How to stop for loop if E approximately = 0.8.?

clc;
clear all;
close all;
[x,y]=meshgrid(-1:2/128:1);
circ=(x.^2+y.^2)<1;
A=ones(129,129);
B=A.*circ;
a1= 12849;

figure(1)
 imagesc(B),colormap gray;axis image;axis off;

   [x1,y1]=meshgrid(-1:2/128:1);
   for r=0:0.001:1
  circ1=(x1.^2+y1.^2)<r;
   C=B.*circ1;
   D=sum(sum(C));
   E=D/a1
  if (E ~= 0.8)
   end
   end
   figure(2)
   imagesc(C),colormap gray;axis image;axis off;

Answer 1

[John D'Errico]

tol = 1.e-12;
[x1,y1]=meshgrid(-1:2/128:1);
for r=0:0.001:1
  circ1=(x1.^2+y1.^2)<r;
  C=B.*circ1;
  D=sum(sum(C));
  E=D/a1;
  if abs(E - 0.8) <= tol
    break
  end
end
figure(2)
imagesc( C ),colormap gray;axis image;axis off;