Reshape 3d matrix to 2d matrix

Question

I have a 3d matrix (n-by-m-by-t) in MATLAB representing n-by-m measurements in a grid over a period of time. I would like to have a 2d matrix, wher

Answer 1

Reshape is of course the standard solution to reshaping an array. (What else would they call it?) There are still a few tricks to uncover.

First of all, the simplest way to turn an array of size [n,m,p] into an array of size [n*m,p]?

B = reshape(A,n*m,p);

But better is this:

B = reshape(A,[],p);

If you leave one of the arguments to reshape empty, then reshape computes the size for you! Be careful, if you try this and the size of A does not conform, then you will get an error. Thus:

reshape(magic(3),[],2)
??? Error using ==> reshape
Product of known dimensions, 2, not divisible into total number of elements, 9.

Logically, we cannot create an array of with two columns from something that has 9 elements in it. I did put a function called wreshape on the MATLAB Central exchange that would pad as necessary to do this operation with no error generated.

Of course, you can always use tricks like

B = A(:);

to create a vector directly from a matrix. This is equivalent to the form

B=reshape(A,[],1);