Software don't response after run this programme!

I am trying to pure white pixel into black pixel in gray scale image. So i make this code. But it is not worked please anybody help

RGB = imread('E:\Working\mapping\first\map.png');
figure,
imshow(RGB),
title('Original Image');
GRAY = rgb2gray(RGB);
g=GRAY;
imtool(GRAY);
title('Gray Image');
[r c]=size(g);
for i=1:r
  for j=1:c
      if (impixel(g,j,i)>254)

                g(i,j)=0;

        end
    end
  end
  imtool(GRAY);

[Image Analyst]

I think you started a duplicate thread which is going on. Let's keep the answer to just that thread. It's best not to start multiple threads on the same question. I may delete this one later, if I remember.