Why does my if condition only work once?

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Alexi 2023-2-26

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https://ww2.mathworks.cn/matlabcentral/answers/1919030-why-does-my-if-condition-only-work-once

编辑： Jan 2023-2-26

In the code here, while I expect the variable g2 to have a value of 1100 * 5, at some point I get a value of 1100 * 1 due to not making an error.

Why does my if condition only work once, what am I missing?

clc;
clear all;
dt=0.01;
time_vec=0:dt:60;
Twin=11;
ors=0.4;
g2=[];
f2=[];
j=1;
l=1;
    
for z = 1:1:length(time_vec)
    time_vec_o(z)=dt*z+Twin*ors;
    f2(j)=1/2*(1-cos(2*pi*(dt*z+Twin*ors)/Twin));
    a(z)= time_vec_o(z);
    b(z)=mod(time_vec_o(z),Twin);
    if  a(z)>=Twin*ors+Twin && b(z)==Twin*ors
        g2(:,l)=f2;
        f2 = [];
        j = 1;
        l=l+1;
    else
        j = j + 1;
    end
end

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Stephen23 2023-2-26

在 MATLAB Online 中打开

On this line:

if a(z)>=Twin*ors+Twin && b(z)==Twin*ors

you are assuming that you can test for exact equivalence of binary floating point values. That is rarely a good approach. The robust approach is to compare the absolute different against a tolerance:

abs(A-B)<tol

Read about the nature of working with binary floating point 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 highly recommended:

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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