From: jzakiya <jzakiya@...>
Date: 2009-12-24T09:55:05+09:00
Subject: Re: Trig value errors

On Dec 23, 7:45 pm, Seebs <usenet-nos...@seebs.net> wrote:
> On 2009-12-24, jzakiya <jzak...@mail.com> wrote:
>
> > Try this (with correct syntax if wrong).
> > Do similar for sin.
> > static VALUE
> > math_cos(VALUE obj, VALUE x)
> > {
> >     Need_Float(x);
> >     cos_value = DBL2NUM(cos(RFLOAT_VALUE(x)));
> >     if (abs(cos_value) < 1e-10) cos_value  0.0;
> >     return cos_value;
> > }
>
> That'd be exceptionally stupid.
>
> Here's the problem:  There exists at least one value for which the correct
> value of cos(x) is about 9e-11, which can be correctly represented (within
> a much, much, smallermargin of error) in a floating point value, and which
> you would then "fix" by changing it from not-zero to exactly-zero.
>
> Which is to say, this would introduce a comparatively HUGE error, much larger
> than the alleged error you're complaining about.
>
> If you want approximate values, round them.
>
> -s
> --
> Copyright 2009, all wrongs reversed.  PeterSeebach/ usenet-nos...@seebs.nethttp://www.seebs.net/log/<-- lawsuits, religion, and funny pictureshttp://en.wikipedia.org/wiki/Fair_Game_(Scientology) <-- get educated!

> Here's the problem:  There exists at least one value for which the correct
> value of cos(x) is about 9e-11, which can be correctly represented (within)

And what (extremely small) angle(s) would that be?
Provide examples please:

If so then do:

if (abs(cos_value) < 1e-14) cos_value = 0.0;

This provides the correct answers for the errors I illustrated.

This makes me happy. :-)