How to create an array out of a matrix based on criteria.

Suppose i have a matrix of

a=[1,2; 
   1,3;
   1,4;
   2,12;
   2,15;
   5,7;
   5,8;
   6,98;
   6.99;
   7,8;
   7,9;
   7,11;
   9,14;
   9,16;
   12,18;
   14,20;
   20,35;
   98,102;
   99,204;
  204,300];

I want to create arrays arrays that will contain elements mentioned in the pair.

b=[1,2,3,4,12,15,18]
c=[5,7,8,9,14,16,20,35]
d=[6,98,99,102,204,300]

The idea is that if 2 is in a pair of 1, then every other element in pair with 2 will end up in the b array. So on and so forth for different categories, b,c,d,.... How can I achieve that kind of iteration in Matlab?

Answer 1

[Thorsten]

b = a(1,:); c = []; 
for i = 2:size(a, 1)
  if ~isempty(intersect(a(i,:), b))
    b = union(b, a(i,:));
  else
    c = union(c, a(i,:));
  end
end

[DuckDuck]

Thanks Thorsten, but this solution does not generalise. What if i'll have more b, c, d, e?

[Thorsten]

Oh, you changed the question. That's not fair! (Just kidding.) The generalized solution is

b{1} = a(1,:);  
for i = 2:size(a, 1)
  bi = 1;
  while bi <= numel(b) && isempty(intersect(a(i,:), b{bi}))
    bi = bi + 1;
  end
  if bi <= numel(b) % append a(i,:) to b{bi}
    b{bi} = union(a(i,:), b{bi});
  else % create bi'th entry in cell b
    b{bi} = a(i, :);
  end
end

for bi = 1:numel(b)
  disp(b{bi})
end