MATLAB: A null assignment can have only one non-colon index

Question

B=zeros(23,23)
for r=1:23;
    for n=1:23;
        if r==n
            B(r,n)=A(r,n);
        else
            B(r,n)=[];
        end
    end
end

A is a symmetric 23*23 matrix and I want to remove all the other values except r=n. It gives me an "A null assignment can have only one non-colon index" error

Answer 1

B=zeros(23,23)
for r=1:23;
    for n=1:23;
        if r==n
            B(r,n)=A(r,n);
        else
            B(r,n)=NaN;
        end
    end
end