Question

Is there any way for me to access specific matrix in the concatenate?
For example:

a = [1,2,3,4:5,6,7,8:9,0,1,2];
b = [3,2,1:4,2,1:5,6,7];
c = [a b];

I wish to access the matrix b from c..

It is very obvious that I can’t access it by c(2); because it is pointing to the second element in c instead.

I heard I could save the information of matrix a and matrix b in the header.

And then I would be able to retrieve it at a later time.
But how it works?

Answer 1

You can save the sizes of the a and b arrays in a separate length la vector, which you can later use as index ranges to access sub-matrices from c. This is kind of like your idea with the header, except that the header is stored as a separate variable la:

% first matrix starts at index 1
la = 1;
a = [1,2,3,4:5,6,7,8:9,0,1,2]; 
% append number of elements in a
la = [la numel(a)];

b = [3,2,1:4,2,1:5,6,7];       
% append number of elements in b
la = [la numel(b)];

c = [a b];
% use cumsum to compute indexes of sub-matrices in c
la = cumsum(la);

% this is your matrix a
c(la(1):la(2)-1)-a
% and this is your matrix b
c(la(2):la(3)-1)-b