Why does this if statement fail?

I have noticed this strange behavior and I just can't figure why this is happening.

I have a script looping through a certain parameter and then saving some output results for certain values of that parameter. The relevant part of the script is:

for Thr=0.4:0.1:0.7
    disp(Thr);
    if Thr==0.5
        disp('but I am here?');
    end
    if Thr==0.6
        disp('am i here?');
    end
end
>> 
    0.4000
    0.5000
but I am here?
    0.6000
    0.7000

For some reason, it the if statement doesnt recognize Thr value of 0.6. If I use a step of 0.01, same happens for Thr values of 0.59, 0.58 and 0.57. I didnt test much further than that.

What is happening here?

[Stephen23]

"What is happening here?"

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

etc. etc.

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

PS: do not test binary floating point numbers for exact equivalence.The correct approach is to compare the absolute difference against a tolerance (chosen to be suitable for your data magnitudes):

abs(A-B)<tol

[Renat Yakupov]

I had no idea. That variable is different from 0.6 by 1e-16. That makes sense now. Thanks for the links!

Answer 1

[KSSV]

for Thr=0.4:0.1:0.7
    disp(Thr);
    if abs(Thr-0.5)<10^-3
        disp('but I am here?');
    end
    if abs(Thr-0.6)<10^-3
        disp('am i here?');
    end
end

Read about comparing floating point numbers. It seems you are checking some options, you need not to do this. Please read abour debugging MATLAB files.

[Renat Yakupov]

Thanks for the tip. In simpler cases like this, rounding to the first decimal also does the trick, but it certainly has to be more flexible for other cases.

[Stephen23]

"In simpler cases like this, rounding to the first decimal also does the trick..."

https://www.mathworks.com/matlabcentral/answers/321709-strange-behaviour-of-matlab-find-command#comment_444258

https://www.mathworks.com/matlabcentral/answers/350303-why-0-6-is-not-equal-to-0-2-3#comment_472139