frequency of 2D matrix in 3D matrix

I have a 3D matrix of size 100X100X40. That means I have 40 100X100 2D matrices. I want to know the number of occurances of each 2D matrix in the 3D matrix.

This is somewhat similar to the unique command we have for elements in 1D array.

How to implement it for 2 D matrices??

Answer 1

[Jos (10584)]

Convert the 3D N-by-M-by-Z matrix into a 2D Z-by-(N*M) matrix and use unique with the rows option on that. An example:

M3D = cat(3, [1 1 ; 1 1], [1 1; 1 1], [2 2 ; 2 2], [1 1 ; 1 1]) 
M2D = reshape(M3D, [], size(M3D, 3)).'
[~, i]  = unique(M2D, 'rows') ;
Mun3D = M3D(:,:,i)
% which you can also do in an incomprehensible one-liner ;-)
% Mun3D = reshape(transpose(unique(transpose(reshape(M3D, ..)) , ..)), ..)