Re: Problems to calculate sin

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 04 Mar 2008 07:15:01 -0800

Message-ID:

<13sqprkd4hajrb5@corp.supernews.com>

Sanny wrote:

On Mar 4, 6:29 pm, Patricia Shanahan <p...@acm.org> wrote:

Sanny wrote:

On Mar 4, 3:40 pm, Steve70 <batst...@libero.it> wrote:

I must create a program that use trigonometry function.
I know sin(30)=0.5 but when I use Math.sin() I can't get it
Math.sin(30*Math.PI/180)=0.49999999999999994
What's the problem?
Thank you
Stefano Buscherini

Math.sin uses Log tables to calculate value of Sin So they are
approximate to 0.0000000000001 Value.

I'm curious. Why log tables?

I don't know how Math.sin is implemented, and don't even assume it is
implemented the same way in all JVMs, but if I had to guess I would have
expected some sort of truncated Taylor series.

Patricia

Using Tables are faster than using a formula to compute a value.

Depends on many things, including the size of the tables. Remember that
one can do quite a lot of simple constant loading and floating point
arithmetic for the cost of one cache miss. Given the max 1 ulp error
requirement for Math.sin, I would expect a polynomial approximation to
be faster than a sufficiently precise table look-up.

By log table I just mean a Table for Sine. Log tables are more
familiar than Log Tables.

I assume you mean something like "Log tables are more familiar than sine
tables." However, my bachelor's degree was in mathematics. I am familiar
with sine tables, series-based approximations to sine, and have read a
little bit about cordic methods.

Do you actually know how Math.sin is implemented, or are you just saying
how you think you would implement it?

Patricia