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eq() and == failing to find matches in generated array.

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Jonathan G-H-Cater
Jonathan G-H-Cater on 10 Mar 2021
Answered: David K. on 10 Mar 2021
I have generated the following vector:
x = -2:0.1:5;
But now when trying to mask the location of various values it seems to fail to find entries that are shown in memory when viewed manually. E.g.
max(x==1.1) % returns 1 as expected.
max(eq(x,1.1)) % returns 1 as expected.
max(x==1.8) % returns 0 but expected 1.
max(eq(x,1.8)) % returns 0 but expected 1.
x(39) % returns 1.8000 as a sanity check to prove it is there.
Im guessing this is some sort of floating point precision issue - but don't know how to avoid it in a reliable way as I thought that was the point of eq().
Have tried to make this example case as simple as possible - so sorry if it seems a little random.
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Jonathan G-H-Cater
Jonathan G-H-Cater on 10 Mar 2021
Here is another one that fails... but now I really am confused:
max((abs(x-1.8)<eps)) % returns 0
max((abs(x-1.1)<eps)) % returns 1
Honestly expected that to solve precision issues.

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Accepted Answer

David K.
David K. on 10 Mar 2021
You are right that it is floating point precision. There are many ways to deal with this and your comment shows one of them with a slight adjustment:
max((abs(x-1.8)<=eps)) % returns 1
You can also use a function called ismembertol and do it as such:
max(ismembertol(x,1.8,eps)); % also returns 1

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