Help me about sphere ?

I have this code to draw haft of sphere x^2 + y^2 + z^2 = 1, z>0. but it have error. help me to fix it. thanks very much.

Answer 1

[Mischa Kim]

Nguyen, use instead

 z = real(sqrt(1 - x.^2 - y.^2));

to retain the real part only.

[Nguyen Trong Nhan]

oh thank. but why I have to use real(). these are positive numbers so the square root of them are also the real number, why does it appear complex number here ?

[Mischa Kim]

In a perfect world, yes. However, in numerical computations zero is not always (exactly) equal to zero. Check out this answer for reference. If you remove the semi-colon from

 z = sqrt(1 - x.^2 - y.^2)

you'll notice that there are small non-zero imaginary parts.

[Nguyen Trong Nhan]

ah Mischa Kim, could you explain me when I need to use the eval and double funtions in matlab ?