How to implement following in matlab?

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.

[dpb]

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]

I do not understand the question.

Answer 1

[Mark Matusevich]

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)

[Pramod Devireddy]

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.

Answer 2

[Pramod Devireddy]

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

Answer 3

[Thorsten]

 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