Pre-Allocating Single Cell's Memory - matlab

Question

I am writing a code and would like to know how to pre-allocate the memory for a single cell. I understand that one can easily pre-allocate an array of cells, but this isn't what I'm looking for.
My code follows the following logic:
for i = 1:numel(k)
R{i} = % Some 4x4 matrix That changes each iteration
end
R = blkdiag(R{:});
The goal here is to build a comma-separated list of an arbitrary amount of matrices, say:
R = {A,B,C,D,E,...}
Such that blkdiag will return the following:
R = [A 0 0...
0 B 0...
0 0 C...
... ]
The code I have right now works fine, I'd just like to know how to pre-allocate memory for this case such that it runs faster if I had an awful lot of matrices.

Answer 1

Putting a matrix into a cell array doesn't cause the cell array to be resized, if your cell array was preallocated. R = cell(numel(k),1) will preallocate the cell array. Each element of this array is a reference to an array (or arbitrary type, could be a numeric array, another cell array, a struct array, an array of custom objects, doesn't matter).
When the cell array is first created, each cell contains an empty matrix, []. Replacing this matrix just changes the reference stored in the cell array to point to a different array. R{3} = x doesn't reallocate memory, it only changes the reference in R{3} to now point to whatever matrix was stored in the variable x.
On the other hand, if you want to add values to the array stored in R{3}, you should preallocate that array:
R{3} = zeros(5,1); % preallocate the array pointed to by R{3}
for ii=1:5
R{3}(ii) = ii;
end