Elseif does not work correctly

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Fatih Enes Kose 2020-5-4

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回答： Image Analyst 2020-5-4

I have a problem with using elseif statement. I have a matrice qq and its values from index 1 to 17 are equal to 0.8. In the 18. index of qq equals to 0.6. I have read these values from workspace. I have a code below and bitsirasi(18)=52 from workspace. qq(18)=0.6, but releated elseif statement does not work. Why?

qqout = zeros(1,3*length(qq));
for j=1:1:length(qq)
        i=bitsirasi(j);     
        if qq(j)==0
            qqout(i)=1;
            qqout(i+1)=1;
            qqout(i+2)=1;
            
        elseif qq(j)==0.2
            qqout(i)=0;
            qqout(i+1)=0;
            qqout(i+2)=0;
            
        elseif qq(j)==0.4
            qqout(i)=0;
            qqout(i+1)=0;
            qqout(i+2)=1;
        
        elseif qq(j)==0.6 %does not work
            qqout(i)=0;
            qqout(i+1)=1;
            qqout(i+2)=0;
        
        elseif qq(j)==0.8
            qqout(i)=0;
            qqout(i+1)=1;
            qqout(i+2)=1;
       
        elseif qq(j)==(-0.2) %does not work, too
            qqout(i)=1;
            qqout(i+1)=1;
            qqout(i+2)=0;
        
        elseif qq(j)==(-0.4)
            qqout(i)=1;
            qqout(i+1)=0;
            qqout(i+2)=1;
       
        elseif qq(j)==(-0.6)
            qqout(i)=1;
            qqout(i+1)=0;
            qqout(i+2)=0;
        end
end

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Stephen23 2020-5-4

"Why?"

Because of how numbers are stored on your computer:

https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal

http://www.mathworks.com/matlabcentral/answers/102419-how-do-i-determine-if-the-error-in-my-answer-is-the-result-of-round-off-error-or-a-bug

http://www.mathworks.com/matlabcentral/answers/140656-about-numerical-difficulty-of-equality-comparison-operation-between-two-double-real-numbers-in-matlab

This is worth reading as well:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

http://faculty.tarleton.edu/agapie/documents/cs_343_arch/papers/1991_Goldberg_FloatingPoint.pdf

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Answer 1

Rik 2020-5-4

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在 MATLAB Online 中打开

I suspect you are encountering the wondrous world of floating point values. Some decimal values do not have an exact binary equivalent.

You need to compare the absolute difference to a tolerance:

elseif abs(qq(j)-(-0.2))<=2*eps

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Guillaume 2020-5-4

编辑：Guillaume 2020-5-4

在 MATLAB Online 中打开

I wouldn't recommend using eps in this case. Particularly, eps(1) which is not really related to the magnitude of the numbers. If the possible values for qq(j) are the only ones listed here then a tolerance of 0.1 would be sufficient.

Note that a bunch of elseif is rarely a good design. That's a lot of typing and doesn't scale well. Look-up tables are usually much better. Consider:

LUT = [0    1 1 1;  ...lookup value, value for i, value for i+1, value for i+2
       0.2  0 0 0;
       0.4  0 0 1;
       0.6  0 1 0;
       0.8  0 1 1;
       -0.2 1 1 0;
       -0.4 1 0 1;
       -0.6 1 0 0];
qqout = zeros(1,3*length(qq));
for j=1:1:length(qq)
        i=bitsirasi(j);     
        [found, where] = ismembertol(qq(j), LUT(:, 1));
        if found
            qqout(i:i+2) = LUT(where, 2:4);
        end
end

Notice how much shorter the code is. And if you want to add conditions, just add rows to the lookup table without having to change any of the actual code.

edit: Oh completely forgot why I started this comment in the first place:

To understand eps, and the whole problem of using == to compare to numbers like 0.2, 0.4, etc. read the floating point guide in particular the basic answer page and the comparison page.

Fatih Enes Kose 2020-5-4