MATLAB：从选定的矩阵元素构造矩阵

Question

I have a 3x3x3 matrix which contains a particular set of elements that I would like to extract. However, I would like for the elements to be ordered in a matrix after selecting them. An example is:

a(1,:,:)=1*[1 2 3; 4 5 6; 7 8 9];
a(2,:,:)=2*[1 2 3; 4 5 6; 7 8 9];
a(3,:,:)=3*[1 2 3; 4 5 6; 7 8 9];

a(a>1.0)

The condition a(a>1.0) gives me a vector of elements, but is there a way to order them in a matrix following their original ordering?

Answer 1

What would you like to do to the elements that don't satisfy your criteria?

You could do something like a(a<=1) = nan;

Answer 2

Suppose 2-D matrix for simplicity:

a = [1 2 3; ...
     4 5 6];

Let's take only even values and keep them in their original shape:

a(mod(a, 2) == 0)

You might want:

  2
4   6

However, in the world of matrix, is there such a matrix which has empty space? Yes, a sparse matrix. But, you must note that a sparse matrix is filled with 0 not just missing.

So, my suggestion is to replace other values with NaN

b = a; % just make a duplicate
b(mod(b, 2) == 1) = nan
b =
    nan    2    nan
      4  nan      6