Forming a block diagonal matrix of one certain matrix?

I have a matrix A which is m*n. I want to create a block diagonal matrix of size 100*100 whose diagonal elements are the matrix A.

   [A,0,0,0
    0,A,0,0
    0,0,A,0
    0,0,0,A
      ...  ]
function, out = blkdiag(A,A,A,A,...) needs writing down the matrix so many times. Is there any other way to do this (not typing so many matrices as input arguments of blkdiag)?

[Teun Burgers]

how about

kron(eye(100),A)

or

kron(eye(100),sparse(A))

[Rik]

Comment posted as flag by @Anubhav Halder:

Worked perfectly for me. Thanks.

Answer 1

[Star Strider]

Cell arrays create comma-separated lists, which are exactly what the blkdiag function wants as its arguments.

See if this does what you want:

A = [1 2; 3 4];                                         % Original Matrix (Created)
N = 3;                                                  % Number Of Times To Repeat
Ar = repmat(A, 1, N);                                   % Repeat Matrix
Ac = mat2cell(Ar, size(A,1), repmat(size(A,2),1,N));    % Create Cell Array Of Orignal Repeated Matrix
Out = blkdiag(Ac{:})                                    % Desired Result

Out =
     1     2     0     0     0     0
     3     4     0     0     0     0
     0     0     1     2     0     0
     0     0     3     4     0     0
     0     0     0     0     1     2
     0     0     0     0     3     4

[David]

Just want to say I really appreciate this answer. Exactly what I needed ... an otherwise tricky step to generalize. Kudos.

[Star Strider]

My pleasure!

I appreciate your compliment!

[Bruno Luong]

The MAT2CELL step can be removed

A = [1 2; 3 4];

Ac = repmat({A}, 1, 3);  
Out = blkdiag(Ac{:})

Answer 2

[Teun Burgers]

how about:

kron(eye(100),A)

or

kron(eye(100),sparse(A))

[Stoyan Margaretov]

Genious, that worked perfectly for me!

Answer 3

[Honglei Chen]

eval(sprintf('Out = blkdiag(A%s);',repmat(',A',1,99)))

HTH

[Stephen23]

The MATLAB documentation for eval recommends that "Whenever possible, do not include output arguments within the input to the eval function, such as eval(['output = ',expression]). The preferred syntax,"

output = eval(expression)

"allows the MATLAB parser to perform stricter checks on your code, preventing untrapped errors and other unexpected behavior." Because the variable Out does not change this could easily have been achieved in this solution, and thus would follow the advice given in the MATLAB help.

Note that Star Strider's solution avoids all of these problems by simply avoiding eval entirely:

https://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html