plotting concentric circles using contour

I'm trying to plot a few concentric circles with the following codes, but what I get is an ellipse, i don't know why:

[x,y] = meshgrid(-3:0.1:3,-3:0.1:3);
z = x.^2+y.^2;
contour(x,y,z)

Answer 1

[Star Strider]

Use the axis function, specifically either:

axis('equal')                           % Function Expression

or:

axis equal                              % Command Expression

.

[Randy Chen]

thanks!

[Randy Chen]

by the way how can I add some radial lines perpendicular to the circles?

[Star Strider]

That will be diffiicult in contour, although relatively straightforward with plot:

[x,y] = meshgrid(-3:0.1:3,-3:0.1:3);
z = x.^2+y.^2;
[cm,ch] = contour(x,y,z);
hold on
axis('equal')
Lvls = ch.LevelList;
LvlIdx = find(ismember(cm(1,:),Lvls));
LvlNrs = cm(2,LvlIdx);
centre = [mean(cm(1,2:LvlNrs(1))); mean(cm(2,2:LvlNrs(1)))];
angls = linspace(0, 2*pi, 33);
XL = get(gca,'XLim');
YL = get(gca,'Ylim');
plot([centre(1);max(XL(:))]*cos(angls), [centre(2);max(YL(:))]*sin(angls), '-g')

Experiment to get the result you want.

[Randy Chen]

thanks a lot!

[Star Strider]

As always, my pleasure!