Unexpected floor() function result?

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Albert Hsueh
Albert Hsueh 2021-5-17
编辑： Adam Danz 2021-5-17
>> (1.2-floor(1.2))*10
ans =
2.0000
>> floor((1.2-floor(1.2))*10)
ans =
1
Have I misunderstood how the floor function interacts with other math operations?
  2 个评论
Albert Hsueh
Albert Hsueh 2021-5-17
>> ((1.2-floor(1.2))*10) == 2
ans =
logical 0
Albert Hsueh
Albert Hsueh 2021-5-17
I'm guessing this is some really small numerical inpercision in the back end. The following results in the expected result
>> round((1.2-floor(1.2))*10)==2
ans =
logical 1

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采纳的回答

Adam Danz
Adam Danz 2021-5-17
编辑：Adam Danz 2021-5-17
Ran in:
This is round-off error and is caused by limitation to representing floating point decimals.
1.2-floor(1.2)
ans = 0.2000
1.2-floor(1.2) - 0.2000 % Reveal round-off error
ans = -5.5511e-17
Likewise,
(1.2-floor(1.2))*10
ans = 2.0000
(1.2-floor(1.2))*10 - 2.000 % Reveal round-off error
ans = -4.4409e-16
Here's another example,
4/3
ans = 1.3333
4/3-1
ans = 0.3333
(4/3 - 1)*3
ans = 1.0000
(4/3 - 1)*3 - 1 % Reveal round-off error
ans = -2.2204e-16
See this answer for a list of references to learn more about round-off error and floating point representation. If there's a specific issue you need to solve to avoid the problem, add some details and I may be able to point you in the right direction.

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