怎么解决，新手无牙

%讨论a，b，c对椭球面x^2/a^2+y^2/b^2+z^2/c^2=1的影响
for i=1:3
a(i)=input('a=');b(i)=input('b=');
c(i)=input('c=');N(i)=input('N=');
x=linspace(-abs(a(i)),abs(a(i)),N(i));
y=linspace(-abs(b(i)),abs(b(i)),N(i));
z=c(i)*sqrt(1-y.*y/b(i)/b(i)-x.*x/a(i)/a(i));
z=real(z);
subplot(1,3,i),
surf(x,y,z),hold on,surf(x,y,-z);
end
%??? Error using ==> surf at 78
%Z must be a matrix, not a scalar or vector.