find function cannot find the value

Hello, I'm trying to use find function to find the value for example from a data with: data = [0, 1.00 2.00 3.00 4.00]; float values for example, if I do:

[~,indx] = find(data == 1.00),

the result would be an empty cell, []. Why is this the problem?

[Stephen23]

This has been clearly explained hundreds of times before, e.g. from https://www.mathworks.com/matlabcentral/answers/321709-strange-behaviour-of-matlab-find-command:

This is not a strange behavior at all, it has nothing to do with find, and it has nothing to do with the changes on your computer. You are the millionth beginner to discover that two different values are not equal. It is not a bug. This behavior is expected when comparing floating point values which are not the same. You might think that they are the same, but that is irrelevant to your computer. Lets have a look in detail:

 >> find(g==p)
 ans = []

Why does find not find any identical values? Because there are no identical values. Lets have a look at the values that you think are the same:

>> fprintf('%.30f\n',p)
2.599999999999999644728632119950
>> fprintf('%.30f\n',g(27))
2.600000000000000088817841970013

Do these look the same to you? This is such a common topic that it has been discussed thousands of times before. Try searching this forum for "floating point equals", or start by reading these:

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

PS: the solution, as every of those links will tell you, is never compare for equality between floating point values, and always use a tolerance:

 >> find(abs(g-p)<0.001)
 ans =  27

[Arbol]

Thank you for the follow up. I understood about that floating point. I even tired to resolved my solution with the tolerance for find but that didn't work.

I did resolved my problem. Thanks btw :)

Answer 1

[Image Analyst]

It works fine:

data = [0, 1.00 2.00 3.00 4.00]; 
[~,indx] = find(data == 1.00)

In the command window:

indx =
   2

I suspect your numbers aren't EXACTLY that and what you're running into is this: http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

[Arbol]

Yes, that's what I thought the problem was too. I wonder if there is a way around it to fix this for the find function. But as of right now, I can use round since my number are like 3.00 to fix the problem :)

[Walter Roberson]

There is no way to "fix" this for find() because it is not a find() issue.

You could consider using ismembertol()

[Arbol]

Thank you, I appreciated your input!