?

Try cnk = nchoosek(1:40, 10) and you'll see how slow it is. Is there an alternative?

Thanks

Answer 1

[dpb]

Well, you're asking for 40!/(10! 30!) elements -->

>> N=prod(31:40)/factorial(10)
N =
 847660528
>>

elements so it's not terribly surprising it might just take a while for the scribes to write 'em all down...

It boils down in the end to

function P = combs(v,m)
    n=length(v);
    P = [];
    if m < n && m > 1
        for k = 1:n-m+1
            Q = combs(v(k+1:n),m-1);
            P = [P; [v(ones(size(Q,1),1),k) Q]]; %#ok
        end
    end
end

which has a big problem in time in that P isn't preallocated.

ADDENDUM

Unfortunately, combnk has the same flaw using almost identically the same code.

Whether somebody has supplied mex file or other solution on FEX I didn't research.

[Matt J]

Well, you're asking for 40!/(10! 30!) elements

Ten times that, in fact. 40!/(10! 30!) is the number of combinations, but there are 10 elements in each combination. Even in uint8, it will be an 8GB array.

[dpb]

Yes, I meant the combinations, not elements.

Answer 2

[Andy]

not if some sort of (in-memory?) compression is applied ... if that's even possible. but looking at the brutal return of say nchoosek(25,15) - there might be a good potential for tokenization of repeating patterns - if that makes sense.

thanks