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Sin(pi) or cos(pi/2) problem with Matlab

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Kamuran
Kamuran 2015-5-10
回答： Karan Gill 2015-7-30
Hi,
I know that not getting ZERO for sin(pi) or cos(pi/2) in Matlab is an ongoing problem. My problem is inputting functions like
f=x^7+cos(x)^4/sin(pi*x) this is an only example input. It can be different but when sin(pi*x) is not equal to zero I will get a really large value but not inf and there is a difference between really large value and inf. Is there way to avoid or automatically filter this error.
the function can be also like f=x^7+cos(x)^4/sin(x) and when/if x=pi I need the function to be inf.
Anybody has a solution for this problem.
Thanks

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回答（2 个）

James Tursa
James Tursa 2015-5-10
编辑：James Tursa 2015-5-10
Pre-test the value of x. E.g., for scalar x:
if( floor(x) == x )
f = inf;
else
f = x^7 + cos(x)^4 / sin(pi*x);
end
xp = x / pi;
if( floor(xp) == xp )
f = inf;
else
f=x^7+cos(x)^4/sin(x)
end
But are you really OK with this behavior, but in cases where x (or xp) is one bit off from being an integer value, f is not inf? Or do you really want f to be inf for some range of small deviations from multiples of pi?
  1 个评论
Kamuran
Kamuran 2015-5-10
I would like to within small deviations of multiples of pi. But my major problem I don't know what function user will input to the code. There might be no sin or cos in the function. So I can not prepare for a specific function. I believe the only way out of this is to identify the problem to the user.

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Karan Gill
Karan Gill 2015-7-30
If you have the Symbolic Math Toolbox, you can use sym to represent pi symbolically and calculate f . Then, use double to convert the answer back into type double. Try:
>> x = 0;
f = double(x^7+cos(x)^4/sin(sym(pi)*x))
f =
Inf

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