MATLAB: Expanding a complex element to a matrix

Question

Hi,

I have a complex matrix and I would like to convert the matrix to the real and imaginary components without changing the location. Say the complex matrix is

[A11 A12;

A21 A22]

each Aij = Gij + 1j* Bij. After expanding the matrix, I would get my desire matrix as

[[G11 -B11;B11 G11] [G12 -B12;B12 G12] ;

[G21 -B21;B21 G21] [G22 -B22;B22 G22]]

In the code below, the variable B indicates what I want to achieve, but I want to do it more efficiently than via a for loop and the i*2 indicator.

clear
clc
nb=2;
A=complex(magic(nb),1./magic(nb));
% tic
Ar=real(A);
Ai=imag(A);
B=zeros(2*nb,2*nb);
for i=1:nb
  for j=1:nb
    B(2*i-1,2*j-1)= Ar(i,j);
    B(2*i-1,2*j)  =-Ai(i,j);
    B(2*i,2*j-1)  = Ai(i,j);
    B(2*i,2*j)    = Ar(i,j);
  end
end
% toc

Any advice would be much appreciated. Thanks for reading.

Answer 1

Ar=real(A);
Ai=imag(A);
B=zeros(2*nb,2*nb);
 B(1:2:end,1:2:end)=Ar;
 B(1:2:end,1:2:end)=Ar;
 B(1:2:end,2:2:end)=-Ai;
 B(2:2:end,1:2:end)=Ai;