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24 Feb 2011 (Updated )

Choose K indices from the set 1:N.

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ICHOOSE(N,K) gives all combinations of choosing K indices
from the set 1:N without order and without repetitions.

ind = ichoose(4,2);
v = 'ABCD';
v(ind) % ['AB';'AC';'AD';'BC';'BD';'CD']

ICHOOSE is certainly not unique. Here is a comparison with other codes
using TIMEIT by Steve Eddins. Note that VCHOOSEK is a MEX-code.

f1 = @() nchoosek(1:24,8);
f2 = @() combinator(24,8,'c');
f3 = @() ichoose(24,8);
f4 = @() VChooseK(int8(1:24),8);

timeit(f1) % 16.1 sec (NCHOOSEK by MathWorks)
timeit(f2) % 1.17 sec (COMBINATOR by Matt Fig)
timeit(f3) % 0.110 sec (ICHOOSE by Jonas Lundgren)
timeit(f4) % 0.066 sec (VCHOOSEK by Jan Simon)


Combinator Combinations And Permutations and V Choose K inspired this file.

MATLAB release MATLAB 7.10 (R2010a)
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Comments and Ratings (2)
18 Mar 2011 Jonas Lundgren

Jan, thank you for the feedback. The compiler is the one that comes with Matlab 2010a. Sorry if that is a slow one. I rarely use MEX-code. I am fully satisfied if ichoose is five times slower than VChooseK.

18 Mar 2011 Jan Simon

A very nice implementation. The speed difference to Matlab's NCHOOSEK is dramatic.
H1 line, short but descriptive help text, author and date mentioned, check of inputs => 5 stars.

How did you compile VChooseK.c? If I use MSVC2008 the Mex I get these timings (Matlab 2009a, 1.5GHz Pentium-M, WinXP 32):
tic; q=ichoose(24, 8);toc
=> 0.255sec
tic; q=VChooseK(uint8(1:):uint8(24), 8); toc
=> 0.049sec

21 Nov 2011

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