Listing elements from one matrix to another

Hello all,

I have a 500 by 1500 matrix.

As an example I got A, a 3 by 9 matrix. The elements displayed are coordinates in form of x y z x y z ...

A = [ 0 0 0 2 1 0 0 0  0; 1 2 0 0 0 0 3 2 0; 0 0 0 2 3 1 0 0 0 ];

I want all relevant elements (x/=0) now listed as below. Also, I do not want any row containg only zeros.

B = [2 1 0; 1 2 0; 3 2 0; 2 3 1]

The order is not important. It is just important that these x y z coordinates stick together and every new point gets a new row.

What I got so far is following code.

    for i=1:size(A,1)
        for j=1:3:size(A,2)-2
if      A(i,j)>0
        B(:,j)=A(i,j);     
        B(:,j+1)=A(i,j+1);
        B(:,j+2)=A(i,j+2);

else    ;   
end
        end
    end

As a result I get following code below.

    B = [1 2 0 2 3 1 3 2 0]

I noticed, that this loop is overwriting numbers of prevoius rows in the same column.

Kind Regards

Answer 1

[Gatech AE]

The formatting in the example you gave us did not translate well; however, the best way to do this can avoid for loops entirely. Using the dimensions of the true matrix, where the columns are a multiple of three, we can do the following.

% Create a column vector of all x,y,z values regardless of matrix size
x = reshape(A(:,1:3:end),[],1);
y = reshape(A(:,2:3:end),[],1);
z = reshape(A(:,3:3:end),[],1);
% Clean up cases where x = 0 with logical indexing, and B is the result of concatenation
mask = x~=0;
B = [x(mask) y(mask) z(mask)];

[Enthusiasten]

Thanks four your answer and your explanation. It worked perfectly. I know this was an easy question, but since my brain stuck in for-loops, I forgot to think 'out of the box'.

Answer 2

[JESUS DAVID ARIZA ROYETH]

A = [0 0 0 2 1 0 0 0 0; 1 2 0 0 0 0 3 2 0; 0 0 0 2 3 1 0 0 0];
B=reshape(A',3,[])';
B(sum(B,2)==0,:)=[]

[Enthusiasten]

Thanks four your answer! It worked like 'Gatech AE' answer perfectly.