Math::BigFloat - bug giving NaN on multiplying 1 * 1e-05

[p5pRT]

Migrated from rt.perl.org#126960 (status was 'rejected')

Searchable as RT126960$

[p5pRT]

From @epa

Created by @epa

Please see the following test program​:

use Math​::BigFloat;
foreach my $use_bigfloat (0, 1) {
  my $x;
  if ($use_bigfloat) {
  print "using BigFloat, ";
  $x = Math​::BigFloat->new(1);
  }
  else {
  print "using ordinary scalar, ";
  $x = 1;
  }
  print "x=$x\n";
  foreach my $t (0.0001, 0.00001) {
  print "t=$t\n";
  my $p = $x + $t;
  print "p=$p\n";
  my $i = int($p);
  print "i=$i\n";
  my $m = $t * $i;
  print "m=$m\n";
  print "\n";
  }
  print "\n";
}

For me this prints the following​:

using ordinary scalar, x=1
t=0.0001
p=1.0001
i=1
m=0.0001

t=1e-05
p=1.00001
i=1
m=1e-05

using BigFloat, x=1
t=0.0001
p=1.0001
i=1
m=0

t=1e-05
p=1.00001
i=1
m=NaN

I can see no reason why in the last section 1e-05 * 1 should give NaN.

Perl Info

Flags:
    category=library
    severity=high
    module=Math::BigFloat

Site configuration information for perl 5.22.1:

Configured by Red Hat, Inc. at Mon Dec 14 11:14:02 UTC 2015.

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Locally applied patches:
    Fedora Patch1: Removes date check, Fedora/RHEL specific
    Fedora Patch3: support for libdir64
    Fedora Patch4: use libresolv instead of libbind
    Fedora Patch5: USE_MM_LD_RUN_PATH
    Fedora Patch6: Skip hostname tests, due to builders not being network capable
    Fedora Patch7: Dont run one io test due to random builder failures
    Fedora Patch15: Define SONAME for libperl.so
    Fedora Patch16: Install libperl.so to -Dshrpdir value
    Fedora Patch22: Document Math::BigInt::CalcEmu requires Math::BigInt (CPAN RT#85015)
    Fedora Patch26: Make *DBM_File desctructors thread-safe (RT#61912)
    Fedora Patch27: Make PadlistNAMES() lvalue again (CPAN RT#101063)
    Fedora Patch28: Make magic vtable writable as a work-around for Coro (CPAN RT#101063)
    Fedora Patch200: Link XS modules to libperl.so with EU::CBuilder on Linux
    Fedora Patch201: Link XS modules to libperl.so with EU::MM on Linux


@INC for perl 5.22.1:
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Environment for perl 5.22.1:
    HOME=/home/eda
    LANG=en_GB.UTF-8
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    SHELL=/bin/bash

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[p5pRT]

From @pjacklam

This behaviour is as documented and as intended. Whether this is the most
sensible behaviour is debatable, but it is at least as it works now.

If we go to the core of what creates the NaN​:
​​
$x = Math​::BigFloat->new(1); # $x is a Math​::BigFloat
$t = 0.00001; # $t is a scalar
$p = $x + $t; # $p is a Math​::BigFloat
$i = int($p); # $i is a Math​::BigInt
$m = $t * $i; # $m is a Math​::BigInt

You see, your call to int() creates a Math​::BigInt. Multiplying 0.00001, or
any other non-integer, with a Math​::BigInt creates a NaN, because a
non-integer can't be represented as a Math​::BigInt.

You can get around this in several ways. One is to use the following at the
top of your program​:

use Math​::BigFloat;
use Math​::BigInt upgrade => 'Math​::BigFloat';

A second way is to use "$i = $p->bint();" rather than "$i = int($p)".

A third way is to use "$t = Math​::BigFloat->new("0.00001");" rather than
letting $t be a scalar.

I hope this helps.

Peter

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'

[p5pRT]

From @epa

But why do 0.0001 and 0.00001 behave differently? That cannot be right.

On 20 Dec 2015, at 14​:15, Peter John Acklam <pjacklam@​gmail.com<mailto​:pjacklam@​gmail.com>> wrote​:

This behaviour is as documented and as intended. Whether this is the most sensible behaviour is debatable, but it is at least as it works now.

If we go to the core of what creates the NaN​:
​​
$x = Math​::BigFloat->new(1); # $x is a Math​::BigFloat
$t = 0.00001; # $t is a scalar
$p = $x + $t; # $p is a Math​::BigFloat
$i = int($p); # $i is a Math​::BigInt
$m = $t * $i; # $m is a Math​::BigInt

You see, your call to int() creates a Math​::BigInt. Multiplying 0.00001, or any other non-integer, with a Math​::BigInt creates a NaN, because a non-integer can't be represented as a Math​::BigInt.

You can get around this in several ways. One is to use the following at the top of your program​:

use Math​::BigFloat;
use Math​::BigInt upgrade => 'Math​::BigFloat';

A second way is to use "$i = $p->bint();" rather than "$i = int($p)".

A third way is to use "$t = Math​::BigFloat->new("0.00001");" rather than letting $t be a scalar.

I hope this helps.

Peter

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[p5pRT]

From @pjacklam

eda@​waniasset.com​:

But why do 0.0001 and 0.00001 behave differently? That cannot be right.

Sorry for not noticing the different results. When I ran your code, I got NaN in both cases. When I ran your code with an older version of the Math-BigInt distribution (which contains the Math​::BigFloat module), I was able to reproduce your result. It turns out that this is a bug that was fixed in version 1.999701 of the Math-BigInt distribution. It was released in Septeber this year. In earlier versions, Math​::BigInt converted all non-integer values to NaNs EXCEPT numbers in the range -1 < x < 1 that were written as decimal numbers, i.e., as "0.0001", not "1e-4"​:

So the reason for the different behaviour in the two cases is that the two numbers stringify differently​: The number 0.0001 becomes the string "0.0001", but 0.00001 becomes the string "1e-5". That difference is essential. The first case hits the bug, but the second doesn't. You can actually see you get m=0 in both cases if you change your code to

  foreach my $t ("0.0001", "0.00001") {

and you get NaN in both cases if you change your code to

  foreach my $t ("1e-4", "1e-5") {

In any case, this is a bug that has been fixed, so I suggest you upgrade to Math-BigInt version 1.999701 or newer.

As a friendly reminder, please include the version number of relevant modules, if you submit another Perl bug report. It just makes life a little bit easier for the developers. :-)

Cheers,
Peter

[p5pRT]

From @epa

Thanks for the explanation. Can the newer version of Math​::BigFloat be added
to the next Perl stable release (5.22.2)? If so this bug can be changed to
PENDING_RELEASE.

--
Ed Avis <eda@​waniasset.com>

[p5pRT]

From @pjacklam

ma. 21. des. 2015 01.46.23 skrev eda@​waniasset.com​:

Thanks for the explanation. Can the newer version of Math​::BigFloat
be added to the next Perl stable release (5.22.2)? If so this bug
can be changed to PENDING_RELEASE.

I don't know about 5.22.2, but 5.23.5 contains the most recent
version of Math​::BigFloat.

[p5pRT]

From @epa

OK, so this will be fixed in perl 5.24 and so can be PENDING_RELEASE.
It will be up to the stable series pumpking (if such a post exists) whether
to include the new Math​::BigFloat in 5.22.2.

--
Ed Avis <eda@​waniasset.com>

[p5pRT]

From @tonycoz

On Mon Dec 21 06​:15​:08 2015, eda@​waniasset.com wrote​:

OK, so this will be fixed in perl 5.24 and so can be PENDING_RELEASE.
It will be up to the stable series pumpking (if such a post exists) whether
to include the new Math​::BigFloat in 5.22.2.

The Math​::Big* modules are now CPAN upstream, so closing this ticket.

Tony

[p5pRT]

@tonycoz - Status changed from 'open' to 'rejected'