Collect conditional matrices from a loop in a 2D or cell array

For a project, I need to draw millions of random 3x3 matrices, take the QR composition of them, transpose the Q matrix and multiply it with a given matrix. Of the resulting matrices (C), I need to store all these matrices that fulfill certain conditions (BB) to do statistical analysis with them (I prefer do that with Excel, so I would export the total array to excel).

My question is how to store these BB matrices from a loop without the last one overwriting the previous one. So far, I have:

B  =[...
    7.797562562, -0.832787948, -1.725054903;...
    2.11093262, 3.138528042, -0.326926679;...
    2.128339023, -0.061787665, 6.644309749];

    for k = 1 : 50;
      rd=rand(3);
      [Q,R]=qr(rd);
      D=Q';
      C=B*D;  
      if C(1,1)>0 && C(2,2)>0 && C(3,3)>0 && C(3,1)<0 && C(3,2)>0,
        BB=C; 
      end  
        
    end

Resulting is a collection (less than the amount of loops) of BB matrices, which I want to export. However, where I save them (save test.mat BB), the last BB matrix overwrites the other one and when I use subscripts to define the loop number, I get this error:

???  In an assignment  A(I) = B, the number of elements in B and
 I must be the same.

Anybody that can help me? I guess it shouldn´t be too difficult, so any answer could help me out!

Answer 1

[José-Luis]

Sounds like you need a cell array

B  =[...
      7.797562562, -0.832787948, -1.725054903;...
      2.11093262, 3.138528042, -0.326926679;...
      2.128339023, -0.061787665, 6.644309749];
  numSim = 50;
  results = cell(numSim,1);
  counter = 1;
      for k = 1 : numSim;
        rd=rand(3);
        [Q,R]=qr(rd);
        D=Q';
        C=B*D;  
        if C(1,1)>0 && C(2,2)>0 && C(3,3)>0 && C(3,1)<0 && C(3,2)>0,
                    results(counter) = {C};
                    counter = counter + 1;         
                    %BB=C; No longer needed

        end  
          
      end
  results(counter + 1 : end) = [];
  %All your results will be in the cell array

And if you need to extract a particular result

BB = results{ii};

Note that you could also save inside the loop, but since C is small, it is probably better this way.