comparing between two identical variables returns logic 0?

Question

Mohamed Salah 2020-5-5

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/523049-comparing-between-two-identical-variables-returns-logic-0

评论： Mohamed Salah 2020-5-5

clc
clear
n=0:63;
x1=0:7;
x=[x1 x1 x1 x1 x1 x1 x1 x1];
y=((-1).^n).*x;
[msg,N1]=find_period(x);
disp(msg);
[msg,N2]=find_period(y);
disp(msg);
%Taking 1 period of Y
y1=zeros(1,N2);
for i = 1:N2
    y1(i)=y(i);
end
if N1==N2
    fprintf('N1=N2=%d \n', N1);
end
ak=our_fourier_series(x1);
bk=our_fourier_series(y1);
for i =1:4
if (bk(i)==ak(4+i))
    disp('bk=ak-N/2')
end
end
bk(6)==ak(2)
bk(6)
ak(2)

as you can see in the command window bk(6) equals ak(2)

but comparing between them returns logic 0

how is that even possible?

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Answer 1

Stephen23 2020-5-5

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https://ww2.mathworks.cn/matlabcentral/answers/523049-comparing-between-two-identical-variables-returns-logic-0#answer_430331

编辑：Stephen23 2020-5-5

"as you can see in the command window bk(6) equals ak(2)"

Nope. Just looking at some floating point values displayed in the command window (or variable viewer, etc.) is not a valid way to determine their exact values (and hence is not valid to decide if they are equal). Those values are certainly not equal, which you will see when you view them at a higher precision, e.g. dowload this:

https://www.mathworks.com/matlabcentral/fileexchange/22239-num2strexact-exact-version-of-num2str

"but comparing between them returns logic 0 how is that even possible?"

Simply because the values are not equal. Read these to know why they are not equal:

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