If you regard A as the points on a plain, than you have the x and y coordinates of those specific points stored for example in matrices X and Y. If you want to make a rectangular grid, you can do that like this:
[X Y] = meshgrid(-10:1:10,-10:1:10); % matrices from the x and y coordinates
Now give the center and radius of the circle, e.g.
xCenter = 3; yCenter = 4; radius = 2;
Finally, find all those indices that fulfil the requirement:
inCircle = (xCenter-X).^2 + (yCenter-Y).^2 < radius^2;
x = X(inCircle);
y = Y(inCircle);
Then you get the corresponding pair of points in vectors x and y. You may display the structure of matrix of the required indices with
spy(inCircle);
