My loop can't (sometimes...) evaluate its variable

Hello,

I have a problem with a program : It was working yesterday, but this morning it didn't anymore... The same problem also happens with a very basic code :

F=0.32;
for x=-0.5:0.01:0.5
    if x==F
       disp(x)
    end
end

For F=0.32 it doesn't disp the value (even if i write 'x" instead of "disp(x)" etc). For F=0.28, it does work... Not for F=0.29.

I'm a bit lost, did I use something wrong ?

Thank you for your help !

Answer 1

[Stephen23]

"did I use something wrong ? "

You are testing for exact equality of binary floating point numbers, which is unlikely to be reliable.

The robust approach is to compare the absolute difference against a tolerance:

tol = 1e-10;
..
if abs(x-F)<tol

Read more about binary floating numbers:

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

[Floriane A]

Thank you very much for your answer.

I wasn't enough aware of that... I'll read this documentation, thanks!