How do I change the squares in this code to circles?

I'm trying to change the shapes this code runs to circles:

clear all
close all
clc

x1 = [-1  1  1  -1 -1]*3-5;
x2 = [-1  1  1  -1 -1]*3+5;

y1 = [-1 -1  1  1  -1]*3;
y2 = [-1 -1  1  1  -1]*3;

dx1=0.4;
dx2=-0.05;

dy1=0.05;
dy2=0.2;

Lx=max(x1)-min(x1);
Ly=max(y1)-min(y1);

for i=1:1000
    figure(1),plot(x1,y1,x2,y2)
    axis equal
    set(gca,'xlim',[-20 20]);
    set(gca,'ylim',[-20 20]);

      x1=x1+dx1;
      x2=x2+dx2;

      y1=y1+dy1;
      y2=y2+dy2;

      if max(x1)>=20
          x1=x1-(max(x1)-20);                   
          dx1=-dx1;           
      elseif min(x1)<=-20
          x1=x1-(min(x1)+20); 
          dx1=-dx1;           
      end

      if max(y1)>=20
          y1=y1-(max(y1)-20); 
          dy1=-dy1;           
      elseif min(y1)<=-20
          y1=y1-(min(y1)+20); 
          dy1=-dy1;           
      end

      if max(x2)>=20
          x2=x2-(max(x2)-20); 
          dx2=-dx2;           
      elseif min(x2)<=-20
          x2=x2-(min(x2)+20); 
          dx2=-dx2;           
      end

      if max(y2)>=20
          y2=y2-(max(y2)-20);
          dy2=-dy2;           
      elseif min(y2)<=-20
          y2=y2-(min(y2)+20); 
          dy2=-dy2;           
      end

      if abs(mean(x1)-mean(x2))<Lx && abs(mean(y1)-mean(y2))<Ly
          title(['close to each other, iteration:' num2str(i)])
          if  max(x1)>min(x2) || max(x2)>min(x1)
              a=dy2;
              dy2=dy1;
              dy1=a;
          end

          if  max(y1)>min(y2) || max(y2)>min(y1)
              a=dx2;
              dx2=dx1;
              dx1=a;
          end

      else
          title(['far from each other, iteration:' num2str(i)]);
      end
      pause(0.05)
  end

I tried using this code for creating circles, but I don't know how to implement it in the previous code.

r=5;
x=-r:0.1:r;
y1=sqrt(r^2-x.^2);
%y2=-sqrt(r^2-x.^2);
y2=-y1;
figure(1), plot(x,y1,x,y2)
axis equal

Answer 1

[Cris LaPierre]

The variables x1, x2, y1 and y2 are what create the box shapes. Replace the values with points that create a circle and run your code. It should work as well on the new vectors as it did on the old.

I tested it with the following:

r = 3;
theta = 0:10:360;
x1 = r*cosd(theta)-5;
x2 = r*cosd(theta)+5;
y1 = r*sind(theta);
y2 = y1;

dx1=0.4;
dx2=-0.05;

dy1=0.05;
dy2=0.2;

Lx=max(x1)-min(x1);
Ly=max(y1)-min(y1);

for i=1:1000

     ...

end

and got the following figure