MATLAB: Anyone can help me to improve this code

if statementloopMATLABmatrix

Dear friends; I have a code which is not suitable for big matrices or unlimited matrice. I would like to make it better because some times i have a very big matrices such as A = (100,100)
A = zeros(n,n) matrix.
R = [ 0 0 0 1 1] vector
c=2
if R(5,1) == 1
    A(5,5) = c
    A(5,4) = - c  
    A(4,5) = - c 
else 
    A(5,5) = 0    
    A(5,4) = 0  
    A(4,5) = 0
end
if R(4,1) == 1
    A(4,4) = c + A(5,5)
    A(3,4) = - c  
    A(4,3) = - c
else 
    A(4,4) = 0    
    A(3,4) = 0  
    A(4,3) = 0
end
.
.
.
ans
A =
     0     0     0     0     0
     0     0     0     0     0
     0     0     0    -2     0
     0     0    -2     4    -2
     0     0     0    -2     2
Thanks in advance, your help always appreciated

Best Answer

[EDIT]
n = 5;           % 
R = [0 0 0 1 1]';% eg
c = 2;           %
A1 = spdiags(-c*R,1,n,n);
A = A1 + A1' + spdiags(c*(R + [R(2:end);0]),0,n,n);
full(A)
OR
Rin(:,[1 3 2]) = c*[-[circshift(R,-1) R],R + [R(2:end);0]];
A = spdiags(Rin,-1:1,n,n);
full(A)
OR
Rin(:,[1 3 2]) = c*[-[circshift(R,-1) R],conv(R,[1;1],'same')];
A = spdiags(Rin,-1:1,n,n);
full(A)
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