Question

I have a variable dimension matrix, X. I want a function that will get the first half of X in one dimension. I.E., I want something like this:

function x = variableSubmatrix(x, d)
    if d == 1
        switch ndims(x)
            case 1
                x = x(1:end/2);
            case 2
                x = x(1:end/2, :);
            case 3
                x = x(1:end/2, :, :);
            (...)
        end
    elseif d == 2
        switch ndims(x)
            case 2
                x = x(:, 1:end/2);
            case 3
                x = x(:, 1:end/2, :);
            (...)
        end
    elseif (...)
    end
end

I’m not quite sure how to do this. I need it to be fast, as this will be used many times in computation.

Answer 1

This should do the trick:

function x = variableSubmatrix(x, d)
  index = repmat({':'},1,ndims(x));  %# Create a 1-by-ndims(x) cell array
                                     %#   containing ':' in each cell
  index{d} = 1:size(x,d)/2;          %# Create an index for dimension d
  x = x(index{:});                   %# Index with a comma separated list
end

The above first creates a 1-by-ndims(x) cell array with ':' in each cell. The cell corresponding to dimension d is then replaced with a vector containing the numbers 1 through half the size of dimension d. Then, the contents of the cell array are output as a comma-separated list (using the {:} syntax) and used as the indices into x. This works because ':' and : are treated the same way (i.e. “all elements of this dimension”) when used in an indexing statement.