RE: Re: float bug? perl 5.8, DBI and oracle 10.2.0

I admire Peter's effort in tracking down the specifics at various
conversion points.

However, there are no documented guarantees of equality from the
library. I hope you're not seriously considering jigging the libraries
to make your program work. If you do that, you'll run up against the
same problem next release.

The more stable solution would be to change your program so it doesn't
depend on equality. Rather you could create a function floatEqual() or
floatZero() that tests whether two float values are nearly equal in
relative terms, or close to zero, without requiring that they be exactly
equal or zero. Then replace the == operator in your application code
with those function calls.

I bet it would be less work to use that solution than to jig the
libraries.

Besides, there are many floating point conversion points, including the
compiler itself, that can change with a new release.

-Will


-----Original Message-----
From: erwan.lemonnier@xxxxxxxxx
[mailto:erwan.lemonnier@xxxxxxxxx] On Behalf Of Erwan Lemonnier
Sent: Wednesday 11 July 2007 04:29
To: dbi-users@xxxxxxxx; hjp@xxxxxxxxx
Subject: Re: Re: float bug? perl 5.8, DBI and oracle 10.2.0


Taking up on Peter's answer to
http://www.nntp.perl.org/group/perl.dbi.users/2007/07/msg31663.html:

This is peculiar, though. DBD::Oracle gets the number from
the database
as a string (this can be confirmed with Devel::Peek::Dump),
so the above
code should be equvialent to:

my ($val) = ('1.73696');

my $sum = 1.73696 - $val;
is($sum,0,"does sum $sum == 0?");

That's indeed what I thought. I tried. But as you mention
below, it isn't.

However, it isn't, for some reason, and I can't see the difference
(upgrading $val to utf8 doesn't make a difference).

Why does perl convert the string '1.73696' to

0011111111111011110010101001011010010001101001110101110011010000

if it comes from an Oracle database, but convert it to

0011111111111011110010101001011010010001101001110101110011010001

if it is hard coded in the script or read from a file?

Thanks! I think you just put your finger on the core of the problem!
This is the very reason why I turned toward the DBI list in the first
hand.

Note that the trailing bit is most probably the one that gets
interpreted as 2^-52 (2.22044604925031e-16) in the faulty $sum result.

I would like to check what binary representation perl 5.6.2 returns
for that same string when fetched from the database. How did you get
the binary strings above?
It itches me that the computation gives the correct result in perl
5.6.2 but not 5.8.8.

The next question would be: what is in charge of converting the data
representation fetched from oracle into perl's native float (leading
to the binary representation you listed above). Would that be
DBD::Oracle? or DBI? or perl itself?

/Erwan Lemonnier




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