Plotting a contour 3D plot in Matlab

Question

I entered this code in Matlab:

[x,y]=meshgrid(-3:1:3,-3:1:3);
z=sqrt((y.*y)-(x.*x))
contour3(x,y,z)

But am getting error for the same. 2D contour plot works out.What's the problem with the given code?

Answer 1

The problem is, that you are introducing complex numbers contour3 cannot handle.

use either

z = abs( sqrt((y.*y)-(x.*x)) )

or

z = real( sqrt((y.*y)-(x.*x)) )

or rethink whether you really want what you are doing.

For the 2D contour command it automatically takes the real part . You could also do something like this to get both plotted.

[x,y] = meshgrid(-3:1:3,-3:1:3);

zr = real( sqrt((y.*y)-(x.*x)) )
contour(x,y,zr,'linewidth',1); hold on

zi = imag( sqrt((y.*y)-(x.*x)) )
contour(x,y,zi,'linewidth',3); hold off

gives:

where the bold lines illustrate the imaginary part and the slim ones the real part.