I have this code which is behaving differently for decimal and integer values. Where am I missing??

Question

akash sonnad 2018-5-31

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/403393-i-have-this-code-which-is-behaving-differently-for-decimal-and-integer-values-where-am-i-missing

编辑： Stephen23 2018-5-31

 x=0.1;
 y=0.1;
 a=[x,y];
 b=[0.2,0.2];
 tf=isequal(a,b);
 while tf==0
     x=x+(0.01);
     y=y+(0.01);
     a=[x,y];
     tf=isequal(a,b);
 end
 disp(a);

if i replace the x, y and incremental values by integers, it works !! for ex as shown below

x=1000;
y=1000;
a=[x,y];
b=[2000,2000];
 tf=isequal(a,b);
 while tf==0
     x=x+1;
     y=y+1;
     a=[x,y];
     tf=isequal(a,b);
 end
 disp(a);

5 个评论
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Paolo 2018-5-31

Thanks for the clarification Stephen, I forgot to take into consideration the accumulating error.

akash sonnad 2018-5-31

Thank you Stephen Cobeldick !!

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Answer 1

Stephen23 2018-5-31

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/403393-i-have-this-code-which-is-behaving-differently-for-decimal-and-integer-values-where-am-i-missing#answer_322707

在 MATLAB Online 中打开

"That is really weird indeed. Perhaps MATLAB struggles to determine equality for floating point numbers"

Not at all: if two numbers are really equal, then MATLAB will correctly identify them as such. The only struggle here is beginners who believe that 0.1 is really stored in their computer, and have yet to learn that floating point number errors accumulate through all computations. Which means that those numbers are NOT equal.

"Where am I missing??"

Never use == to test for equivalence of decimal values. Always compare the difference against a tolerance:

abs(A-B)<tol

You need to learn about the practical limits of floating point numbers:

http://www.mathworks.com/help/matlab/matlab_prog/floating-point-numbers.html

http://www.mathworks.com/matlabcentral/answers/69-why-does-1-2-3-1-3-not-equal-zero

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

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

http://www.mathworks.com/company/newsletters/news_notes/pdf/Fall96Cleve.pdf

http://www.mathworks.com/matlabcentral/answers/41536-why-is-2-24-10-22-4-not-equal-to-0

https://www.mathworks.com/matlabcentral/newsreader/view_thread/66553

http://blogs.mathworks.com/loren/2006/08/23/a-glimpse-into-floating-point-accuracy/