烂设计——被复杂化的索引

好的设计是：

先生产节点编号A_ind，在索引

       % upper half
        for i=1:m/2-1
            for j=1:n
                M=m/2-1;
                Begin=1; End= M*n;
                id=Begin-1+(j-1)*M +i; 
                if(id+1<=End)
                    A(id, id+1)=-1;
                end
                if(id-1>=Begin)
                    A(id, id-1)=-1;
                end
                if(id+m<=End)
                    A(id, id+M)=-1;
                end
                if(id-m>=Begin)
                    A(id, id-M)=-1;
                end
                A(id, id  )=4;
                k=k+1;
            end
        end
        for i=1:m/2-1   % -1
            for j=1:n
                M2=m/2-1;   % shift in id
                Begin2=End; % line 28
                End2= End+M2*n;
                id=Begin2-1+(j-1)*M +i; 
                if(id+1<=End2)
                    A(id, id+1)=-1;
                end
                if(id-1>=Begin2)
                    A(id, id-1)=-1;
                end
                if(id+M<=End2)
                    A(id, id+M2)=-1;
                end
                if(id-M>=Begin2)
                    A(id, id-M2)=-1;
                end
                A(id, id  )=4;
                k=k+1;
            end
        end

        % interface
        % error
        for i=m
            for j=1:n
                M=m/2;
                id= (j-1)*M +i; 
                if(1)%id+1<=mn/2)
                    %A(id, id+1)=-1;
                    index_j= id+ m*n/2- (i-1);
                    A(id, index_j)=-1;
                end
                if(id-1>=1)
                    A(id, id-1)=-1;
                end
                if(id+M<=mn/2)
                    A(id, id+M)=-1;
                end
                if(id-M>=1)
                    A(id, id-M)=-1;
                end
                A(id, id  )=4;
                k=k+1;
            end
        end
        %}