why this logical expression is wrong?

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mohammad fallah
mohammad fallah 2019-7-18
评论： Peter Jarosi 2019-7-19
Hi,
why this statment is wrong,
>> sind(30)==0.5
ans =
0
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Walter Roberson
Walter Roberson 2019-7-18
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 2019-7-18
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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Peter Jarosi
Peter Jarosi 2019-7-18
编辑：Peter Jarosi 2019-7-18
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:
https://en.wikipedia.org/wiki/Floating-point_arithmetic#Machine_precision_and_backward_error_analysis
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Peter Jarosi
Peter Jarosi 2019-7-18
I voted your question. It's a million dollar question, and the problem is general, not Matlab's fault.
Walter Roberson
Walter Roberson 2019-7-18
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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更多回答（1 个）

Steven Lord
Steven Lord 2019-7-19
This is a bug. See Bug Report 1839169.
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Peter Jarosi
Peter Jarosi 2019-7-19
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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