28 views (last 30 days)

Show older comments

I want to plot the function The function is defined for . With

f = @(x,y)sqrt(y-x.^2);

fmesh(f,[-3 3 0 9])

I get a mesh plot of the function in the rectangle [-3,3]X[0,9]. I want to plot the function using meshgrid. The following code results to complex values error message.

x = -3:1:3

y = 0:1:9

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

Z = sqrt(Y-X.^2)

mesh(X,Y,Z)

The problem is that Z is evaluated for not suitable pairs of (X,Y). What am I doing wrong?

Star Strider
on 6 Apr 2021

Edited: Star Strider
on 6 Apr 2021

Try this:

x = -3:1:3;

y = 0:1:9;

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

Y = Y.*(Y>=X.^2); % Select Y ≥ X^2

Z = sqrt(Y-X.^2);

Zre = Z.*(imag(Z)==0); % Select Only Real Values Of ‘Z’

figure

mesh(X,Y,Zre)

EDIT — (5 Apr 2021 at 15:56)

Added:

Y = Y.*(Y>=X.^2); % Select Y ≥ X^2

to accord with the edited Question.

Star Strider
on 6 Apr 2021

As always, my pleasure!

The easiest solution is to use fmesh. It is possible to examine the fmesh code using edit or type (however that is not very revealing, since it calls fsurf, that it is posible to examine in some detail) to see exactly how it works. The fsurf function then calls a number of other funcitons that I have not taken the time to examine closely. My information on the ‘f’ plotting functions derives from other discussions in MATLAB Central that I have read over the years.

VBBV
on 6 Apr 2021

%f

x = -3:1:3

y = 0:1:9

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

Z = real(sqrt(Y-X.^2))

mesh(X,Y,Z)

Plot only the real part

Bruno Luong
on 6 Apr 2021

Here is the surface in meshgrid form

s = -1:0.01:1;

y = 0:0.1:9;

[S, Y] = meshgrid(s,y);

X = S.*sqrt(Y);

Z = sqrt(max(Y-X.^2,0));

figure

mesh(X,Y,Z)

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!