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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 

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:

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

   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.


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