Asked by luis
on 18 Sep 2013

How do I create a vector combinations in pairs ,with no repeated elements ,for example :

A(1,2,3,4)

1,2

1,3

1,4

2,1

2,3

2,4

3,1

3,2

3,4

4,1

4,2

4,3

I tried to use some commands like : perms And combnk , thanks in advanced

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Answer by Azzi Abdelmalek
on 18 Sep 2013

Edited by Azzi Abdelmalek
on 18 Sep 2013

Accepted answer

a=fliplr(fullfact([4 4])) a(~diff(a')',:)=[]

or

[ii,jj]=ndgrid(1:4,1:4); a=[jj(:) ii(:)]; a(~(a(:,1)-a(:,2)),:)=[]

Azzi Abdelmalek
on 19 Sep 2013

Answer by Roger Stafford
on 19 Sep 2013

What you are asking for in this comment are known as the partial permutations. I don't know if matlab has such a routine but you can use 'nchoosek' and 'perms' to create one. Let A be a row vector of n elements and let the number of these to be selected in each permutation be called r.

c = nchoosek(A,r)'; ncr = size(c,2); p = perms([1:r]); pr = size(p,1); p = reshape(p',1,[]); B = zeros(ncr*pr,r); for k = 1:ncr B((k-1)*pr+1:k*pr,:) = reshape(c(p,k),r,[])'; end

B will be the desired list of partial permutations.

Answer by Roger Stafford
on 19 Sep 2013

Here is a more compact way of using 'nchoosek' and 'perms'.

c = nchoosek(A,r)'; B = reshape(c(perms(1:r)',:),r,[])';

where A, r, and B are as before.

Answer by Andrei Bobrov
on 19 Sep 2013

Edited by Andrei Bobrov
on 19 Sep 2013

d = fullfact([4 4]); out = d(diff(d,[],2)~=0,:);

and

A = [8 2 9 6 1]; n = 3; ix = fullfact(ones(1,n)*numel(A)); out = A(ix(all(diff(sort(ix,2),[],2),2),:));

and using the ideas by Roger Stafford (they very nice)

c = nchoosek(A,r)'; p = perms([1:r]); s = size(c); c(reshape(bsxfun(@plus,p',reshape((0:s(2)-1)*s(1),1,1,[])),s(1),[])');

Answer by Jos (10584)
on 19 Sep 2013

The simplest way is to create all N*N combinations and weed out the N ones that have the same value.

N = 4 ; A = 1:N ;

[b2,b1] = ndgrid(A) ; % generalization q = b1~=b2 ; % q = ~eye(N) ; % they are all on the diagonal B = [b1(q) b2(q)]

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