Random dots to make a circle

This code works excellent to make a circle with randomized dots. My problem now is that I want a inner circle were these randomized dots can't be placed. So it will be like this

but have I drawn a circle.

Answer 1

[Alan Stevens]

Try something like

n = 1000;
t = 2*pi*rand(n,1);

Ri = 1;
Ro = 2;
r = Ri + (Ro - Ri)*rand(n,1);
x0 = 0; y0 = 0;
x = x0 + r.*cos(t);
y = y0 + r.*sin(t);

plot(x,y,'.b')

[Lucas Sigfridsson]

Thanks that works excellent. Do you have any ideas how to not have the inner circle in the middle of the bigger one?

[Alan Stevens]

How about

n = 2000;
t = 2*pi*rand(n,1);

Ri = 1;
Ro = 3;
r = Ro*rand(n,1);
x0 = 0; y0 = 0;
x = x0 + r.*cos(t);
y = y0 + r.*sin(t);

xoffset = 1;
yoffset = -0.5;
theta = 0:pi/180:2*pi;
xi = xoffset + Ri*cos(theta);
yi = yoffset + Ri*sin(theta);

plot(x,y,'.b')
hold on
fill(xi,yi,'w')

Note that, in order to get a uniform distribution of dots in a circle you need to choose R^2, not R, randomly (though this might not be important for your application, and I've not bothered to do that in the listing here).