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How to implement following in matlab?

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Pramod Devireddy
Pramod Devireddy on 1 Jul 2015
Edited: Thorsten on 2 Jul 2015
suppose I have a matrix:
A=[1 2;2 2;5 2;6 2;3 1;4 1]
I want the matrix: B=[1 2;1 5;1 6;2 5;2 6;5 6] in 1st iteration which are the combinations of first column elements of rows that have the second column as 2. and B=[1 3;2 3;5 3;6 3;1 4;2 4;5 4; 6 4;] in the second iteration which are the combinations of first column elements of rows that have the second column as 2 and 1. and B=[3 4] in the third iteration which are the combinations of first column elements of rows that have the second column as 1.

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dpb
dpb on 1 Jul 2015
I can't figure out what your description means by comparing to the above B and A. Show us precisely the rule for the rearrangement.
Jan
Jan on 2 Jul 2015
I do not understand the question.

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Answers (3)

Mark Matusevich
Mark Matusevich on 2 Jul 2015
Make a function which gets a vector and creates all combinations, then call it 3 times:
B1 = combinations(A(1,A(:,2)==2));
B2 = combinations(A(1,ismember(A(:,2),[1 2])));
B2 = combinations(A(1,A(:,2)==1));
Implement the 'combinations' function using 'ndgrid' and
tril(ones(n,n),-1)

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Pramod Devireddy
Pramod Devireddy on 2 Jul 2015
But i want the matrix B to change in iterations and i want to implement it for a bigger matrix in which the second column elements can start from higher number not just 2.

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Pramod Devireddy
Pramod Devireddy on 2 Jul 2015
Edited: Pramod Devireddy on 2 Jul 2015
is there any way that i can improve it:
A=[1 2;2 2;5 2;6 2;3 1;4 1];
[svals,idx] = sort(A(:,2),'descend');
f=sort(unique(svals),'descend');q=1;m=[];
for i=1:size(f,1)
for k=q:size(f,1)
m=[m;f(i),f(k)];
end
q=q+1;
end
for j=1:size(m,1)
if m(j,1)==m(j,2)
B=combnk(A(idx((svals==m(j,1))),1),2)
else
B=combvec(A(idx((svals==m(j,1))),1)',A(idx((svals==m(j,2))),1)')'
end
end

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Thorsten
Thorsten on 2 Jul 2015
Edited: Thorsten on 2 Jul 2015
You can do this using nchoosek and ismember:
A = [1 2;2 2;5 2;6 2;3 1;4 1];
col2val{1} = 2;
col2val{2} = [1 2];
col2val{3} = 1;
B = A;
for i = 1:numel(col2val);
x = A(ismember(A(:,2), col2val{i}), 1)
if numel(x) < 2, break; end % cannot generate pairs of less than 2 numbers
pairs = nchoosek(1:numel(x), 2);
B = x(pairs)
end

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