The answer is NaN

I don't know why the answer of this code is NaN. Help me please. thanks so much

 for a = 01:20
   filename = [num2str(a,'%02d') '_training' '.tif'];
   img = imread(filename);
   img1=imrotate(img,90,'bilinear');
   C=rgb2gray(img1);
%co-occurrence matrix
glcm=graycomatrix(C,'GrayLimits',[0 255],'NumLevels',256,'Offset',[-1 -1]);
p=glcm/sum(glcm(:));
entropy=0;
for i=1:256
    for j =1:256
    entropy=entropy-p(i,j)*log2(p(i,j));
    end
end
kq(a,:)=[entropy];
end

[Matt J]

What variable represents "the answer"?

Answer 1

[Matt J]

What prevents p(i,j) from being 0 in

 entropy=entropy-p(i,j)*log2(p(i,j));

Note

>> 0*log(0)

ans =

     NaN

[Huy]

how can i fix it? how can i overcome this problem?

[Matt J]

if p(i,j)==0

   blablabla

end

or, discarding the for-loops,

entropy=p.*log(p);

entropy(isnan(entropy)) = blablabla

[Huy]

thanks so much

Answer 2

[Image Analyst]

Often a small number is added to values before taking log

smallNumber = 1; % % Whatever...
p(p==0) = smallNumber;

Now do your log.

[Huy]

thanks for your help