Why the division of two positive values gives negative value?

Question

Youssra 2016-5-5

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https://ww2.mathworks.cn/matlabcentral/answers/282723-why-the-division-of-two-positive-values-gives-negative-value

编辑： Stephen23 2016-5-5

Hello,I'm using the following code to find a positive factor:

[U S V] = svd(image, 'econ'); % calculate the SVD of the image
level=4;
factorJND=jnd(image, level) ; % calculate the JND values of the image
f=factorJND/abs(U*V)  % divid the JND value by the multiplication of U and V matrices ( they have the same size)

knowing that factorJND and abs(U*V) are both positive, it gives me positive and negative numbers!! I don't know why!

f= -7.2851    6.4520
   -7.7509    5.5236
   -7.3374    4.1684
   -5.6905    5.0915

I even try to do :

f=abs(factorJND)/abs(U*V)

But still gives me the same result while it should be all positive values!

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Answer 1

Stephen23 2016-5-5

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编辑：Stephen23 2016-5-5

You are using the wrong divide: you probably intended to use array right divide ./ instead of matrix right divide / (yes, they are different, and it is your job to pick the right one).

And read about the difference:

http://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

That is why MATLAB has documentation: so that we know how to use it. You should read it whenever you use any function or operator, so that you understand how to use MATLAB. You don't need to just guess or ask random strangers on internet forums.