Plotting a surface in MATLAB

I would like to plot

f(x,y) = x^2*y/(x^2 + y^2)

How do I do this in matlab?

[sally_wu]

Maybe like this?

x=2;

y=3;

f(x,y) = x^2*y/(x^2 + y^2);

linspace(x,y)

[Anthont Goddard]

I am just getting a load of text output. Not a 3d plot

Answer 1

[Star Strider]

You need to use the meshgrid (or equivalently, ndgrid) function:

x = linspace(-1, 1);
y = linspace(-2, 2);

[X,Y] = meshgrid(x,y);

f = @(x,y) x.^2.*y./(x.^2 + y.^2);

figure(1)
surf(X,Y,f(X,Y))
grid on

I created a function for ‘f’ here, but you could as easily use it simply as:

f = X.^2.*Y./(X.^2 + Y.^2);

and then plot as:

plot(X,Y,f)

I ‘vectorised’ your code to do element-wise operations. See the documentation on Array vs. Matrix Operations for a full discussion.

[Anthont Goddard]

When I run that script I get an error. Subscript indices must either be real positive integers or logicals.

[Star Strider]

This code:

x = linspace(-1, 1);
y = linspace(-2, 2);

[X,Y] = meshgrid(x,y);

f = @(x,y) x.^2.*y./(x.^2 + y.^2);

figure(1)
surf(X,Y,f(X,Y))
grid on

ran for me without error, and produced this plot:

[Anthont Goddard]

Ah that worked. Thanks :)

[Star Strider]

My pleasure!