I want to create a block matrix using some loop (and logical conditions). I wrote the following code that works when "p" and "q" are numbers. How can I make it work in the same way, when "p" and "q" are matrices?

I want to create a large matrix c, where p, q and r are small matrices (of same size). The form of "c" shown here is not fixed, rather it would depend on some logic in my code. For example in this case, the diagonal blocks must be "p" and so on.

c=
    [p r q r r
     r p r q r
     q r p r q
     r q r p r
     r r q q p]

I wrote the following code, that works only if "p", "q" and "r" are numbers. But they don't work if I put them to be matrix. How can I fix it to make it work for matrices?

clc;
  a=1:5;
  %p=[2 1;1 2];
  %q=[0.3 0.2;0.2 0.3];
  %r=[0 0;0 0];
  p=5;
  q=2;
  r=0;
  for(i=1:5)
  for(j=1:5)
    if (abs(a(i)-a(j))==0)
      c(i,j)=p;
    elseif (abs(a(i)-a(j))==2)
      c(i,j)=q;
      else
      c(i,j)=r;
    endif
  endfor
  endfor
  c

Output:

c =

     5   0   2   0   0
     0   5   0   2   0
     2   0   5   0   2
     0   2   0   5   0
     0   0   2   0   5

Answer 1

[Rik]

clc;
p=[2   1  ;1   2  ];
q=[0.3 0.2;0.2 0.3];
r=[0   0  ;0   0  ];
c=[p r q r r;...
   r p r q r;...
   q r p r q;...
   r q r p r;...
   r r q q p];
c=cell2mat(c);

[Md Nur Kutubul Alam]

Thank you very much. In the above code we have to define the cell size beforehand while in my case the cell size is unknown. In fact it can change dynamically. For matrix with elements in it, I can easily add new elements/matrix in the existing one like-

if (my_variable==2)
c=[c a q;...
   a c q;...
   q a c];

Can I do similar things if I go ahead with the cell style? In brief i need full freedom as I have in dealing with matrices.

[Rik]

Sure, you can do what you wrote:

c={1};a={2};q={3};
c=[c a q;...
 a c q;...
 q a c];
c=cell2mat(c);

As long as you make sure that the elements you put in are all equal sizes, you should be golden.

PS if you found this answer useful, please mark it as accepted answer. It will give us both reputation points and will make it easier for other people with the same question to find an answer.

Answer 2

[Stephen23]

Here is a simple solution using indexing:

>> C = {[2,1;1,2],[0.3,0.2;0.2,0.3],[0,0;0,0]};
>> V = [1,3,2,3,3]; % define your logic here
>> M = toeplitz(V,V); % create index matrix
>> cell2mat(C(M)) % create output matrix
ans =
 2.00000   1.00000   0.00000   0.00000   0.30000   0.20000   0.00000   0.00000   0.00000   0.00000
 1.00000   2.00000   0.00000   0.00000   0.20000   0.30000   0.00000   0.00000   0.00000   0.00000
 0.00000   0.00000   2.00000   1.00000   0.00000   0.00000   0.30000   0.20000   0.00000   0.00000
 0.00000   0.00000   1.00000   2.00000   0.00000   0.00000   0.20000   0.30000   0.00000   0.00000
 0.30000   0.20000   0.00000   0.00000   2.00000   1.00000   0.00000   0.00000   0.30000   0.20000
 0.20000   0.30000   0.00000   0.00000   1.00000   2.00000   0.00000   0.00000   0.20000   0.30000
 0.00000   0.00000   0.30000   0.20000   0.00000   0.00000   2.00000   1.00000   0.00000   0.00000
 0.00000   0.00000   0.20000   0.30000   0.00000   0.00000   1.00000   2.00000   0.00000   0.00000
 0.00000   0.00000   0.00000   0.00000   0.30000   0.20000   0.00000   0.00000   2.00000   1.00000
 0.00000   0.00000   0.00000   0.00000   0.20000   0.30000   0.00000   0.00000   1.00000   2.00000

MATLAB is a beautiful high-level language, it is not a low-level language like C, and as such it does not rely on ugly loops for solving every task.