central difference coefficient matrix
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Hi im trying to code the following coefiicient matrix A in a for loop in Matlab. N is the total number of elements. And i and j are obtained from k. could someone help me please code the A matrix correctly for any N value.
for k=1:N
A(k,k) = 4; % the diagonal element, corresponding to Tij, is always 4
% In the following, we look at the four neighbours of (i,j) that are
% involved in the 5-point formula
i = mod(k-1,nx)+1; j = (k-i)/nx+1; %get (i,j) from k
%boundary condtion
if i==1
b(k,1)=Tl;
elseif i==nx
b(k,1)=Tr;
elseif j==1
b(k,1)=Tb;
elseif j==ny
b(k,1)=Tt;
end
%A matrix construction
if i>1 && j>1
if i==j
A(j-1,i)=-1;
A(j,i+1)=-1;
A(j,i-1)=-1;
A(j+1,i)=-1;
end
end
end
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/290063/image.png)
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