Loop matrices into a structure - matlab

Question

I'm not even sure if the title describes what I want to do, so let me try to elaborate.
Each iteration of the loop creates an X matrix. As it stands now, only the final iteration of the X matrix remains after the loop finishes.
However my desired final output is 6 X matrices. One for each of the loop iterations.
Basically, I want to save all 6 of the X matrices, not overwrite them after each iteration.
for col=2:7
logprice=log(ret(1:end,col));
logret=diff(logprice);
exret=logret-logRFree;
price_mat(:,col-1)=logprice;
ret_mat(:,col-1)=logret;
exret_mat(:,col-1)=exret;
X=[ret_mat(:,col-1) termspread creditspread inflation realrate ];
end

Answer 1

If the size of X is different on each iteration (and it looks that way), it may be easiest to store them in a cell array:
% before loop
X = cell(N,1);
% in loop, with a counter ii ...
X{ii} = [...];
If the X matrix were the same size on each iteration of the loop, you could preallocate a 3D array and save to each slice (e.g. X=zeros(R,C,P); ... X(:,:,ii) = [...];);

Answer 2

There are two most common ways. Cell arrays is one of them. The other one is 3-D matrices. It can be generalized to n-D matrices too.
Cell arrays can handle different size of matrix X on every iteration. 3-D matrices cannot. All matrices have to be of the same size. You can pre-allocate cells as well as 3-D matrices to save execution time on large loops. Also, it is a good practice to follow.
Preallocate cell array
Preallocate matrix
It can be done as follows:
count=0;
for col=2:7
count=count+1;
logprice=log(ret(1:end,col));
logret=diff(logprice);
exret=logret-logRFree;
price_mat(:,col-1)=logprice;
ret_mat(:,col-1)=logret;
exret_mat(:,col-1)=exret;
%%%%%%Execute only one of the following two statements%%%%%%%
X{count}=[ret_mat(:,col-1) termspread creditspread inflation realrate ];
X(:,:,count)=[ret_mat(:,col-1) termspread creditspread inflation realrate ];
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
end