% The whole K is participated in caculation.
% So K(fixeddofs) should be 1 in diag and 0 otherwise
function K = boundaryK(K, fixeddofs)
    K(fixeddofs, :) = 0;
    K(:, fixeddofs) = 0;

    for i = 1:size(fixeddofs, 2)
        dof = fixeddofs(i);
        K(dof, dof) = 1;
    end
end