Use a vector as an index to a matrix

Question

I'm writing a MATLAB function to read out data into an n-dimensional array (variable dimension size). I need to be able to access a specific point in the Matrix (to write to it or read it, for example), but I don't know ahead of time how many indexes to specify.

Currently I have a current_point vector which I iterate through to specify each index, and a max_points vector which specifies the size of the array. So, if for example I wanted a 3-dimensional array of size 1000-by-15-by-3, max_points = [1000 15 3], and current_point iterates from [1, 1, 1] to [1000, 15, 3] ([1, 1, 1] -> [1000, 1, 1] -> [1, 2, 1] -> [1000, 2, 1] ->...). What I'd like to be able to do is feed current_point as an index to the matrix like so:

output_matrix(current_point) = val

But apparently something like output_matrix([1 2 3]) = val will just set outputmatrix(1:3) = 30. I can't just use dummy variables because sometimes the matrix will need 3 indexes, other times 4, other times 2, etc, so a vector of variable length is really what I need here. Is there a simple way to use a vector as the points in an index?

Answer 1

Using the function sub2ind to create a linear index is the typical solution to this problem, as shown in this closely-related question. You could also compute a linear index yourself instead of calling sub2ind.

However, your case may be simpler than those in the other questions I linked to. If you're only ever indexing a single point with your current_point vector (i.e. it's just an n-element vector of subscripts into your n-dimensional matrix), then you can use a simple solution where you convert current_point to a cell array of subscripts using the function num2cell and use it to create a comma-separated list of indices. For example:

current_point = [1 2 3 ...];        % A 1-by-n array of subscripts
subCell = num2cell(current_point);  % A 1-by-n cell array of subscripts
output_matrix(subCell{:}) = val;    % Update the matrix point

The operation subCell{:} creates the equivalent of typing subCell{1}, subCell{2}, ..., which is the equivalent of typing current_point(1), current_point(2), ....

Answer 2

I know it is too late but for anybody who will find this topic. the easiest way that work for me is to use: diag(A (x(:),y(:)) );

unfortunately this works only if you need to get values from the matrix, not for changing values