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

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Jonathan G-H-Cater
Jonathan G-H-Cater 2021-3-10
回答： David K. 2021-3-10
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 2021-3-10
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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采纳的回答

David K.
David K. 2021-3-10
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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