MATLAB: Substract nd array slices

Question

Hi,

I have a n-dimensional array which may contain singleton dimensions and I would like to substract the second page of dimension i from the first page of dimension i. My current solution looks something like this:

function A = substract_page( A, dim )
  orig_size = size(A);    % store the original shape
  A = shiftdim(A, dim);   % shift the dimension to be substracted to the end
  shifted_size = size(A); % aliter: shifted_size = circshift orig_size(dim); ?
  A = reshape(A, [], shifted_size(end));  % collapse first dimensions for easier substraction
  A = A(:, 2) - A(:, 1);
  shifted_size(end) = shifted_size(end) - 1;  % substraction leads to dimension loss
  A = reshape(A, shifted_size);               % restore shape
  lost_dimensions = length(orig_size) - ndims(A);
  A = shiftdim(A, -lost_dimensions);         % make sure that  singular dimensions  in the end don't get lost
  A = shiftdim(A, length(orig_size) - dim + lost_dimensions);    % restore order
end

But it seems way to complicated and I feel there is a much easier and more elegant solution.

Answer 1

Does your code perform this:

dim = 3;
A   = rand(2,3,4,5);
A   = A(:, :, 2, :) - A(:, :, 1, :)

? Then:

function A = substract_page(A, dim)
C1      = repmat({':'}, 1, ndims(A));
C2      = C1;
C1{dim} = 1;
C2{dim} = 2;
A       = A(C2{:}) - A(C1{:});
end