Find function in Matlab

Question

Trung Ngo 2017-12-19

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/373782-find-function-in-matlab

编辑： Stephen23 2017-12-20

采纳的回答： Star Strider

在 MATLAB Online 中打开

Dear all ,

I have problem with this code

clc 
close all
Signal = Hole1_mid;
Compare(:,1)=Signal(:,2);
value1 = 1.9752;
index1 = find(ismember(Compare,value1),n);
Hole1_6m(:,end) = Signal(1:index1,end);

Find function is not working in this case and I have no idea why

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Rik 2017-12-19

What would you mean with 'not working'? If it is a true bug, it is very unlikely it wouldn't have been found yet in a popular function like find, but it's not impossible. If not a bug, you must not be understanding what it does.

Try to create a stand-alone piece of code we can run to replicate what you have. Point out what you get, and how it differs from what you want.

Have a read here and here. It will greatly improve your chances of getting an answer.

Stephen23 2017-12-20

编辑：Stephen23 2017-12-20

"Find function is not working in this case"

Actually it is working correctly.

"and I have no idea why"

Your code tests for equivalence of floating point values, which is a very buggy and unreliable way to write code. You need to change your algorithm to take into account floating point error:

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/69-why-does-1-2-3-1-3-not-equal-zero#answer_271

https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal

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

https://www.mathworks.com/matlabcentral/answers/20830-1-0010-does-not-equal-1-0010

and a thousand other threads where this has been discussed. You could easily have searched this forum yourself and found many answers telling what is happening. This is worth reading as well:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

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Answer 1

Star Strider 2017-12-19

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https://ww2.mathworks.cn/matlabcentral/answers/373782-find-function-in-matlab#answer_297033

在 MATLAB Online 中打开

Try this:

tol_val = 0.001;
index1 = find(ismembertol(Compare,value1), n, tol_val);

See the documentation on ismembertol (link) for details. Choose the value for ‘tol_val’ that works with your data.

See the discussion in Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero? (link) for the reason ismembertol is preferable in situations such as those you are experiencing.

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Answer 2

Walter Roberson 2017-12-19

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/373782-find-function-in-matlab#answer_297032

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

1.9752 is not exactly representable in binary floating point, so there is a good chance that whatever value is in Compare is not exactly the same as the bit-for-bit exact value stored in value1

Consider using ismembertol()