why this logical expression is wrong?

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Hi,
why this statment is wrong,
>> sind(30)==0.5
ans =
0
  4 Comments
Walter Roberson
Walter Roberson on 18 Jul 2019
I seem to remember seeing a sind bug in the bug reports a couple of releases ago, but I cannot find that information now.
Peter Jarosi
Peter Jarosi on 18 Jul 2019
I think it's not a bug, because it depends on the method of approximation. That's an interesting question how developers of sind() function fixed sind(30). Whether there is an if statement in the code of sind() in order to set sind(30) exactly 1/2. :-)

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Accepted Answer

Peter Jarosi
Peter Jarosi on 18 Jul 2019
Edited: Peter Jarosi on 18 Jul 2019
because of the accuracy problem of a floating-point system
>> format longE
>> sind(30)
ans =
4.999999999999999e-01
Never, ever compare two floating-point numbers with == operand!
See also:
  11 Comments
Peter Jarosi
Peter Jarosi on 18 Jul 2019
I voted your question. It's a million dollar question, and the problem is general, not Matlab's fault.
Walter Roberson
Walter Roberson on 18 Jul 2019
The result of sind(30) should be exactly 1/2, but it appears that in some older releases it was not exactly that.
It is generally better to not compare for floating point equality except when you know that the exact bit-pattern occurs somewhere. For example, it is fair to test
A = min(B);
B == A
because you know that A will be a bit-for-bit copy of one of the values that is in B. (Well, except for some obscure cases involving non-default nan values... and in those cases, nan == nan is always false anyhow.)

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More Answers (1)

Steven Lord
Steven Lord on 19 Jul 2019
This is a bug. See Bug Report 1839169.
  1 Comment
Peter Jarosi
Peter Jarosi on 19 Jul 2019
How did you fix it? Did you use a better approximation method (for instance longer Taylor series) or just put an if statement in the code of function sind()? :-)
(I'm joking)

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