sybperl-l Archive

From: Steve Baumgarten <sbb at panix dot com>
Subject: Re: DBD::Sybase 1.07 install on Solaris 8 - failing t/exec.t test #11
Date: Nov 2 2005 9:42PM

Greg Earle wrote:

>> Floats tend to do this from time to time.
>> Reason I tend use decimal instead.
> 
> So, what's the consensus - do I just change the test in exec.t
> to match my returned mutant results?  :-)  And do a "make test"
> and "make install" manually, ignoring the mismatched results?

Mix floats and the == operator and you're bound to be surprised sooner 
or later. One must compare to a certain desired precision. You want 
something like:

   my $epsilon = 0.001;
   if( abs( $a - $b ) > $epsilon ) {
     # numbers are different
   } else {
     # numbers are the same
   }

Here's an excellent, easy-to-read discussion, "The Perils of Floating 
Point":

http://www.lahey.com/float.htm

Examples are given in Fortran, but even those not familiar with it can 
make sense of what the code is doing.

An excerpt from the article:

   At the heart of many strange results is one fundamental:
   floating-point on computers is usually base 2, whereas the external
   representation is base 10. We expect that 1/3 will not be exactly
   representable, but it seems intuitive that .01 would be. Not so! .01
   in IEEE single-precision format is exactly 10737418/1073741824 or
   approximately 0.009999999776482582. You might not even notice this
   difference until you see a bit of code like the following:

   REAL X
   DATA X /.01/
   IF ( X * 100.d0 .NE. 1.0 ) THEN
      PRINT *, 'Many systems print this surprising result. '
   ELSE
      PRINT *, 'And some may print this.'
   ENDIF

   Base-10 floating-point implementations don't have this anomaly.
   However, base-10 floating-point implementations are rare because
   base-2 (binary) arithmetic is so much faster on digital computers.

Feel free to blame base 2 floating point arithmetic. It's fast, it's a 
standard (IEEE 754), but it causes no end of grief when people attempt 
to use the == operator to determine whether two floating point numbers 
are equal.

SBB







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