Use a vector to index a matrix without linear index
- by David_G
G'day,
I'm trying to find a way to use a vector of [x,y] points to index from a large matrix in MATLAB.
Usually, I would convert the subscript points to the linear index of the matrix.(for eg. Use a vector as an index to a matrix in MATLab) However, the matrix is 4-dimensional, and I want to take all of the elements of the 3rd and 4th dimensions that have the same 1st and 2nd dimension. Let me hopefully demonstrate with an example:
Matrix = nan(4,4,2,2); % where the dimensions are (x,y,depth,time)
Matrix(1,2,:,:) = 999; % note that this value could change in depth (3rd dim) and time (4th time)
Matrix(3,4,:,:) = 888; % note that this value could change in depth (3rd dim) and time (4th time)
Matrix(4,4,:,:) = 124;
Now, I want to be able to index with the subscripts (1,2) and (3,4), etc and return not only the 999 and 888 which exist in Matrix(:,:,1,1) but the contents which exist at Matrix(:,:,1,2),Matrix(:,:,2,1) and Matrix(:,:,2,2), and so on (IRL, the dimensions of Matrix might be more like size(Matrix) = (300 250 30 200)
I don't want to use linear indices because I would like the results to be in a similar vector fashion. For example, I would like a result which is something like:
ans(time=1)
999 888 124
999 888 124
ans(time=2)
etc etc etc
etc etc etc
I'd also like to add that due to the size of the matrix I'm dealing with, speed is an issue here - thus why I'd like to use subscript indices to index to the data.
I should also mention that (unlike this question: Accessing values using subscripts without using sub2ind) since I want all the information stored in the extra dimensions, 3 and 4, of the i and jth indices, I don't think that a slightly faster version of sub2ind still would not cut it..
