Doesnt overwrite Variable inside of Loop

Question

Raz134 2021-1-21

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/723528-doesnt-overwrite-variable-inside-of-loop

编辑： Stephen23 2021-1-22

采纳的回答： Matt Gaidica

在 MATLAB Online 中打开

I am trying to overwrite the variable "anzahl_gamma" when gamma on the position t is equal to "g_s", g_s is 0.05

so "anzahl_gamma" should be 4 after my loop but it doesnt overwrite.

gamma = [0.005,0.01,0.025,0.0500,0.1,0.9,0.95,0.975,0.99,0.995]; 
signifikanzniveau = 0.95; 
g_s = 1 - signifikanzniveau;   % 0.05
anzahl_gamma=0;
for t = 1:10
    if gamma(t)== g_s
    anzahl_gamma = t;
    end
end

Why does this give me anzahl_gamma=0;

and when i change g_s to 0.05 directly it works:

for t=1:10
    if gamma(t)==0.05
anzahl_gamma=t;
end
end

with 0.05 directly i get anzahl_gamma=4;

What am i missing here? why is the first one (gamma(t)==g_s) not working?

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Answer 1

Matt Gaidica 2021-1-22

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/723528-doesnt-overwrite-variable-inside-of-loop#answer_603428

编辑：Matt Gaidica 2021-1-22

在 MATLAB Online 中打开

gamma = int32([0.005,0.01,0.025,0.0500,0.1,0.9,0.95,0.975,0.99,0.995] * 1000);
signifikanzniveau = 0.95; 
g_s = int32((1 - signifikanzniveau) * 1000);   % 0.05
anzahl_gamma = 0;
for t = 1:10
    if gamma(t) == g_s
    anzahl_gamma = t;
    end
end

There are a few ways to solve this. See here about floating point numbers. Also, beware gamma is a reserved function, I try to stay away from using those as variable names.

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Raz134 2021-1-22

Thank you this worked. I didnt know that matlab cannot compare vectors if they do not consist of integer numbers. I think thats whats happening here?

But thank you very much!

Stephen23 2021-1-22

编辑：Stephen23 2021-1-22

在 MATLAB Online 中打开

"I didnt know that matlab cannot compare vectors if they do not consist of integer numbers"

It can. But operations on the binary floating point numbers that your computer hardware supports (and that enable MATLAB to perform very fast and efficient HW operations for many numeric computations) by their very nature accumulate floating point error. This is simply a feature of binary floating point numbers.

It is up to the programmer to understand the behaviors of binary floating point numbers. For example, Iinstead of testing for exact equality, the recommended approach is to compare the absolute difference against a tolerance:

abs(A-B)<tol

Read more about 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 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

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