Re: problem with cos ( pi/2 )
Re: problem with cos ( pi/2 )
- Subject: Re: problem with cos ( pi/2 )
- From: Tommy Nordgren <email@hidden>
- Date: Sun, 18 Jun 2006 18:33:14 +0200
On 16 jun 2006, at 17.07, billmonk wrote:
Not sure if this is the right list...searching it hasn't turned up
any likely entries...
When calling cos( pi/2 ), I'm getting a tiny number, rather than
zero, for
cos( M_PI_2 ); // pi/2, 1.57079632679489661923 from <math.h>
pi/2 is not an exactly representable number in floating point
binary. (not in any base in fact)
The only way to get the Mathematical result is by a symbolic math
package.
However the deviation appears about one order of magnitude too big,
since it IS possible
to represent pi/2 with an accuracy of about 1e-16
I've added "-llibm" to "Other C flags", as alluded to here,
http://developer.apple.com/documentation/Darwin/Reference/ManPages/
man3/intro.3.html
but am still getting 2.6484540326036093e-14 instead of 0.
My old IM:PowerPC Numeric book, and the OS X pdf here
http://developer.apple.com/documentation/Performance/Conceptual/
Mac_OSX_Numerics/Mac_OSX_Numerics.pdf
say cos( pi/2 ) is a special case (for all the reasons given in
PPCNumerics) but that an IEEE-compliant math lib should return
-0.0, with the "inexact" flag set, for cos( pi/2 ).
So it looks like I'm not actually using the IEEE libm. What am I
doing wrong?
As a data point, I notice Calculator.app in Degrees mode correctly
gives 0 for cos( 90 ), but in Radians mode gives a tiny number for
cos ( pi /2 ); a different one than I get for cos( M_PI_2 ). That's
kind of weird...
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