MATLAB: Diagonal matrix help, this is the code, not sure what to put for the D(i,j) part

Question

format long;
if (n~=4)&(n~=50) 
    'Input value must be 4 or 50'
    return;
end
load(['data',num2str(n)],'A','b');   % load matrix A and b according to n
D=zeros(n,n);  %Initialize D 
%%%%%%%%%%%In the double-loop below, extract from A its diagonal part D, 
for i=1:n
   for j=1:n
   if  (i==j)
   D(i,j) = ???;
   end
      end
  end

Answer 1

I would see if:

D(i,j) = A(i,j);

did what I wanted.