What is wrong with this grey level sliced without background code?

aa=input('image.jpg');
z=aa;
e=input ('Enter lower threshold value ');
f=input ('Enter higher threshold value ');
%Grey level without background
    mask = (z>e) & (z<f);
    z(mask)=255;
    mask1=(z<e) & (z>f);
    z(mask1)=0;
    subplot(2,2,4);
    imshow(z);
    title ('Grey level sliced image without background');

Answer 1

[Walter Roberson]

Suppose the user entered 17 and 222. Now when you compute mask1, which pixels in the image will have a value that is simultaneously less than 17 & greater than 222?