How to plot function with conditions (3d)

Suppose I want to plot the function

%  f(x,y) = x.*y by saying:
[x,y] = meshgrid (-2:2:2);
z = x.*y;
surf(x,y,z)

But what if I want the x and y values also to fulfill the equation x + y <= 1 ?

Answer 1

[Ameer Hamza]

You can replace the unwanted values with nan.

[x,y] = meshgrid (-2:2:2);
z = x.*y;
mask = x + y  <= 1;
z(~mask) = nan;
surf(x,y,z)

[Niklas Kurz]

Thank you !