How to collapse an array across a specific set of dimensions?

Question

In Matlab, we can collapse across dimensions of an array like this:

M     = rand(3,4,5);
myvec = M(:);        % gives a 60-element vector

I think it's called serialising or flattening. The order of the elements is dim1 first, then dim2, then dim3 -- so you get [M(1,1,1); M(2,1,1); M(3,1,1); M(1,2,1); ...].

But what I want to do is collapse just along the first two dimensions:

mymatrix = M( :: , : ); % something that works like this?

to give a 12 x 5 matrix. So, for example, you get

[M(1,1,1)  M(1,1,2)  M(1,1,3)  M(1,1,4)  M(1,1,5)
 M(2,1,1)  M(2,1,2)  M(2,1,3)  M(2,1,4)  M(2,1,5)
 M(3,1,1)  M(3,1,2)  M(3,1,3)  M(3,1,4)  M(3,1,5)
 M(1,2,1)  M(1,2,2)  M(1,2,3)  M(1,2,4)  M(1,2,5)
 ...
]

So that the first dimension of mymatrix is the "flattened" 1st and 2nd dimensions of the original M, but preserving any other dimensions.

I actually need to do this for the "middle 3 dimensions" of a 5-dimensional array, so a general solution would be great! e.g. W=rand(N,N,N,N,N); mymatrix = W( :, :::, : ) should give a N x N^3 x N matrix if you see what I mean.

Thanks

Answer 1

Use reshape with square brackets ([]) as a placeholder for one of the dimension length arguments:

sz = size( M );
mymatrix = reshape( M, [], sz(end) );        % # Collapse first two dimensions

or

mymatrix = reshape( M, sz(1), [], sz(end) ); % # Collapse middle dimensions

The placeholder [] tells reshape to calculate the size automatically. Note that you can use only one one occurrence of []. All other dimension lengths must be specified explicitly.