Find function in Matlab

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Trung Ngo
Trung Ngo 2017-12-19
编辑： Stephen23 2017-12-20
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
  2 个评论
Rik
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
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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采纳的回答

Star Strider
Star Strider 2017-12-19
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.

更多回答（1 个）

Walter Roberson
Walter Roberson 2017-12-19
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()

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