Find a subset of unique permutations
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I have many arrays A of varying length. For any given A, I'd like to find all the unique sets of three elements of A. For example, if A has five elements:
A = [5 6 2 4 7];
one combination of three elements in A is [5 6 2], another combination is [5 6 4], and so on. When I work this out manually I get these combinations:
5 6 2
5 6 4
5 6 7
5 2 4
5 2 7
5 4 7
6 2 4
6 2 7
6 4 7
2 4 7
The appearance of the solutions above makes me think there may be a clever way to use an identity matrix, but I can't quite think of how. Or perhaps there's a way to use perms, perhaps with sort and unique?
Can you think of a way to get all 3-element subsets of an array?
3 Comments
Joseph Cheng
on 7 May 2015
hmm... well to use what you ask i've only gotten this far
A=[5 6 2 4 7]
pA = perms(A);
sA = sort(pA(:,1:3),2);
uA =unique(sA,'rows')
but it wouldn't be in the same number order of A.
Accepted Answer
Jos (10584)
on 7 May 2015
Why not use NCHOOSEK?
A = [5 6 2 4 7];
nchoosek(A,3)
ans =
5 6 2
5 6 4
5 6 7
5 2 4
5 2 7
5 4 7
6 2 4
6 2 7
6 4 7
2 4 7
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