Your b = cat(1, a(:, :, i)) is incorrect and is just equivalent to b = a(:, :, i). If you were to use a loop you could do it like this:
b = [];
for i = 1:size(a, 3)
b = [b; a(:, :, 1)]; %which is cat(1, b, a(:, :, i))
end
This would be extremely innefficient and matlab's editor will warn you that you're resizing b on every iteration. The proper way to do it in matlab is:
b = reshape(permute(a, [1 3 2]), [], size(a, 2))
without a loop.
