[Numpy-discussion] what goes wrong with cos(), sin()

Christopher Barker Chris.Barker@noaa....
Wed Feb 21 13:54:14 CST 2007

Anne Archibald wrote:
> Or, to see more clearly, try taking (on a pocket calculator, say)
> sin(3.14) (or even sin(pi)).

This is an interesting point. I took a class from William Kahan once 
(pass/fail, thank god!), and one question he posed to us was:

How many digits of pi is used in an HP calculator?

I never figured out the answer myself, and the question involved the 
info that certain calculations involving pi being accurate to a certain 
degree. I don't have my HP calculator with me right now but I suspect 
that sin(pi) might well evaluate to exactly zero, or closer than 1e-16 

I wonder if there are any C math libs that do a better job than you'd 
expect from standard FP? (short of unlimited precision ones)


Christopher Barker, Ph.D.

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