How to find difference in mask within a for loop?

I have a code. How i will find difference mask(k)-mask(k-1)?

 [xgrid, ygrid] = meshgrid(1:size(img,2), 1:size(img,1));
 for i = 1 : length(rk) % BUG found

    %     mask = ((xgrid - x).^2 + (ygrid - y).^2) <= rk(i).^2;

    if (i == 1)
        % Create the mask for first radius value
        mask = ((xgrid - x).^2 + (ygrid - y).^2) <= rk(i).^2;
        imshow(mask); % Debugging point
        title ('For first radius');
        tempVar = img(mask);

        % Putting the value in the matrix
        tempV(1:size(tempVar,1),i) = tempVar ;
    else
        mask2 = ((xgrid - x).^2 + (ygrid - y).^2) <= rk(i).^2;
        figure; imshow(mask2); % Debugging point
        title ('New Mask');
        % Insert mask subtraction here
        tempMaskSub = logical(mask2 - mask);
        figure;imshow(tempMaskSub);
        title ('Difference');
    end
 end